Rare Earths / Rare Earth Elements / REE

Rare earths, each member of the group of chemical elements consisting of three elements of the group 3 (scandium [Sc], yttrium [Y] and lanthanum [La]) and the first extended series of elements below the main body of the periodic table (Cer [Ce ] by Lutetium [Lu]). The elements cerium by lutetium are called lanthanides, but many scientists also call these elements, though erroneously, rare earths.

The rare earths are generally trivalent elements, but a few have different valences. Cer, praseodymium and terbium can be tetravalent, while samarium, europium and ytterbium can be bivalent. Many introductory scientific books consider rare earths to be so chemically similar that they can be considered together as one element. To some extent, that's correct - about 25 percent of their uses are based on this close similarity - but the other 75 percent of rare earth uses are based on the unique properties of each element. In addition, a close examination of these elements reveals great differences in their behavior and properties; for example, the melting point of lanthanum, the prototype element of the lanthanide series (918 ° C or 1.684 ° F), is much lower than the melting point of lutetium, the last element in the series (1.663 ° C or 3.025 ° F). This difference is much larger than for many groups in the periodic table; for example, the melting points of copper, silver and gold only vary by about 100 ° C (180 ° F).

The name rare earths itself is a mistake. At the time of their discovery in the 18th century, they were found to be part of complex oxides, which were then referred to as "earths". In addition, these minerals seemed to be in short supply, and so these newly discovered elements were called "rare earths". In fact, these elements are quite abundant and exist in many functioning deposits around the world. The 16 naturally occurring rare earths fall in the 50th percentage point of elemental abundance. At the beginning of the 21st century, China became the world's largest producer of rare earth elements. Australia, Brazil, India, Kazakhstan, Malaysia, Russia, South Africa, and the United States also extract and refine significant quantities of these materials.

Many people are unaware of the enormous impact rare earth elements have on their daily lives, but it is almost impossible to use a piece of modern technology that does not contain one. Even a product as simple as a light flint contains rare earth elements. The modern automobile, one of the largest consumers of rare earth products, illustrates their consistency. Dozens of electric motors in a typical automobile, as well as the speakers in its sound system, use neodymium-iron-boron permanent magnets. Electrical sensors use yttria-stabilized zirconia to measure and control the oxygen content of the fuel. The three-way catalyst is based on cerium oxides to reduce nitrogen oxides to nitrogen gas and to oxidize carbon monoxide to carbon dioxide and unburned hydrocarbons to carbon dioxide and water in the exhaust products. Phosphors in optical displays contain yttrium, europium and terbium oxides. The windshield, mirrors and lenses are polished with cerium oxides. Even the gasoline or diesel fuel that powers the vehicle has been refined with rare earth cracking catalysts that contain lanthanum, cerium or mixed rare earth oxides. Hybrid cars are powered by a rechargeable nickel-lanthanum metal hydride battery and an electric traction motor with permanent magnets with rare earth elements. In addition, modern media and communication devices - cell phones, televisions, and computers - use rare earths as magnets for speakers and hard drives, and phosphors for optical displays. The amounts of rare earths used are quite small (0,1-5 wt%, with the exception of permanent magnets, which contain about 25 percent neodymium), but they are critical and any of these devices would not work as well or would be essential harder if it weren't for the rare earths.

The periodic table with elements of strategic metals and rare earths

1 H

2 He

3 Li

4 Be

5 B

6 C

7 N

8 O

9 F

10 Ne

11 Na

12 Mg

13 Al

14 Si

15 P

16 S

17 Cl

18 Ar

19 K

20 Ca

21 Sc

22 Ti

23 V

24 Cr

25 Mn

26 Fe

27 Co

28 Ni

29 Cu

30 Zn

31 Ga

32 Ge

33 As

34 Se

35 Br

36 Kr

37 Rb

38 Sr

39 Y

40 Zr

41 Nb

42 Mo

43 Tc

44 Ru

45 Rh

46 Pd

47 Ag

48 Cd

49 In

50 Sn

51 Sb

52 Te

53 I

54 Xe

55 Cs

56 Ba

57 La

58 Ce

59 Pr

60 Nd

61 Pm

62 Sm

63 Eu

64 Gd

65 Tb

66 Dy

67 Ho

68 Er

69 Tm

70 Yb

71 Lu

72 Hf

73 Ta

74 W

75 Re

76 Os

77 Ir

78 Pt

79 Au

80 Hg

81 Tl

82 Pb

83 Bi

84 Po

85 At

86 Rn

87 Fr

88 Ra

89 Ac

90 Th

91 Pa

92 U

93 Np

94 Pu

95 Am

96 Cm

97 Bk

98 Cf

99 Es

100 Fm

101 Md

102 No

103 Lr

104 Rf

105 Db

106 Sg

107 Bh

108 Hs

109 Mt

110 Ds

111 Rg

112 Cn

113 Nh

114 Fl

115 Mc

116 Lv

117 Ts

118 Og

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Discovery and history of the rare earths

Although the rare earths have existed since Earth's birth, their existence did not end until the 18. Century known. 1787 discovered the Swedish army lieutenant Carl Axel Arrhenius in a small quarry in Ytterby (a small town near Stockholm) a unique black mineral. This mineral was a mixture of rare earths, and the first single element isolated was 1803 the element cerium.

The history of each rare earth element is complex and confusing, largely because of their chemical similarity. Many "newly discovered elements" were not one element, but mixtures of up to six different rare earth elements. In addition, there was evidence of the discovery of a large number of other "elements" that should, but were not, members of the rare earth series.

The last naturally occurring rare earth element (lutetium) was discovered by 1907, but research into the chemistry of these elements was difficult, as no one knew how many real rare earth elements there were. Fortunately, 1913-14 solved the situation with the research of Danish physicist Niels Bohr and English physicist Henry Gwyn Jeffreys Moseley. Bohr's theory of the hydrogen atom allowed the theorists to show that only 14 lanthanides exist. Moseley's experimental studies confirmed the existence of 13 of these elements and showed that the 14. Lanthanide element must be 61 and lies between Neodymium and Samarium.

In the 1920 years, the search for Element 61 was intense. 1926 claimed that groups of scientists at the University of Florence, Italy, and at the University of Illinois discovered the element 61 and named the element Florentium or Illinium, but their claims could not be independently verified. The fury of these allegations and counterclaims eventually returned to 1930. It was not until 1947, after the fission of uranium, that this 61 element was isolated from scientists at the Oak Ridge National Laboratory of the US Atomic Energy Commission in Tennessee and named Promethium. (See the articles about these elements for more details on the discovery of each element.)

In the 160 years of discovery (1787-1947), the separation and purification of rare earth elements has been a difficult and time-consuming process. Many scientists spent their entire lives obtaining an 99 percent pure rare earth, mostly by fractional crystallization, which exploits the small differences in the solubility of a rare earth salt in an aqueous solution compared to a neighboring lanthanide element.

Since the rare earth elements were found to be fission products of the cleavage of a uranium atom, the US Atomic Energy Commission made great efforts to develop new methods for separating the rare earth elements. However, 1947 simultaneously published results from Gerald E. Boyd and colleagues at the Oak Ridge National Laboratory and Frank Harold Spedding, as well as colleagues from the Ames Laboratory in Iowa, which showed that ion exchange processes provide a much better way to separate the rare earths.

Frequency, occurrence and reserves of rare earths

As mentioned earlier, the rare earths are quite abundant, but their availability is somewhat limited, mainly because their concentration in many ores is quite low (less than 5 percent by weight). An economically viable source should contain more than 5 percent rare earths, unless they are mined with another product - e.g. zirconium, uranium or iron - which enables the economic recovery of ore bodies with concentrations of only 0,5 percent by weight.

Of the 83 naturally occurring elements, the 16 naturally occurring rare earth elements fall in the 50th percentile of element abundances. Promethium, which is radioactive, with the most stable isotope with a half-life of 17,7 years, is not considered to be naturally occurring, although traces have been found in some radioactive ores. Cer, the most common, is ranked 28th and thulium, the least common, is ranked 63rd. Taken together, rare earths are the 22nd most common "element" (at the 68th percentile mark). The non-lanthanide rare earth elements yttrium and scandium are the 29th and 44th in their abundance.

Lanthanum and the light lanthanides (cerium by europium) are more abundant than the heavy lanthanides (gadolinium by lutetium). Thus, the individual light lanthanide elements are generally cheaper than the heavy lanthanide elements. In addition, even-order metals (cerium, neodymium, samarium, gadolinium, dysprosium, erbium, and ytterbium) are more abundant than their odd-numbered neighbors (lanthanum, praseodymium, promethium, europium, terbium, holmium, thulium, and lutetium).

Rare earth ore deposits are found all over the world. The main ores are found in China, the United States, Australia and Russia, while other orebodies are found in Canada, India, South Africa and Southeast Asia. The most important minerals contained in these ores are bastnasite (fluorocarbonate), monazite (phosphate), loparite [(R, Na, Sr, Ca) (Ti, Nb, Ta, Fe3 +) O3] and laterite (SiO2, Al2O3 and Fe2O3).

Chinese deposits made 2018 about 82 percent of the world's rare earth minerals extracted (112.000 tonnes of rare earth oxide). About 94 percent of China's mined rare earths come from bastnasite deposits. The largest deposit is located in Bayan Obo, Inner Mongolia (83 percent), while smaller deposits in the provinces of Shandong (8 percent) and Sichuan (3 percent) are mined. About 3 percent comes from Laterittonen (ion absorption), which are located in the provinces of Jiangxi and Guangdong in southern China, while the remaining 3 percent are produced in different locations.

Officially, 2018 130.000 tons of REO equivalent have been mined, but a rare earth black market is expected to produce another 25 percent of that amount. Most rare earth materials are smuggled out of China.

The Chinese monopoly allowed prices for various rare earth materials from 2009 to 2011 to be increased by hundreds of percent and export quotas for many of these products as well. This led to a major change in the dynamics of the rare earth markets. The dismantling of Bastnasit was resumed at 2011 Mountain Pass, California, after a nine-year hiatus, and Monazit mining began in Mount Weld, Australia, the same year. At the same time, Loparit was mined in Russia, while monazite was mined in India, Vietnam, Thailand and Malaysia. These and other mining companies brought a new balance between supply and demand, in which China was still the primary supplier of rare earth minerals, but companies either sought alternative sources, used less, or recycled more rare earths.

Quick overview of the rare earths

Z	sym bowl	Name	etymologie	selected uses
21	Sc	scandium	from latin Scandia 'Scandinavia', where the first ore was discovered	Stadium lighting, fuel cells, racing bikes, X-ray technology, laser
39	Y	Yttrium	after the discovery of the rare earth ore at Ytterby, Sweden	Fluorescent lamp, LCD and plasma screens, LEDs, fuel cell, Nd: YAG laser
57	La	lanthanum	from Greek lanthanein 'To be hidden'.	Nickel-metal hydride batteries (e.g. in electric and hybrid cars, laptops), catalysts, Particulate filters, fuel cells, high refractive index glasses
58	Ce	cerium	after the dwarf planet Ceres.	Car catalytic converters, particulate filters, ultraviolet radiation protective glasses, polishing agents
59	Pr	praseodymium	from greek Prasinos , Allium green ' didymos 'Double' or 'twin'	Permanent magnets, aircraft engines, electric motors, glass and enamel dyeing
60	Nd	Neodym	from greek neos 'New' and didymos 'Double' or 'twin'	Permanent magnets (e.g. in electric motors, wind turbines, Magnetic resonance tomographs, hard disks), glass staining, laser, CD player
61	Pm	promethium	from Prometheus, a titan of Greek mythology	Luminescent numbers, heat sources in space probes and satellites (radioactive element)
62	Sm	Samarium	after the mineral Samarskit, which in turn named after the Mountain engineer WM Samarski	Permanent magnets (in dictation machines, headphones, hard disk drives), Space, glasses, lasers, medicine
63	Eu	europium	besides americium, the only element named after one continent	LEDs, fluorescent lamps, plasma television (red phosphor)
64	Gd	gadolinium	after Johan Gadolin (1760-1852), the namesake of Gadolinits	Contrast agent (magnetic resonance imaging), radar screens (green phosphor), Nuclear fuel elements
65	Tb	terbium	after the Swedish locality Ytterby	Phosphors, permanent magnets
66	Dy	dysprosium	from Greek δυσπρόσιτος, inaccessible '	Permanent magnets (e.g. wind turbines), phosphors, lasers, nuclear reactors
67	Ho	holmium	from Stockholm (lat. Holmia) or a derivation of the chemist Holmberg	High-performance magnets, medical technology, lasers, nuclear reactors
68	Er	erbium	after the Swedish locality Ytterby	Laser (medicine), fiber optic cable
69	Tm	thulium	to Thule, the mythical island on the edge of the world	Fluorescent lamps, X-ray technology, televisions
70	Yb	Ytterbium	after the Swedish locality Ytterby	Infrared laser, chemical reducing agent
71	Lu	lutetium	after the Roman name of Paris, Lutetia	Positron emission tomograph

 

Until 2017, the world's known reserves of rare earth minerals amounted to around 120 million tons of REO. China has the largest share (37 percent), followed by Brazil and Vietnam (each 18 percent), Russia (15 percent) and the rest of the world (12 percent). With such large reserves, the world would not run out of rare earths for more than 900 years if the demand for minerals remained at the level of 2017. Historically, however, rare earth demand has increased by about 10 per year. If demand on this scale continued to rise and no recycling of the produced rare earths were carried out, the known world reserves would probably come sometime after the middle of the 21. Century exhausted.

In view of the limited reserves and high value of rare earth metals, recycling of these elements from end-of-life consumer goods is likely to become more important. Currently, only scrap metals, magnetic materials and compounds used in the manufacture of phosphors and catalysts are recycled. However, products containing relatively large amounts of rare earths could be instantly recycled using existing techniques. These include rechargeable nickel-metal hydride batteries, which contain a few grams to a few kilograms of LaNi5-based alloys as hydrogen absorbers, as well as large permanent magnets based on SmCo5 and Nd2Fe14B. All of these materials contain 25-30 weight percent light lanthanides - much more than even the best rare earth ore (see below). However, the majority of consumer electronic devices contain only small amounts of rare earths. For example, the spindle magnet of a hard drive contains only a few grams of Nd2Fe14B. A cell phone speaker magnet makes up less than 0,1 percent of the total mass of the phone. A compact fluorescent lamp has only a fraction of a gram of lanthanide metals in its phosphorus. Given the complexity of many modern electronic devices, recycling of rare earths must happen at the same time as recycling of other valuable resources and potentially hazardous substances. These include precious metals (such as silver, gold, and palladium), non-ferrous metals (such as aluminum, cobalt, nickel, copper, gallium, and zinc), carcinogens (such as cadmium), toxins (such as mercury, lead, and beryllium), plastics, glass, and ceramics. Numerous scientific and technical questions must therefore be solved, firstly in order to create consumer goods that are easily recyclable at the end of their life, and secondly in order to make the recycling of rare earths meaningful and economical and thus the rare earths - an extremely valuable, but nature's limited resource - to use optimally.

Minerals and ores of the rare earths

The content of the individual rare earth elements varies greatly from mineral to mineral and from deposit to deposit. The minerals and ores are generally classified as "light" or "heavy"; in the former group most of the elements present are the light atomic elements (i.e. lanthanum, cerium, praseodymium, neodymium, samarium and europium), while most of the elements in the latter group are the heavy atomic elements, gadolinium, terbium, dysprosium, holmium, erbium , Thulium, ytterbium and lutetium, as well as yttrium, which are considered members of the heavy group. The geochemistry of scandium differs significantly from the geochemistry of the other rare earth elements. Information about its ores and minerals can be found in the article scandium. Essentially, no scandium is found in any of the minerals listed below.

Of the approximately 160 minerals known to contain rare earth, only four are currently mined for their rare earths: Bastnasite, Lateritton, Monazit, and Loparit. With the exception of laterite clay, these minerals are good sources of light lanthanides and lanthanum and account for about 95 percent of the rare earths used. Laterittone are a commercial source for the heavy lanthanides and yttrium.

Other minerals that have been used as a source of rare earths include apatite, euxenite, gadolinite and xenotime. Allanite, fluorite, perovskite, spherical and zircon have the potential to be future sources of rare earths. (In addition, uranium and iron residues have been used in the past as a source of the heavy lanthanides plus yttrium and light lanthanides plus lanthanum.) Many of these minerals, such as apatite and euxenite, are processed for other ingredients, and the rare earths could be by-produced become. In addition to the minerals that occur in the earth's crust, there are also some deep-sea slimes, such as near Minamitori Iceland, Japan, which contain rare earth elements. The concentrations vary between hundreds and thousands of parts per million, and these sludges may someday be a source of rare earths.

The idealized chemical compositions of these 13 minerals, which are rare earth sources, are listed in the table.

Composition of primary rare earth content

Name	optimal composition	Primary content of REE
allanite	(Coffee shop2+) (R, Al, Fe3+)3Si3O13H	R = light lanthanides
apatite	Ca5(PO4)3F	R = light lanthanides
bastnasite	RCO3F	R = light lanthanoid (60-70%)
euxenite	R (Nb, Ta) TiO6 â € xH2O	R = heavy lanthanoids plus Y (15-43%)
fluorite	CaF2	R = heavy lanthanoids plus Y
gadolinite	R2(Fe2+, Be)3Si2O10	R = heavy lanthanoids plus Y (34-65%)
laterite clays	SiO2, Al2O3Fe2O3	R = heavy lanthanoids plus Y
loparite	(R, Na, Sr, Ca) (Ti, Nb, Ta, Fe3+)O3	R = light lanthanoid (32-34%)
monazite	RPO4	R = light lanthanoid (50-78%)
perovskite	CaTiO3	R = light lanthanides
sphene	CaTiSiO4X2 (X = ½O2−OH-, or F-)	R = light lanthanides
xenotime	RPO4	R = heavy lanthanoids plus Y (54-65%)
zircon	ZrSiO2	R = both light and heavy lanthanoids plus Y

Composition of selected rare earth minerals

bastnasite

Bastnasite, a fluorocarbonate, is the major source of rare earths. Approximately 94 percent of the rare earths used worldwide come from mines in Mountain Pass, California, USA, Bayan Obo, Inner Mongolia, China, Shandong Province, China, and Sichuan Province, China. The Bayan Obo deposit is a bit richer in praseodymium and neodymium than the bastnasite in the mountain pass, especially at the expense of the lanthanum content, which is 10 percent higher in the ore in the mountain pass. The rare earth contents of the Shandong and Sichuan minerals differ slightly from those of the Bayan Obo minerals and also those of the other. The Shandong Bastnasite is similar to the mineral of the Mountain Pass. The Sichuan ore contains more lanthanum, less praseodymium and neodymium and about the same amount of cerium as the Bayan Obo deposit.

The rare earth content in selected minerals, including some bastnasites, is given in the table.

Rare Earth Element Bastnasit (Mountain Pass, California) Bastnasit Monazit (Mount Weld, Australia) Xenozeit (Lehat, Malaysia) High-Y Laterite Loparit (Kola Peninsula , Russia)

 

Laterite clay (s)

The Laterittone (also known as Ionenabsorptionstone) consist mainly of silica, alumina and iron oxide; those that also contain viable amounts of rare earths are found only in Jiangxi Province in southeastern China. Of the Jiangxi deposits, the clays of Longnan are quite rich in heavy lanthanides and yttrium. The clays in Xunwu have a very unusual distribution of rare earths, rich in lanthanum and neodymium with a relatively high yttrium content. Also noteworthy is the low concentration of cerium and praseodymium in both clays, especially in the Xunwu clay, compared to the normal rare earth distribution in the other minerals. These clays are the major source of heavy elements used in rare earth containing products, eg dysprosium in Nd2Fe14B permanent magnets.

 

monazite

Monazite, a phosphate, is the third most important ore source for rare earths. In the 1980 years, it accounted for 40 percent of world production, but contributed to 2010 only a small part of the mined rare earths. There were two reasons for this change: First, it is more expensive to process monazite from the ore body into a rare earth concentrate than Bastnasite; secondly, monazite contains a significant amount of radioactive thoria (Thoxnumx) compared to bastnasite, requiring special environmental handling and storage procedures. However, it is expected that Monazit will contribute to a growing share of mined rare earths as activities in Mount Weld, Australia (Lynas), are ramped up to full production by the end of 2.

Monazit is widespread; In addition to Australia, it is found in India, Brazil, Malaysia, the Commonwealth of Independent States, the United States, Thailand, Sri Lanka, the Democratic Republic of the Congo, South Korea and South Africa.

loparit

Loparite is a complex mineral that is mined mainly for its titanium, niobium and tantalum content, with the rare earth minerals extracted from the ore being by-produced. This ore occurs mainly on the Kola Peninsula in northwestern Russia and in Paraguay. Its rare earth distribution is similar to that of Bastnasit, except that it has significantly higher concentrations of heavy lanthanides and yttrium.

Xenotime

Xenotime is a phosphate mineral similar to monazite but enriched with heavy lanthanides and yttrium. It has been mined for many years but has contributed only about 1 percent of the rare earth mined since the 1970 years. Xenotime contains minor amounts of the radioactive compounds U3O8 and Thoxnumx as monazite. Due to its high concentrations of yttrium and heavy lanthanides, xenotime is used as the starting material for the individual rare earth elements and not as a mixture of heavy rare earths. The main producer of Xenotime is Malaysia; Deposits are also reported in Norway and Brazil.

Electronic structure and ionic radius

The chemical, metallurgical and physical behavior of rare earths is determined by the electron configuration of these elements. In general these elements are trivalent, R3 +, but some of them have different valences. The number of 4f electrons of each lanthanide is given in the table of the number of 4f electrons and ionic radii for the R3 + ion. The 4f electrons have lower energies than the outer three valence electrons and lie radially inward of the outer three valence electrons (ie 4f electrons are “localized” and part of the ion nucleus), and are therefore not directly involved in bonding with other elements, if one Connection is formed. Therefore, the lanthanides are chemically similar and difficult to separate and occur together in different minerals. The outer or valence electrons for the 14 lanthanides and lanthanum are equal, 5d6s2; for Scandium 3d4s2; and for yttrium 4d5s2. There are some differences in the chemical properties of the lanthanides due to the lanthanide contraction and the hybridization or mixing of the 4f electrons with the valence electrons.

The systematic and even decrease of lanthanum to lutetium is called lanthanide contraction. It is due to the increase in nuclear charge, which is not completely shielded by the extra 4f electron when going from one lanthanide to the next. This increased effective charge pulls the electrons (both the core and outer valence electrons) closer to the nucleus, taking into account the smaller radius of the higher atomic number lanthanides. The lanthanide contraction also contributes to the reduction of lanthanum to lutetium basicity and is the basis for various separation techniques.

As the 4f electrons are added as one moves through the lanthanide series from lanthanum to cerium to praseodymium and so on, the electrons that have a magnetic moment due to the electron spin maintain the same direction of rotation and the moments are aligned parallel to each other, until the 4f plane is half full, ie with seven 4f electrons in the gadolinium. The next electron must be aligned anti-parallel according to the Pauli exclusion principle, so that two 4f electrons are paired. This continues until the 14. Electron is added to the lutetium, where all 4f electron spins are paired and the lutetium does not have 4f magnetic moment.

The 4f electron configuration is extremely important and determines the magnetic and optical behavior of the lanthanide elements; For example, the particular properties of strong Nd2Fe14B permanent magnets are due to the three 4f electrons in neodymium, and the red color in optical displays using cathode ray tubes is provided by the europium ion in a host compound while the green color is provided by terbium.

As already mentioned, several lanthanides can have a different valence state, R4 + for R = cerium, praseodymium and terbium and R2 + for R = samarium, europium and ytterbium. These additional valence states are a striking example of Hundes Rule, which states that empty, half-filled and fully-filled electronic levels are usually more stable states: Ce4 + and Tb4 + give up an f electron to an empty and half-filled 4f level, respectively and Eu2 + and Yb2 + gain an f electron to make a half-filled and fully-filled 4f level, respectively. Pr4 + and Sm2 + can in rare cases gain additional stability by giving up or gaining an f-electron. In both of these cases, they tend to, but do not reach the empty or half-full level. By giving up a 4f electron to an R4 + ion, the radii of cerium, praseodymium and terbium become smaller, 0,80, 0,78 and 0,76 Å. Conversely, samarium, europium and ytterbium gain a 4f electron from the valence electrons to an R2 + ion, and their radii increase to 1,19, 1,17 and 1,00 Å, respectively. Chemists have used these valence changes to separate Ce4 +, Eu2 +, and Yb2 + from the other trivalent R3 ions by relatively cheap chemical methods. CeO2 (where Ce is tetravalent) is the normal stable oxide form, while the oxides of praseodymium and terbium have the stoichiometry Pr6O11 and Tb4O7, which contain both the tetra and trivalent states - i.e. 4PrO2 ∙ Pr2O3 and 2TbO2 ∙ Tb2O3. The divalent ions Sm2 +, Eu2 + and Tb2 + form dihalides - eg SmCl2, EuCl2 and YbCl2. Several europium oxide stoichiometries are known: EuO (Eu2 +), Eu2O3 (Eu3 +) and Eu3O4 (e.g. EuO ∙ Eu2O3).

The ionic radius of scandium is much smaller than that of the smallest lanthanide, lutetium: 0,745 Å versus 0,861 Å. The radius of scandium is slightly larger than that of the usual metal ions - e.g. Fe3 +, Nb5 +, U5 + and W5 +. This is the main reason why scandium is essentially not found in any of the normal rare earth minerals, usually no more than 0,01 percent by weight. However, scandium is obtained as a by-product in the processing of other ores (e.g. wolframite) and from mining waste (e.g. uranium). On the other hand, the radius of yttrium, 0,9 Å, is almost identical to that of holmium, 0,901 Å, and this explains the presence of yttrium in the heavy lanthanide minerals.

Most rare earth metals have a valency of three; however, Cer's is 3,2, and europium and ytterbium are bivalent. This becomes clear when the metallic radii are represented as a function of the atomic number. The metallic radii of the trivalent metals have the normal lanthanide contraction, but a clear deviation occurs in cerium, where its radius falls below the line defined by the trivalent metals, and in europium and ytterbium, where their radii are far above this line.

The melting points for europium and ytterbium are significantly lower than those of the adjacent trivalent lanthanides when plotted against atomic number, which is also consistent with the divalent nature of these two metals. Anomalies are also observed in other physical properties of europium and ytterbium compared to trivalent lanthanide metals (see below for properties of the metals).

The table shows the number of 4f electrons and the radius of the R3 + ion for the rare earth elements.

Number of 4f electrons and Ionic radii for the R3 + ion
Rare earth Element	number of 4f electrons	number of unpaired 4f electrons	Ionic Radius (Å)
La	0	0	1.045
Ce	1	1	1.01
Pr	2	2	0.997
Nd	3	3	0.983
Pm	4	4	0.97
Sm	5	5	0.958
Eu	6	6	0.947
Gd	7	7	0.938
Tb	8	6	0.923
Dy	9	5	0.912
Ho	10	4	0.901
Er	11	3	0.890
Tm	12	2	0.880
Yb	13	1	0.868
Lu	14	0	0.861
Sc	0	0	0.745
Y	0	0	0.900

 

Processing of rare earth ores

All rare earth ores contain less than 10 percent of REO and need to be upgraded to about 60 percent for further processing. They are first ground to a powder and then separated from the other materials in the orebody by various standard processes, including magnetic and / or electrostatic separation and flotation. In the case of Mountain Pass Bastnasit, a hot foam floatation process is used to remove the heavier products, barite (BaSO4) and Celestite (SrSO4), by settling as the bastnasite and other light minerals are driven off. The 60 percent REO concentrate is treated with 10 percent HCl to dissolve the calcite (CaCO3). The insoluble residue, now 70 percent REO, is roasted to oxidize the Ce3 + to the Ce4 + state. After cooling, the material is leached with HCl and dissolves the trivalent rare earths (lanthanum, praseodymium, neodymium, samarium, europium and gadolinium) to leave the cerium concentrate, which is refined and marketed to various qualities. The europium can be easily separated from the other lanthanides by reducing the europium to divalent form and the remaining dissolved lanthanides are separated by solvent extraction. The other bastnasites are treated in a similar way, but the exact reagents and procedures differ from the other components of the various orebodies.

Monazite and xenoite ores are treated essentially the same as both are phosphate minerals. The monazite or xenotime is separated from the other minerals by a combination of gravity, electromagnetic and electrostatic techniques and then split by either the acidic process or the basic process. In the acid process, the Monazite or Xenotime is treated with concentrated sulfuric acid at temperatures between 150 and 200 ° C (302 and 392 ° F). The solution contains soluble rare earth and thorium sulfates and phosphates. The separation of thorium from the rare earths is quite complicated as the solubilities of both the thorium and the rare earths vary depending on the temperature and acidity. Separation is not possible with very low and medium acid values. When the acid content is low, the thorium phosphate precipitates out of the solution and rare earth sulfates remain in the solution, while when the acid content is high, the opposite occurs - the rare earth sulfate is insoluble and thorium is soluble. After the thorium has been removed from the rare earths, these are used as a mixed concentrate or processed for the individual elements (see below).

In the basic process, finely ground monazite or xenotime is mixed with an 70 percent sodium hydroxide solution (NaOH) and held for several hours in an autoclave at 140-150 ° C (284-302 ° F). After the addition of water, the soluble sodium phosphate (Na3PO4) is by-produced from the insoluble R (OH) 3, which still contains 5-10 percent thorium. Two different methods can be used to remove thorium. In one process, the hydroxide is dissolved in hydrogen chloride (HCl) or nitric acid (HNO3) and then the thorium hydroxide (Th (OH) 4) is selectively precipitated by the addition of NaOH and / or ammonium hydroxide (NH4OH). In the other method, HCl is added to the hydroxide to lower the pH to about 3 to dissolve the RCl3, and the insoluble Th (OH) 4 is deposited. The thorium-free rare earth solution is converted into the hydrated chloride, carbonate or hydroxide and sold as a mixed concentrate, or it can be used as a starting material for the separation of the individual elements (see below).

Separation chemistry of the rare earths

The rare earth separation processes used today were developed during and shortly after World War II in several laboratories of the US Atomic Energy Commission (AEC). Work on the ion exchange process was performed at the Oak Ridge National Laboratory (Oak Ridge, Tennessee) by Gerald E. Boyd and coworkers and at the Ames Laboratory (Ames, Iowa) by Frank Harold Spedding and coworkers. Both groups showed that the ion exchange process would work at least on a small scale to separate rare earths. In the 1950s, the Ames group showed that it was possible to separate kilograms of high-purity (> 99,99 percent) individual rare earth elements. This was the beginning of the modern rare earth industry, when large quantities of high purity rare earth elements became available for electronic, magnetic, phosphor and optical applications.

Donald F. Peppard and colleagues from the Argonne National Laboratory (near Chicago, Ill.) And Boyd Weaver and Oak Ridge National Laboratory staff developed the liquid-liquid solvent extraction method for rare earth separation in the mid-XNXX's. With this method, all rare earth producers separate mixtures into the individual elements with purities ranging from 1950 to 95 percent. The ion exchange process is much slower, but higher purities of more than 99,9 percent (ie 99,9999999 nines or better) can be achieved. For optical and phosphorous-containing materials where purities of 5 to 5 nines are required, the single rare earth element is first purified by solvent extraction to a purity of about 6 percent and then further processed by ion exchange to the purity required for the particular application.

ion exchange

In the ion exchange process, a metal ion, R3 +, in solution exchanges with three protons on a solid ion exchanger - a natural zeolite or a synthetic resin usually called resin. The tenacity with which the cation is held by the resin depends on the size of the ion and its charge. A separation of the rare earths is not possible, however, because the resin is not selective enough. Separation is possible through the introduction of a complexing agent; when the strength of the R3 + ion complex of neighboring lanthanide ions varies sufficiently from one rare earth to another, the separation occurs. Two common complexing agents for separating rare earths are ethylene diamine tetraacetate (EDTA) and hydroxyethylene diamine triacetate (HEDTA).

0,1 mm (0,004 inch) diameter resin beads are packed in a long column and the resin bed is made by passing an acid through the column. It is then loaded with a mixed rare earth acid solution containing the complexing agent and a retention ion such as Cu2 + or Zn2 +. The retention is needed to prevent the first rare earth ion from spreading and being lost during the separation process. An eluent, ammonium (NH4), pushes the rare earths through the ion exchange columns. The most stable complex arises first, ie the copper or zinc complex, followed by lutetium, ytterbium, the other lanthanides (and yttrium, which occurs mostly near dysprosium and holmium, depending on the complexing agent), and finally lanthanum. The individual rare earth complexes R3 + form rectangular bands with a minimum overlap of adjacent bands. The given rare earth solution is collected and the R3 + ion is precipitated from the solution with oxalic acid. The rare earth oxalate is converted to the oxide by heating in air to 800-1.000 ° C (1.472-1.832 ° F).

Solvent extraction

The liquid-liquid solvent extraction process uses two immiscible or partially immiscible solvents that contain dissolved rare earths. The two liquids are mixed, the solutes are distributed between the two phases until equilibrium is established, and then the two liquids are separated. The concentrations of the solutes in the two phases depend on the relative affinity for the two solvents. The product (liquid) that contains the desired solute is called "extract" by convention, while the residue left in the other phase is called "raffinate". The best way to influence the separation of the rare earths is to use a multi-stage countercurrent separator in continuous operation with many mixer settler tanks or cells. In the event that A has a greater affinity for the organic phase and B has a greater affinity for the aqueous phase, the organic phase is enriched in A and the aqueous phase in B. For the rare earth elements, it is much more complex as there are several rare earths that are separated at the same time, not two as in the example above. Tributyl phosphate (TBP) is used as the organic phase to extract the rare earth ion from the strongly acidic aqueous phase of nitric acid. Other extractants such as di-2-ethylhexyl orthophosphoric acid and long chain amines have also been used.

Preparation of rare earth metals

Depending on the melting and boiling point of the particular metal (see below properties of the metals) and the purity of the metal required for a particular application, there are various methods for producing the individual rare earth metals. For high purity metals (99 percent or better), the calc-thermal and electrolytic processes for the low-melting lanthanides (lanthanum, cerium, praseodymium and neodymium), the calc-thermal processes for the refractory metals (scandium, yttrium, gadolinium, terbium, dysprosium, holmium, erbium and lutetium and another process (the so-called lanthanothermic process) for high pressure metals (samarium, europium, thulium and ytterbium), all of which are used to produce commercial grade metals (95-98 percent pure).

Kalzothermic process

The calcothermal process is used for all rare earth metals with the exception of the four with high vapor pressures - ie low boiling points. The rare earth oxide is converted to the fluoride by heating it with anhydrous hydrogen fluoride (HF) gas to form RF3. The fluoride can also be prepared by first dissolving the oxide in aqueous HCl acid and then adding aqueous HF acid to precipitate the RF3 compound out of solution. The fluoride powder is mixed with calcium metal, placed in a tantalum crucible and, depending on the melting point of R, heated to 1.450 ° C (2.642 ° F) or higher. The calcium reacts with the RF3 to form calcium fluoride (CaF2) and R. Since these two products do not mix, the CaF2 floats on the metal. When cooling to room temperature, the CaF2 is easily separated from R. The metal is then heated in a tantalum crucible in a high vacuum to above its melting point in order to evaporate the excess calcium. At this point, R can be further purified by sublimation or distillation. This process is used to make all rare earths with the exception of samarium, europium, thulium, and ytterbium.

In China, commercial-scale calc-thermal reduction is often carried out in graphite crucibles. This leads to a strong contamination of the produced metals with carbon, which easily dissolves in the molten rare earth metals. Conventional oxide crucibles such as alumina (Al2O3) or zirconia (ZrO2) are unsuitable for the calcineric reduction of rare earth metals because molten rare earths rapidly reduce aluminum or zirconium from their oxides and form the corresponding rare earth element.

Electrolytic process

The low melting metals (lanthanum, cerium, praseodymium and neodymium) can be prepared from the oxide by one of two electrolytic processes. The first method is to convert the oxide into the chloride (or fluoride) and then reduce the halide in an electrolytic cell. An electric current with a current density of about 10 A / cm2 is passed through the cell to reduce the RCl3 (RF3) to Cl2 (F2) gas at the carbon anode and the liquid R metal at the molybdenum or tungsten cathode. The electrolyte is a molten salt consisting of RCl3 (RF3) and NaCl (NaF). The electrolytically produced lanthanides are not as pure as those prepared by the calc-thermal process.

The second electrolytic process reduces the oxide directly in an RF3-LiF-CaF2 molten salt. The main problem with this process is that the oxide solubility is quite low and it is difficult to control the oxygen solubility in the molten salt solution.

The electrolytic process is limited to the rare earth metals, which melt below 1.050 ° C (1.922 ° F), because those that melt much higher react with the electrolysis cell and the electrodes. As a result, the electrolytic cell and the electrodes must be exchanged frequently, and the generated rare earth metals are highly contaminated.

Large commercial applications use the individual metals lanthanum for nickel-metal hydride batteries, neodymium for Nd2Fe14B permanent magnets and mischmetal for alloying agents and lighter flints. Mixture metal is a mixture of rare earth elements reduced from a rare earth concentrate where the rare earth content is the same as in the mined ores (ie, generally about 50 percent cerium, 25 percent lanthanum, 18 percent neodymium and 7 percent praseodymium). The lanthanum and neodymium metals are mostly produced by the direct electrolytic reduction of the oxides. Mischmetal is generally made by the electrolysis of the mixed RCl3.

Production of samarium, europium, thulium and ytterbium: lanthanum thermic process

The divalent metals europium and ytterbium have high vapor pressures - or lower boiling points than the other rare earth elements, as can be seen from the representation depending on the atomic number - which makes their production by metallothermal or electrolytic processes difficult. Samarium and thulium also have low boiling points compared to the other lanthanide metals as well as scandium and yttrium. The four high vapor pressure metals are made by mixing R2O3 (R = samarium, europium, thulium and ytterbium) with fine chips of lanthanum metal and placing the mixture in the bottom of a tall tantalum crucible. The mixture is heated to 1.400-1.600 ° C (2.552-2.912 ° F) depending on the R. The lanthanum metal reacts with R2O3 to form lanthanum oxide (La2O3), and R evaporates and collects on a condenser at the top of the crucible, which is about 500 ° C colder than the reaction mixture at the bottom of the crucible. The four metals can be further purified by subliming the metal again.

 

Properties of rare earth metals

As mentioned earlier, the rare earth elements - especially the lanthanides - are quite similar. They occur together in nature, and their complete separation is difficult to achieve. However, there are some noticeable differences, especially in the physical properties of the pure metallic elements. For example, their melting points differ by almost a factor of two and the vapor pressures by a factor of more than a billion. These and other interesting facts are explained below.

crystal structures

All rare earth metals except europium crystallize in one of four densely packed structures. As the one proceeds along the lanthanide series from lanthanum to lutetium, the crystal structures change from face centered cubic (fcc) to hexagonal close packed (hcp), with two intermediate structures consisting of a mixture of fcc and hcp layers, one to 50 percent consists of each layer (double hexagonal [dhcp]) and the other one-third consists of fcc and two-thirds hcp (sm-type). The two intermediate structures are unique among the crystal structures of all metallic elements, while the fcc and hcp structures are quite common.

Several elements have two densely packed structures: lanthanum and cerium have the fcc and dhcp structures, samarium has the sm and hcp structures, and ytterbium has the fcc and hcp structures. The existence of these structures depends on the temperature. In addition to densely packed structures, most rare-earth metals (scandium, yttrium, lanthanum through samarium and gadolinium through dysprosium) have a high-temperature cubic (bcc) polymorphic body centering agent. The exceptions are europium, which is from 0 K (-273 ° C or -460 ° F) to its melting point at 822 ° C (1.512 ° F) bcc, and holmium, erbium, thulium and lutetium associated with the hcp- Structure are monomorphic. Cerium, terbium and dysprosium have low temperature transformations (below room temperature). That of Cer is due to a valence change, while that of terbium and dysprosium are of magnetic origin.

melting points

The melting points of the lanthanide metals increase rapidly with increasing atomic number of 798 ° C (1.468 ° F) for cerium to 1.663 ° C (3.025 ° F) for lutetium (a doubling of the melting point temperatures), while the melting points of scandium and yttrium with those of the last Elements of trivalent lanthanide metals are comparable. The low melting points for the light to medium lanthanides are considered to be due to an 4f electron contribution to the bond, which is a maximum at cerium and decreases as the atomic number increases to about zero at erbium. The low melting points of europium and ytterbium are due to their divalence.

boiling

The boiling points of rare earth metals vary by almost a factor of three. Those of lanthanum, cerium, praseodymium, yttrium and lutetium are among the highest of all chemical elements, while those of europium and ytterbium can be classified in the group of metals with the lowest boiling points. This large difference results from the difference in the electronic structures of the atoms in the solid metal and in the respective gas. For the trivalent solid metals with the highest boiling points, the gas atom has three outer electrons 5d16s2, while the bivalent solid metals with the low boiling points have gas atoms with only two outer electrons 6s2. The lanthanides with intermediate boiling points are trivalent solids, but their gaseous forms have only two outer electrons, 6s2. This difference in the electronic states of the solid metals compared to those of the corresponding gaseous atoms explains the observed behavior.

Electrical Properties

The electrical resistances of the rare earth metals are between 25 and 131 micro-ohm-cm (μΩ-cm), which fall in the middle of the electrical resistance values ​​of the metallic elements. Most trivalent rare earth metals have room temperature values ​​from about 60 to 90 μΩ-cm. The low value of 25 μΩ-cm is for divalent fcc-ytterbium metal, while the two largest values, gadolinium (131 μΩ-cm) and terbium (115 μΩ-cm), are due to a magnetic contribution to electrical resistance, the arises near the magnetic order temperature of a material.

Lanthanum metal is the only superconducting (ie, no electrical resistance) rare earth metal at atmospheric pressure, while scandium, yttrium, cerium, and lutetium are also superconducting but under high pressure. The fcc modification of lanthanum becomes superconducting at Ts = 6,0 K (-267,2 ° C or -448,9 ° F), while the dhcp polymorph has a Ts of 5,1 K (-268,1 ° C or -450,5 ° F).

Magnetic properties

The magnetic properties of rare earth metals, alloys and compounds are highly dependent on the number of unpaired 4f electrons. The metals without unpaired electrons (scandium, yttrium, lanthanum, lutetium, and bivalent ytterbium) are weakly magnetic, as are many of the other non-earth metals. The remainder of the lanthanides, cerium by thulium, is highly magnetic because they have unpaired 4f electrons. This makes the lanthanides the largest family of magnetic metals. The magnetic ordering temperature usually depends on the number of unpaired 4f electrons. Ce with an unpaired electronic order at about 13 K (-260 ° C, or -436 ° F) and gadolinium with seven (the maximum possible number) orders at room temperature. All other magnetic ordering temperatures of lanthanide fall between these two values. Gadolinium is ferromagnetic at room temperature and is the only element that does so besides the 3D electronic elements (iron, cobalt, and nickel). Magnetic strength, as measured by its effective magnetic moment, has a more complicated correlation with the number of unpaired 4f electrons since it also depends on its orbital motion. Taking this into account, one finds the maximum effective magnetic moment in dysprosium with holmium in a very close second, 10,64 versus 10,60 drilling magnetons; the value of gadolinium is 7,94.

The rare earth metals have exotic (and sometimes complicated) magnetic structures that change with temperature. Most lanthanides have at least two magnetic structures. Gadolinium has the simplest structure at room temperature. All 4f spins are aligned in a direction parallel to each other; this structure is called ferromagnetic gadolinium. Most other lanthanide metals sometimes have 4f spins aligned antiparallel to each other, but mostly only partially; these are all referred to as antiferromagnetic metals, regardless of whether the spins are fully or partially compensated. In many of the antiferromagnetic structures, the spins form helical structures.

Thermal expansion

When comparing the LCTE values ​​of the hexagonal metals, the thermal expansion in the densely packed direction is always greater than in the planes (A, B and C layers). The abnormally large LCTE values ​​for europium and ytterbium confirm again the divalent character of these two metals.

Elastic properties

As with most other rare earth element properties, the moduli of elasticity of the rare earth metals fall in the middle percentile of the other metal elements. The values ​​for scandium and yttrium are about the same as those of the end limbs of the lanthanides (erbium to lutetium). There is a general increase in Young's modulus with increasing atomic number. The anomalous values ​​for cerium (about 4f binding) and ytterbium (divalence) are obvious.

Mechanical properties

The rare earth metals are neither weak nor particularly strong metallic elements, and they have a low ductility. Since the mechanical properties are very much dependent on the purity of the metals and their heat development, it is difficult to compare the values ​​given in the literature. Breaking strength varies from about 120 to about 160 MPa (megapascals) and ductility from about 15 to 35 percent. The ytterbium (europium not measured) strength is much smaller, 58 MPa, and the ductility is higher, about 45 percent, as expected for the divalent metal.

Chemical properties

The reactivity of the rare earth metals with air shows a significant difference between the light lanthanides and the heavy ones. The light lanthanides oxidize much faster than the heavy lanthanides (gadolinium by lutetium), scandium and yttrium. This difference is due in part to the variation of the oxide product formed. The light lanthanides (lanthanum by neodymium) form the hexagonal R2O3 A-type structure; the middle lanthanides (samarium through gadolinium) form the B-type monoclinic R2O3 phase; while the heavy lanthanides, scandium and yttrium form the C-type cubic R2O3 modification. The A-type reacts with water vapor in the air to form an oxyhydroxide that will chip off the white coating and allow oxidation by exposing the fresh metal surface. The C-type oxide forms a dense, coherent coating that prevents further oxidation, similar to the behavior of aluminum. Samarium and gadolinium, which form the B-type R2O3 phase, oxidize slightly faster than the heavier lanthanides, scandium and yttrium, but still form a cohesive coating that prevents further oxidation. For this reason, the light lanthanides have to be stored in a vacuum or under a protective gas atmosphere, while the heavy lanthanides, scandium and yttrium can remain outdoors for years without oxidation.

The bcc-structured europium metal oxidizes the fastest of the rare earths with moist air and must be treated at all times under a protective gas atmosphere. The reaction product of europium under exposure to humid air is a hydrated hydroxide, Eu (OH) 2-H2O, which is an unusual reaction product since all other rare earth metals form an oxide.

The metals react violently with all acids except hydrofluoric acid (HF), release H2 gas, and form the corresponding rare earth anion compound. When incorporated into hydrofluoric acid, the rare earth metals form an insoluble RF3 coating that prevents further reaction.

The rare earth metals react readily with hydrogen gas to RH2 and under heavy hydration conditions to the RH3 phase, with the exception of scandium which does not form a trihydride.

Connections

The rare earth elements form tens of thousands of compounds with all the elements to the right of them - including the metals of group 7 (manganese, technetium and rhenium) in the periodic table as well as beryllium and magnesium, which are on the far left in group 2 - important series of compounds and some individual compounds with unique properties or unusual behavior are described below.

Oxide

The largest family of inorganic rare earth compounds investigated so far are the oxides. The most common stoichiometry is the R2O3 composition, but since a few lanthanide elements have other valence states besides 3+, other stoichiometries exist - for example cerium oxide (CeO2), praseodymium oxide (Pr6O11), terbium oxide (Tb4O7), europium oxide (EuO) and Eu3O4. Most of the discussion will focus on the binary oxides, but ternary and other higher order oxides will also be briefly covered.

sesquioxides

All rare earth metals form the sesquioxide at room temperature, but it may not be the stable equilibrium composition. There are five different crystal structures for the R2O3 phase. They are referred to as A, B, C, H, and X types (or shapes), and their existence depends on the rare earth element and temperature. The A-type exists for the light lanthanides, and they transform into the H-type via 2.000 ° C (3.632 ° F) and then into the X-type 100-200 ° C (180-360 ° F) higher. The B-type exists for the middle lanthanides, and they also transform into the H-type via 2.100 ° C (3.812 ° F) and then into the X-type near the melting point. The C structure is found for heavy lanthanides as well as for Sc2O3 and Y2O3. The C-type R2O3 compounds are converted to the B-type upon heating between 1.000 and 2.000 ° C (1.832 and 3.632 ° F) and then to the H-type before melting. The R2O3 phases are refractory oxides with melting temperatures between 2.300 and 2.400 ° C (4.172 and 4.352 ° F) for the light and heavy R oxides, respectively, but have limited applications as refractories due to the above structural transformations.

The sesquioxides are among the most stable oxides in the periodic table; the more negative the value of the free energy of formation (ΔGf0), the more stable the oxide. The interesting feature is the anomalous free energies of the formation of Eu2O3 and ytterbium oxide (Yb2O3), because one might think that they should be at or near the line defined by the other trivalent R2O3 phases, since europium and Ytterbium are both trivalent in these compounds. The less negative ΔGf0 values ​​result from the fact that europium and ytterbium are both divalent metals and, when reacted with oxygen to form the trivalent R oxide, an energy is needed to turn the divalent europium or ytterbium to the trivalent state to convict.

There are a number of important applications concerning the R2O3 compounds; Generally, they are used in combination with other compounds or materials. The oxides without unpaired 4f electrons, lanthanum oxide (La2O3), lutetium oxide (Lu2O3) and gadolinium oxide (Gd2O3) are added to optical glasses used as lenses; the task of the R2O3 is to increase the refractive index. The same oxides plus yttrium oxide (Y2O3) are used as support materials for rare earth-based phosphors; they are usually mixed with other oxide materials to optimize their optical properties. Yttrium vanadate (YVO4) is one of the most popular hosts alongside yttrium oxysulfide (Y2O2S).

Some of the lanthanide ions with unpaired 4f electrons have electronic transitions which, when activated by electrons or photons, give intense and sharp colors and are used in televisions with cathode ray tubes, optical displays, and fluorescent lamps; these are Eu3 + (red), Eu2 + (blue), Tb3 + (green) and Tm3 + (blue). The respective activators R2O3-oxides are added to the carrier in 1-5 percentages to produce the corresponding phosphor and corresponding color light. The Eu3 + ion causes an intense red color, and its discovery in 1961 led to a major transformation in the TV industry. Before the introduction of Europium, the color image on television was rather boring. When using the new europium phosphor, the color was much brighter and more intense, which made the color television even more enjoyable. This application was the beginning of the modern rare earth industry. The annual production rate of individual rare earth elements increased significantly, the products had higher purities, and the amount of mined rare earths increased dramatically in the following years.

Y2O3 oxide is added to ZrO2 to stabilize the cubic form of ZrO2 and introduce oxygen vacancies, resulting in a material with high electrical conductivity. These materials (5-8 percent Y2O3 in ZrO2) are excellent oxygen sensors. They are used to determine the oxygen content in the air and to control the rich-to-lean ratio of fuels.

The addition of about 2% by weight of R2O3 (R = lanthanum, cerium and unseparated R) to zeolites (3SiO2 / Al2O3) has the catalytic activity of Fluid Catalytic Cracking (FCC) catalysts by a factor of two to three over non-rare zeolites Earth improved. FCC catalysts have been one of the largest rare earth markets (1964-15 percent) since their invention in 18 year. The main functions of the rare earths are the stabilization of the zeolite structure, which increases its life before it needs to be replaced, and the improvement of the selectivity and effectiveness of the FCC catalyst.

One of the oldest applications of 1912 rare earth oxides is the coloration of glass: neodymium oxide (Nd2O3), for colors ranging from a subtle pink coloration in low concentrations to a blue violet in high concentrations, samarium oxide (Sm2O3) for yellow and erbium oxide ( Er2O3) for light pink. Didymium oxide, Di2O3 (Di is a mixture of about 25 percent praseodymium and 75 percent neodymium), is used in glassblowers and welder glasses because it absorbs the intense yellow light that emits sodium in sodium-based glasses very effectively. (The use of CeO2-Ce2O3 in decolorizing glass is discussed in the next section.)

Higher oxides

As a result of the tendency towards completely empty or half-filled 4f mirrors (see above Electronic structures and ionic radius), cerium, praseodymium and terbium tend to form tetravalent or partially tetravalent compounds - namely CeO2, Pr6O11 and Tb4O7. However, the free energies of formation of the R2O3 of cerium, praseodymium and terbium are close to those of the higher oxides, and a whole series of oxide intermediate phases, ROx (where 1,5 <x <2), were observed, depending on the temperature, the oxygen pressure and the thermal history of the sample. There are at least five intermediate phases in the CeOx system. The CeOx compounds were used as a portable source of oxygen. By far the most important use of CeOx compounds is in vehicle catalytic converters, which largely eliminate the environmentally harmful carbon monoxide and nitrogen oxides from gasoline-powered vehicles.

Another important application of CeO2 is as a polishing agent for glass lenses, front panels of monitors, semiconductors, mirrors, gemstones and windscreens in the automotive sector. CeO2 is much more effective than other polishes (eg iron oxide [Fe2O3], ZrO2 and silica [SiO2]) because it is three to eight times faster, while the quality of the final polish is equal to or better than the other oxide polishes. The exact mechanism of the polishing process is not known, but it is believed that it is a combination of mechanical abrasion and chemical reaction between CeOx and the SiO2 glass, with water playing an active role.

CeO2 is an important glass additive that is suitable for various applications. It is used to decolorize glass. It prevents the browning of glass from X-rays, gamma rays and cathode rays and absorbs ultraviolet radiation. These applications use the oxidation-reduction behavior of CeO2-Ce2O3. Since iron oxide is always present in the glass, the role of CeO2 is to oxidize the Fe2 +, which gives the glass a bluish hue, the Fe3 +, which has a pale yellow color. Selenium is added to the glass as a complementary dye in order to “neutralize” the Fe3 + color. Glass tans easily due to the formation of color centers when it is exposed to various types of radiation. The Ce4 + ions act like electron traps in the glass and absorb the electrons released by the high-energy radiation. Cerium is found in the non-tanning glass of televisions and other cathode ray screens, as well as in radiation-repellent windows in the nuclear power industry. CeO2 is added to the glass containers in order to protect the product from damage by long-term exposure to ultraviolet radiation from sunlight, again using the oxidation-reduction pair Ce4 + -Ce3 +.

In the systems PrOx and TbOx, seven or four intermediate phases between 1,5 <x <2,0 were found. Some of the compositions and crystal structures are the same as in the CeOx system. However, since the proportion of praseodymium and especially of terbium is much lower than that of cerium in the common ore sources, little or no commercial application was developed with the PrOx and TbOx systems.

Lower oxides

An NaCl-type RO phase has been reported for virtually all rare earth elements, but it has been found that these are ternary phases stabilized by nitrogen, carbon, or both. The only real binary RO connection is EuO. This oxide is a ferromagnetic semiconductor (Tc = 77 K [-196 ° C, or -321 ° F]), and this finding has had a pronounced impact on the theory of solid state magnetism because there are no overlapping conduction electrons previously reported for the occurrence of ferromagnetism were considered necessary. It is assumed that the ferromagnetism in EuO is due to the oxygen-mediated cation-cation exchange (Eu2 + -Eu2 +). Subsequently, ferromagnetism was found in EuS and EuSe and antiferromagnetism in EuTe.

Europium also forms another suboxide, Eu3O4, which can be considered as mixed material containing Eu3 + and Eu2 + -ie, Eu2O3-EuO.

Ternary and higher quality oxides

The rare earth oxides together with other oxides form tens of thousands of ternary and higher valued compounds such as alumina (Al2O3), iron oxide (Fe2O3), cobalt sesquioxide (Co2O3), chromium sesquioxide (Cr2O3), gallium sesquioxide (Ga2O3), and manganese sesquioxide (Mn2O3). The two most common structures formed by the rare earth ternary oxides are the perovskite, RMO3, and the garnet, R3M5O12, where M is a metal atom.

The perovskite structure is a closed lattice with the R located at the eight corners of the unit cell. The M atoms, which are smaller than the R atoms and generally trivalent, are located in the center of the unit cell, and oxygen atoms occupy the centers of the six faces. The basic structure is a primitive cube, but there are tetragonal, rhombohedral, orthorhombic, monoclinic and triclinic distortions. Other elements can be replaced, in whole or in part, to allow M and R a wide variety of feature ladders, semiconductors, insulators, dielectrics, ferroelectrics, ferromagnets, ferromagnets, antiferromagnets, and catalysts. Some of the more interesting applications are LaGaO3, LaAlO3 or YAlO3 epitaxial layers for high temperature oxide superconductors, magnetoresistive layers and GaN layers; Cathode and compounds of (La, M) MnO3 and (La, M) CrO3 for solid oxide fuel cells; lanthanum-modified lead zirconate lead titanate (commonly known as PLZT) as a transparent ferroelectric ceramic for thermal and lightning protection devices, data recorders and goggles; and (Pr, Ca) MnO3, which has a colossal magnetoresistance and is used in switches.

Garnets have a much more complex crystal structure than the perovskites: 96 oxygen sites, while the metal atoms occupy 24 tetrahedral sites, 16 octahedral sites, and 24 dodecahedral sites (64 total). The general formula is R3M5O12, where R occupies the tetrahedral sites and M atoms occupy the other two sites. M is generally a trivalent ion of aluminum, gallium or iron. One of the most important rare earth grenades is Yig (Yttrium Iron Garnet), which is used in a variety of microwave devices such as radars, dampers, filters, circulators, isolators, phase shifters, power limiters and switches. YIG is also used in integrated microwave circuits where thin films are deposited on garnet substrates. The properties of these materials can be altered by replacing gadolinium with yttrium and aluminum or gallium with iron.

The quaternary oxide YBa2Cu3O7 is the best known of the higher-grade oxides and has a layered perovskite-like structure. It was found that this material showed 1987 77 K (-196 ° C or -321 ° F) superconductivity (ie it has no electrical resistance). This discovery sparked a revolution, as 77 K's Tc enabled cooling with low-cost liquid nitrogen. (Prior to 1986, the highest known superconducting transition temperature was 23 K [-250 ° C]). YBa2Cu3O7 (YBCO, also known as Y-123) not only broke a temperature record, but that it was an oxide was probably rather a surprise, since all good superconductors used to be metallic materials. This material was rapidly commercialized and is now used to generate high magnetic fields in research equipment, magnetic resonance imaging (MRI) and electrical power transmission lines.

hydrides

The rare earth metals react easily with hydrogen to RH2, and by increasing the hydrogen pressure, the trivalent R metals (other than scandium) also form the RH3 phase. Both the RH2 and RH3 phases are non-stoichiometric (ie the number of atoms of the elements present can not be expressed as the ratio of the small integers). The RH2 phase has the CaF2 fluoride structure for trivalent R, and for divalent europium and ytterbium, the dihydride crystallizes in an orthorhombic structure that has the same structure as the alkaline earth metal hydrides. The RH3 phases have two different crystal structures. For the light lanthanides (lanthanum by neodymium), the RH3 has the fluoride-like structure and forms a continuous solid solution with RH2. For heavy lanthanides (samarium by lutetium) and yttrium, RH3 crystallizes with a hexagonal structure. The rare earth hydrides are air sensitive and must be handled in glove boxes.

The electrical resistance of RH2 is about 75 percent lower than that of pure metals. However, the electrical resistance increases as more hydrogen is added beyond RH2 and approaches that of a semiconductor at RH3. For lanthanum hydride (LaH3), the compound is diamagnetic and not just a semiconductor. Most RH2 compounds where R is a trivalent rare earth are antiferromagnetic or ferromagnetic. However, the divalent europium dihydride EuH2 is ferromagnetic for 25 K (-248 ° C or -415 ° F).

When a thin film of YHx or LaHx protected by a thin film of palladium metal was hydrogenated, the metal phase became with x <2,9 reflected light, but the film became transparent when x approached 3,0. By reducing the hydrogen content, the transparent YHx (LaHx) film became a mirror again. Since then, a number of other hydrogen-containing, switchable mirror materials have been developed - all trivalent rare earth elements and the R-magnesium alloys as well as the magnesium alloys with vanadium, manganese, iron, cobalt and nickel additives.

halides

The three most important stoichiometries in the halide systems (X = fluorine, chlorine, bromine and iodine) are trihalides (RX3), tetrahalides (RX4) and reduced halides (RXy, y <3). The trihalides are known for all rare earths except europium. The only known tetrahalides are the RF4 phases, where R = cerium, praseodymium and terbium. The dihalids RX2, where R = samarium, europium and ytterbium, have been known for a long time, are stable compounds and can be easily produced. A number of "RX2" compounds have been reported in the literature for most lanthanides, but subsequent research has shown that these phases were in fact ternary compounds, stabilized by interstitial impurities such as hydrogen and carbon. This also applies to other reduced halides (2 <x <3) - e.g. Gd2Cl3.

The RF3 connections behave very differently than RCl3, RBr3 and RI3. The fluorides are air stable, non-hygroscopic (ie they do not easily absorb water), insoluble in water and mild acids. The fluorides are made by converting the oxide to RF3 by reacting with ammonium bifluoride (NH4HF2). The RF3 phases crystallize in two modifications - the trigonal LaF3 structure (lanthanum by promethium) and the orthorhombic YF3 structure (samarium by lanthanum and yttrium). The RF3 compounds are alloyed with other floating fluorides - namely ZrF4 and ZrF4-BaF2 - which are classified as heavy metal fluoride glasses (HMFG). Many HMFGs are transparent from the ultraviolet to the mid-infrared range and are used as fiber optic materials for sensors, communications, windows, light guides and prisms. These materials are characterized by good glass-forming properties, chemical resistance and temperature resistance. One of the most important compositions is 57 percent ZrF4, 18 percent BaF2, 3 percent LaF3, 4 percent AlF3, and 17 percent NaF (with some slight variations from these percentages) and is known as ZBLAN.

The compounds RCl3, RBr3 and RI3 behave very differently than the compounds RF3 because they are hygroscopic and hydrolyze rapidly in the air. As you would expect, the RX3 (X = chlorine, bromine and iodine) are well soluble in water. The trihalides are generally prepared from the respective oxide by dissolving R2O3 in an HX solution and crystallizing the RX3 compound from solution by dehydrogenation. The dewatering process must be carried out carefully as otherwise the RX3 phase contains some oxygen. The dewatering process becomes more difficult with increasing atomic number of lanthanide and also of X. The RCl3 and RBr3 compounds have three different crystal structures from the light to the middle and heavy lanthanides (which includes YX3), while the RI3 compounds have only two different crystal structures along the series.

Metal and complex compounds

Among the many rare earth intermetallic compounds that are formed, some are characterized by unusual applications or interesting scientific knowledge. Six of these applications are explained below.

Permanent Magnets

The best-known rare earth intermetallic compound is Nd2Fe14B, which is ferromagnetic and, with appropriate heat treatment, becomes the hardest known magnetic material. Therefore, this intermetallic compound is used as a permanent magnet in many applications. Main applications include electric motors (eg the modern automobile contains up to 35 electric motors), spindles for computer hard disk drives, speakers for mobile phones and portable media players, direct-drive wind turbines, actuators and MRI devices. SmCo5 and Sm2Co17 are also permanent magnets. Both have higher Curie temperatures (magnetic order) than Nd2Fe14B, but are not as strong magnetic.

Rechargeable batteries

Another important compound, namely a hydrogen absorber for green energy, is LaNi5. It is a major component in nickel-metal hydride batteries used in hybrid and all-electric vehicles. LaNi5 absorbs and dissolves hydrogen fairly easily at room temperature and absorbs six hydrogen atoms per LaNi5 molecule at low hydrogen pressure. This is one of the most important markets for rare earths.

electron guns

The next compound, lanthanum hexaboride (LaB6), only has a small market but is critical for electron microscopy. It has an extremely high melting point (> 2.500 ° C or> 4.532 ° F), a low vapor pressure and excellent thermal emission properties, making it the material of choice for the electron guns in electron microscopes.

Micro Kelvin cooling

The metallic compound PrNi5 is also a small market material, but it is a world record holder. It has the same crystal structure as LaNi5, does not magnetically align itself into the microkelvin region (0,000001 K [-273,14999999 ° C, or -459,669998 ° F]) and is an excellent candidate for cooling by nuclear adiabatic demagnetization. PrNi5 was used as the first stage together with copper as the second stage to reach an operating temperature of 0,000027 K (-273,149973 ° C or -459,669951 ° F). At this temperature, experimental measurements on materials other than the magnetic refrigerant itself were made for the first time. There are many low-temperature laboratories in the world that use PrNi5 as a refrigerant.

magnetostriction

All magnetically ordered materials that are exposed to an applied magnetic field expand or contract depending on the orientation of the sample with respect to the magnetic field direction. This phenomenon is called magnetostriction. It is quite small for most materials, but 1971 found that TbFe2 has a very large magnetostriction, about 1.000 times larger than normal magnetic substances. Today, one of the best commercial magnetostrictive materials Tb0.3Dy0.7Fe1.9, called Terfenol D, is used in devices such as sonar systems, micropositioners and fluid control valves.

Giant magnetocaloric effect

Magnetic materials that undergo a magnetic transition usually heat up (although a few substances cool down) when exposed to an increasing magnetic field, and when the field is removed, the opposite happens. This phenomenon is called magnetocaloric effect (MCE). 1997 was found by Gd5 (Si2Ge2) from American materials scientists Vitalij K. Pecharsky and Karl A. Gschneidner, Jr., to show an exceptionally large MCE called the giant magnetocaloric effect (GMCE). The GMCE is due to a simultaneous crystallographic and magnetic transition when the Gd5 (Si2Ge2) magnetically orders, which can be controlled by variation of the magnetic field. This discovery gave a great boost to the possibility of using GMCE for magnetic cooling. Since then, about six more GMCE materials have been discovered, and one of the most promising materials is another lanthanide compound, La (FexSix) 13.

Magnetic cooling has not yet been commercialized, but many testers and prototype chillers have been built. If magnetic cooling becomes viable, it should reduce energy consumption and cooling costs by about 20 percent. It is also a much more environmentally friendly technology because it eliminates the environmentally harmful ozone depleting and greenhouse gases used in today's gas-compression refrigeration technology.

complex

The rare earth elements react with many organic molecules and form complexes. Many of them have been willing to help with the separation of rare earth elements through ion exchange or solvent extraction processes in the 1950 and 1960 years, but since then they have been studied on their own and for other applications such as light bulbs, lasers and nuclear magnetic resonance. Magnetic resonance imaging (MRI) is an important medical probe for examining patients. The most important materials for enhancing the MRI image are gadolinium-based complexes, such as Gd (dtpa) -1, where dtpa is the shorthand notation for diethylenetriamine-N, N, N ', N', N ', N "-pentaacetate. Millions of cans (vials) are administered annually around the world. Each vial contains 1,57 grams (0,06 ounce) of gadolinium.

Nuclear properties

As a group, the rare earth elements are rich in the total number of isotopes, ranging from 24 for scandium to 42 for cerium, averaging about 35 each without counting core isomers. The elements with odd ordinal numbers have only one or at most two stable (or very long-lived) isotopes, but those with even ordinal numbers have four to seven stable isotopes. Promethium has no stable isotopes; Promethium-145 has the longest half-life of 17,7 years. Some of the unstable isotopes are weakly radioactive and have an extremely long half-life. The unstable radioactive isotopes are produced in a variety of ways - for example by fission, neutron bombardment, radioactive decay of neighboring elements and bombardment of neighboring elements with charged particles. The lanthanide isotopes are of particular interest to nuclear scientists as they offer a rich field for the study of theories about the nucleus, particularly because many of these nuclei are not spherical, a property that has a critical impact on nuclear stability. When either the protons or the neutrons complete a nuclear envelope (that is, reach certain fixed values), the nucleus is exceptionally stable; the number of protons or neutrons needed to complete a shell is called the magic number. A special magic number - 82 for neutrons - occurs in the lanthanide series.

Some of the lanthanide elements have large thermal neutron sensing cross sections, ie they absorb a large number of neutrons per unit area. Cross-sectional values ​​for naturally occurring samarium, europium, gadolinium and dysprosium are 5.600, 4.300, 49.000 and 1.100 barns. Some of these elements are therefore incorporated into control rods that regulate or shut down the operation of nuclear reactors (europium and dysprosium) when they get out of control (gadolinium). Naturally occurring europium absorbs 4,0 neutrons per atom, dysprosium 2,4, samarium 0,4 and gadolinium 0,3 before they become worthless as neutron absorbers. Therefore, europium and dysprosium are used in control rods and not in samarium or gadolinium. In addition, the lanthanides can be used as combustible neutron absorbers to keep the reactivity of the reactor almost constant. When uranium is subjected to fission, it produces some fission products that absorb neutrons and tend to slow down the nuclear reaction. If the right amount of lanthanides are present, they will burn about as fast as the other absorbers. Most other rare earths are quite transparent to thermal neutrons with cross-sections from 0,7 barns for cerium to 170 for erbium.

Some of the most important radionuclides are Yttrium-90 (cancer therapy), Cer-144 and Promethium-147 (industrial meters and power sources), Gadolinium-153 (industrial X-ray fluorescence) and Ytterbium-169 (portable X-ray source).

toxicity

The rare earths have low toxicity and can be safely treated with normal care. Solutions injected into the peritoneum cause hyperglycemia (a surplus of sugar in the blood), a drop in blood pressure, splenic degeneration and fatty liver. When solutions are injected into the muscle, about 75 percent of the rare earths remain in place, while the remainder goes to the liver and skeleton. When taken orally, only a small percentage of a rare earth element is absorbed into the body. Organically complexed ions are slightly more toxic than solids or inorganic solutions. As with most chemicals, dust and vapors should not be inhaled or absorbed. Eye-sprayed solutions should be washed out and metal chips should be removed.