THE PHYSICAL METALLURGY OF THE RARE EARTH METALS

Karl A. Gschneidner, Jr.Department of Materials Science and Engineering

Iowa State UniversityAmes, Iowa 50011-2300,USA

Res MetallicaKatholieke Universiteit

Leuven, BelgiumMay 23, 2012

IOWA STATE UNIVERSITYOF SCIENCE AND TECHNOLOGY

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AS A CONSUMER – IThe automobile is the largest rare earth containing product you will purchase(or contains a product derived by using rare earths)

GAS OR DIESEL POWERED AUTOMOBILE

PRODUCTSElectric motors (~35 in an average car) [Nd,Pr,Dy]Speakers for sound system [Nd,Pr,Dy]Sensors to measure and control oxygen content in fuel [Y]3-way catalytic converter [Ce]Optical displays – phosphors [Y,Eu,Tb]

Ni-metal-hydride battery [Hybrid Vehicles] [La,MM]Electric traction motor [Hybrid Vehicles] [Nd,Dy]

DERIVED PRODUCTSGasoline – FCC cracking catalysts [La,Ce, mixed REO]Windshield, mirrors – polishing [Ce]

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The automobile is the largest rare earth containing product you will purchase (or contains a product derived by using rare earths)

ALL ELECTRICAL VEHICLE(a trade-off)

ADD Traction Motors [Nd,Pr,Dy]

TOSS 3-way Catalytic converter [Ce] Oxygen sensors [Y]

AS A CONSUMER – II

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AMES LABORATORY - IHow Did Rare Earths Get to Ames? Spedding – Assoc. Prof. of Chemistry, Iowa State College – 1937 spectrascopist, but had to separate and purify his REs

First Atomic Reactor – Stagg Field, Univ. Chicago needed 6 tons uranium metal developed a new and better way to make U Aug.-Nov. 1942 delivered 2 tons U

Delivered 1,000 tons U for other reactors by Dec. 1945Delivered 300 tons Th after World War II

Ames Laboratory became an Atomic Energy Commission laboratory May 17, 1947.

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Discovered and developed ion exchange chromatography separate and purify REs up to 99.9999% pure first time scientists had high purity REs in reasonable quantities available to study applications soon followed, e.g. Eu for color TV

Analytical Chemistry developed new analytical techniques four different methods – in 1940s-1950s ICP-MS – inductively coupled plasma-mass spectrometry (1970-80s)

Process Metallurgy high purity metals >99.8 atomic % pure with respect to all elements (routine) special – 99.99 atomic % pure 15 tons Y metal (nuclear aircraft) lanthothermic process – preparation: Sm, Eu, Tm, Yb

Basic Research chemistry, metallurgy, materials science, condensed matter physics

Interdisciplinary Research magnetism X-ray crystallography

AMES LABORATORY - II

6

2

4

5

6

7

1

3

GroupPeriod

3 Li

4 Be

5 B

6 C

7 N

8 O

9 F

10 Ne

57 La 58

Ce

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

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

1 H

2 He

11 Na

12 Mg

13 Al

14 Si

15 P

16 S

17 Cl

18 Ar

1

2

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13 14 15 16 17

3 4 5 6 7 8 9 10 11 12

LANTHANIDES

59 Pr 60 Nd 61

Pm 62 Sm 63

Eu 64 Gd 65

Tb67

Ho 68 Er

66 Dy

70 Yb 71

Lu

69 Tm

PERIODIC TABLE

RARE EARTHS (R): Sc, Y, + Lanthanides (Ln)

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ELECTRONS DO IT ALLFour kinds of electrons

s – fast moving electrons; weak bonds (high electrical conductivity in copper)

p – moderately fast moving electrons; form strong bonds

d – slow moving electrons; form very strong bonds highest melting metal - 3380°C

second highest boiling point - 5725°C

f – do not move, sit by the nucleus; little if any bonding lower melting point

no effect on boiling point

tungsten

lanthanides

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LOCATION OF ELECTRONS IN AN ATOM

N = nucleus

Outer electrons: valence electrons – bonding conduction electrons – conduct electricity, heat lanthanides – 6s and 5d; Y – 5s,4d; Sc – 4s,3d

Inner electrons: core electrons – s p d f where the 4f electrons reside

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ELECTRON WAVE FUNCTIONS OF AN ISOLATED GADOLINIUM ATOM

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(f) Eu2+J = O M = OGd3+ M7/2; Lu3+

In the absence of crystal field

Crystal field – due to the electric charges on other atoms in a solid and their locations in the unit cell

oblate (red shading)

prolate (green shading

spherical (blue shading)

SHAPES OF THE 4f CHARGE DENSITIES

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ELECTRONIC STRUCTURES OF THE RARE EARTH GROUND STATES

ElementNeutral AtomConfiguration

Sc 3d4s2

Y 4d5s2

La 4f0 5d6s2

Ce 4f1 5d6s2

Pr 4f3 6s2

Nd 4f4 6s2

Pm 4f5 6s2

Sm 4f6 6s2

Eu 4f7 6s2

Gd 4f7 5d6s2

Tb 4f9 6s2

Dy 4f10 6s2

Ho 4f11 6s2

Er 4f12 6s2

Tm 4f13 6s2

Yb 4f14 6s2

Lu 4f14 5d6s2

Configuration found in many text books and handbooks

Generally not important to most scientists – who work with solid or liquid materials

Important in chemical thermodynamic cycles if the R(g) state is involved

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ELECTRONIC STRUCTURES OF THE RARE EARTH IONS

4f Configuration of Known

Oxidation StatesElement M+2 M+3 M+4

Sc - 0 -Y - 0 -La - 0 -Ce - 1 0Pr - 2 1Nd - 3 -Pm - 4 -

Sm 6 5 -Eu 7 6 -Gd - 7 -Tb - 8 7Dy - 9 -Ho - 10 -Er - 11 -Tm - 12 -Yb 14 13 -Lu - 14 -

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ELECTRONIC STRUCTURES OF THE RARE EARTH STANDARD STATES

Metallic State

No. of Electrons

Element Valence 4f

Sc 3 0

Y 3 0

La 3 0

Ce 3 (3.1) 1

Pr 3 2

Nd 3 3

Pm 3 4

Sm 3 5

Eu 2 7

Gd 3 7

Tb 3 8

Dy 3 9

Ho 3 10

Er 3 11

Tm 3 12

Yb 2 14

Lu 3 14

T = 298 K, P = 1 atm

Standard state starting point for thermodynamic calculations, e.g. free energy formation of a RXn compound

For most rare earth elements it is the trivalent state, for Eu and Yb it is the divalent state

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USES THAT DEPEND UPON VALENCE AND SIZE

Mixed rare earthsPetroleum cracking catalyst (also La, Ce)Mischmetallighter flintsalloy additive

Individual rare earth elementsNickel-metal(La)-hydride batteriesAlloying agent (La, Ce, Nd, Y)

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USES THAT DEPEND ON 4f ELECTRONS

Permanent Magnets (purity not a problem 95%R) Nd, Pr, Sm, Dy

Phosphors (very sensitive to impurities, 99.999%R; i.e. 5 nines)Eu (red, blue)Tb (green)fluorescent lampsoptical displays (TV, etc.)

Fiber optics (very sensitive to impurities, 99.999%R; i.e. 5 nines)Er

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USES THAT DEPEND ON THE ABSENCE OF ELECTRONIC TRANSITIONS IN UV, OPTICAL

AND IR WAVE LENGTHSHigh Purities Required 99.999%R

Optical lensesLa, Gd, Lu

Phosphor hostsY, Gd

Artificial gem stonesY

___________________________Y3+ and La3+ - no 4f electronsGd3+ lowest transition in the very short ultraviolet regionLu3+ completely filled 4f level, all 4f electrons paired up

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USES THAT DEPEND UPON VALENCE CHANGES

Moderately Pure R Required > 98% pure)

Ce3+ Ce4+ 4f1 4f0

Automotive 3-way emission catalysts

UV light absorption

Polishing compounds

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ABUNDANCES OF NATURALLY OCCURRING ELEMENTS IN THE LITHOSPHERE

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SELECTED RARE EARTH MINERALSName Idealized Composition Primary Rare Earth Content

Allanite (Ca,Fe2+)(R,Al,Fe3+)3Si3O13H R = light lanthanides

Apatite Ca5(PO4)3F R = light lanthanides

Bastnasite RCO3F R = light lanthanides (60-70%)

Euxenite R(Nb,Ta)TiO6·xH2O R = heavy lanthanides plus Y (15-43%)

Fluorite CaF2 R = heavy lanthanides plus Y

Gadolinite R2(Fe2+,Be)3Si2O10 R = heavy lanthanides plus Y (34-65%)

Laterite clays SiO2, Al2O3, Fe2O3 R = heavy lanthanides plus Y

Loparite (R,Na,Sr,Ca)(Ti,Nb,Ta,Fe3+)O3 R = light lanthanides (32-34%)

Monazite RPO4 R = light lanthanides (50-78%)

Perovskite CaTiO3 R = light lanthanides

Sphene CaTiSiO4X2 (X = ½O2-, or F-) R = light lanthanides

Xenotime RPO4 R = heavy lanthanides plus Y (54-65%)

Zircon ZrSiO2 R – both light and heavy lanthanides plus Y

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RARE EARTH CONTENT IN SELECTED MINERALS

Rare earth

element

BastnasiteMountain

Pass,USA

BastnasiteBayanObo,China

MonaziteMt.

Weld,Australia

XenotimeLehat,

Malaysia

High YRE

lateriteLongnan,

China

Low YRE

lateriteXunwu,China

LopariteKola

Peninsula,

RussiaLa 33.8 23.0 25.5 1.2 1.8 43.4 25.0Ce 49.6 50.0 46.7 3.1 0.4 2.4 50.5Pr 4.1 6.2 5.3 0.5 0.7 9.0 5.0Nd 11.2 18.5 18.5 1.6 3.0 31.7 15.0Sm 0.9 0.8 2.3 1.1 2.8 3.9 0.7

Eu 0.1 0.2 0.4 Trace 0.1 0.5 0.1Gd 0.2 0.7 <0.1 3.5 6.9 3.0 0.6Tb 0.0 0.1 <0.1 0.9 1.3 Trace TraceDy 0.0 0.1 0.1 8.3 6.7 Trace 0.6Ho 0.0 Trace Trace 2.0 1.6 Trace 0.7

Er 0.0 Trace Trace 6.4 4.9 Trace 0.8Tm 0.0 Trace --- 1.1 0.7 Trace 0.1Yb 0.0 Trace --- 6.8 2.5 0.3 0.2Lu Trace Trace --- 1.0 0.4 0.1 0.2Y 0.1 Trace <0.1 61.0 65.0 8.0 1.3

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PROCESSING AND SEPARATION - I

Greatly Depends Upon Source

Only Mined for Rare Earths Lights Bastnasite Monazite

Heavies Ionic Clays (laterite) Xenotime

Co-mined for Rare Earths Separate non-REs from REs then process for REs

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Ores (1 8% REO) Mineral (15 80% REO) Concentrate (~70% REO) use as mixed rare earth Individual RE Available purities: 90 99.999% – What do you need? That’s what you buy.

PROCESSING AND SEPARATION - II

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Original solution

Liq.Sol.

Sol. Liq.Combine

Redissolveand crystallize

Combine Combine

Evaporate someH2O

liquid solid

Sol. Liq.

Redissolveand crystallize

Evaporate someH2O

liquid solidliquid solid

La3+ Pr3+ Nd3+ Sm3+

SCHEMATIC OF SEPARATING TWO RARE EARTHS BY FRACTIONAL CRYSTALLIZATION

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SEPARATION OF RARE EARTH ELEMENTSChemically very similar

To get a given rare earth elementNo specific chemical can pull out a desired REMust separate all of the elements before it in the series

ExceptionsCe – make use of its dual valence (3+/4+) oxidize it to 4+ state and separate by precipitation this reduces the amount of rare earth to be separated by ~50%

Eu – make use of its dual valence (2+/3+) carry out extraction process to remove La, Nd, Pr this increases the amount of Eu in the solution of the remaining REs reduce it to 2+ and precipitate EuSO4

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DTPA

EDTA

HEDTA

STABILITY CONSTANTS OF COMPLEXING AGENTS

The slope is more important than the magnitude.

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THE SPECIAL CASE OF YTTRIUM

Change complexing agents

Use DTPA or HEDTA Moves Y into the lights and separates Y from the heavies

Then use EDTA To separate Y from the lights

DTPA

EDTA

HEDTA

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PROBLEM: SEPARATION SEQUENCE

Mountain Pass, CaliforniaTo get to Eu, Tb and Dy need to remove ~99% other RERemove Ce by oxidation process, cuts problem in half

Bokan Mountain, AlaskaTo get to Eu, Tb and Dy and Y need to remove ~52% of the lighter REs, below Eu.Remove Ce by oxidation process, cut problem in halfAfter getting Eu Y, can stop separation process and save Ho and Er

Still no simple solution to get the critical elements

R: La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

% 25 49 4 19 1 0.5 1 0.1 0.1 -- 0.1 -- -- --

R: La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

% 10 24 3.1 12 3.1 0.2 3.5 0.8 4.2 8.8 2.4 -- -- --

Mountain Pass, California

Bokan Mountain, Alaska

Y<0.1

Y27

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ELECTROLYTIC PREPARATION OF RARE EARTH METALS

C1100Amps10,000

NdF3-LiF Flux 3O24Nd2 Nd2O3+¾¾¾¾ ¾

°

(g)Electrolytic Cell

+ some F2(g)

COMMENT: oxide solubility in the flux is small; control of amount oxide is critical and difficult to do; some NdF3 may be reduced to Nd + F2(g) when Nd2O3 is consumed.

For R with melting point less than 1050°C (La, Ce, Pr, Nd, MM)

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CALCIOTHERMIC PREPARATION OF RARE EARTH METALS

2C15003 3CaF2R3Ca2RF +¾¾ ¾+ °

O3H2RCl6HClOR 23C600~32 +¾¾¾ ¾+ °

2C12003 3CaCl2R3Ca2RCl +¾¾¾ ¾+ °

OR

323C700432 3NHO3H2RFHF)F3(NHOR ++¾¾ ¾+ °

For R with melting point greater than 1050°C (R = Sc, Y, Gd Er, Lu)

Ta

CaF2

Gd

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LANTHANOTHERMIC PREPARATION OF Sm, Eu, Tm, Yb

La* + R2O3 R + La2O3

R Boiling point

(°C)Melting Point

(°C)

La 3464 918

Sm 1794 1074

Eu 1527 822

Tm 1950 1545

Yb 1196 819

*Could also use Ce and MM (some Nd impurity) instead of La

Dy distillate

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SUBLIMATION/DISTILLATION OF RARE EARTH METALS

To improve the metal purity R + i Rg (i remains in residue)

R rare earth metal i impurities (O, C, N, Ta, Mo)

heat

Sublimation DistillationSc YDy GdHo TbEr Lu

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c

ba

c

ba

c

ba

A

A

C

B

A

A

B

A

C

A

B

A

A

B

A

B

C

B

C

A

C

A

hcp: Sc, Y, Sm, Gd, Tb,Dy, Ho, Er, Tm, Yb, Lu

fcc: La, Ce, Yb

dhcp: La, Ce, Pr, Nd, Pm

Sm-type: Sm

[111]

CLOSE-PACKED CRYSTAL STRUCTURES OF THE RARE EARTH METALS

Eu is body-centered cubic (not shown)

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METALLIC RADIUS OF THE RARE EARTH METALS

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THE MELTING AND TRANSFORMATION TEMPERATURES OF THE RARE EARTH METALS

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THE BOILING POINTS OF THE RARE EARTH METALS

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OXIDESNormal Oxides – Sesquioxide R2O3

Among the most stable oxides

Other Valence State Oxides Tetravalent or partially tetravalent

CeO2 all 4+ Pr6O11 4PrO2 + Pr2O3

Tb4O7 2TbO2 + Tb2O3

Divalent or partially divalent EuO 2+ Eu3O4 EuO + Eu2O3

Notable CeO2 (with Ce2O3) automotive 3 way emission catalyst EuO – ferromagnetic semiconductor

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THE FREE ENERGY OF FORMATION AT 298 K OF THE R2O3 PHASE

Eu2O3 and Yb2O3 are Anomalous

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RARE EARTH HYDRIDES - IMetals readily react with hydrogen to form RH2 and RH3

Exceptions: Sc, Eu and Yb only form RH2

Structures RH2 CaF2 type structure (fluoride)

RH3 fluorite – La, Ce, Pr, Nd hexagonal HoH3-type – Y, Sm, Gd-Tm, Lu

RH2 Eu, Yb: Orthorhombic, isostructural with alkaline earth MH2

Reactive in air RHx + O2 R2O3 + H2O

RH3 semiconductors: R3+ + 3H-

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RARE EARTH HYDRIDES - IISemiconductors x < 2.7 metallic conduction x > 2.8 semiconductor at x 2.8 metal to semiconductor transition

Switchable Mirrors thin films of RHx x 3 x < 2.9 films reflect light (mirrors) x = 3.0 films are transparent

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RARE EARTH HALIDES - IX = F, Cl, Br and I

RX3 normal phase X = F – stable in air; insoluble in H2O and acids X = Cl, Br, I – hygroscopic

RX4

only R = Ce, Pr and Tb

RX2 R = Sm, Eu, Yb reported “RX2” – not true RX2 phases, stabilized by interstitial elements (H, C)

RF3 or RCl3 start material for making R metals

RF3 – component of heavy metal fluoride glasses ZrF4-BaF2-LaF3-AlF3

fiber optic glasses

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Fluoride Melting Points Halide Melting and Boiling Points

RARE EARTH HALIDES - II

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PHOSPHORS, LASERSCritically depend on the 4f electronsSharp transitionsEach lanthanide (Ln) is uniqueMust be 99.99+ pureY, La, Lu are hosts (no unpaired 4f electrons; also must be pure)0.5 to 10% Ln in hosts

V B G R

4f electron energy levels in the absence of a crystal field

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MAGNETIC PROPERTIES - ICurie temperature TC

ferromagnetic below TC spins aligned parallel to one another paramagnetic above TC spins randomly orientated

Neél temperature TN

antiferromagnetic below TN spins aligned antiparallel to one another paramagnetic above TN spins randomly orientated

Sometimes above TN – paramagnetic below TN – antiferromagnetic changes to ferromagnetic (Tc)

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Quantum Numbers for Trivalent Lanthanides

SPIN S ½n (n = 1,2 . . .7), where n is the number of unpaired 4f electrons

ORBITAL L li = 3,2,1,0,-1,-2,-3; L = li

TOTAL J J = L ± S - for less than half filled (Ce Eu) + for more than half filled (Tb Yb)

for Gd: L = 0 and J = S = 7/2

DIVALENT LANTHANIDES Eu2+ 4f7 same as Gd3+

Yb2+ 4f14 same as Lu3+

MAGNETIC PROPERTIES - II

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Effective Magnetic Moment, peff

Paramagnetic region: peff = g[J(J+1)]½

Ferromagnetic region at T 0K: peff = gJ

The larger peff the stronger the magnet

MAGNETIC PROPERTIES - IIIMagnetic Strength

Gyromagnetic Ratio, g

1)2J(J1)L(L - 1)S(S 1)J(J 1 g

+++++

+

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Magnetic Ordering TemperaturesDeGennes Factor: (g-1)2J(J+1)

MAGNETIC PROPERTIES - IV

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Magnetic Entropy: SM = Rln(2J+1)

MAGNETIC PROPERTIES - V

Important in magnetic refrigeration, the larger the entropy change the larger the cooling power

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PERMANENT MAGNET COMPOUNDS

R La Pr Nd Sm Gd Tb Dy Ho Er

x 0 0 0 0 0 0.1 0.2 0.5 1.0

Magn. Mom. 7.1 9.0 9.1 7.2 2.6 1.7 3.2 4.6 5.6

Saturation Magnetic Moments = S3d JR

For 3d = Co in RCo5+x

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Terfenol D (Tb0.3Dy0.7)Fe1.9 Giant magnetostrictive material

LaNi5Hy Nickel-metal-hydride battery

LaB6 Electron gun for electron microscopes

Giant Magnetocaloric Effect Materials Gd5(Si2Ge2) La(Fe1-xSix)13

INTERESTING INTERMETALLIC COMPOUNDS

Magnetic Refrigeration/Heating

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TOXICITY

LowCan be safely handled with ordinary care

Organically complexed ions are more toxic than solids or inorganic solutions

Dust and vapors should not be inhaled – true for most chemicals

Solutions splashed into eyes should be washed out – true for most chemicals

Splinters of metal should be removed – true for most metals