THE PHYSICAL METALLURGY OF THE
RARE EARTH METALS
Karl A. Gschneidner, Jr.
Department of Materials Science and Engineering
Iowa State University
Ames, Iowa 50011-2300,USA
Res Metallica
Katholieke Universiteit
Leuven, Belgium
May 23, 2012
IOWA STATE UNIVERSITY
OF SCIENCE AND TECHNOLOGY
AS A CONSUMER – I
The 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
PRODUCTS
Electric 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 PRODUCTS
Gasoline – FCC cracking catalysts
[La,Ce, mixed REO]
Windshield, mirrors – polishing [Ce]
2
AS A CONSUMER – II
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]
3
AMES LABORATORY - I
How 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. 1945
Delivered 300 tons Th after World War II
Ames Laboratory became an Atomic Energy Commission
laboratory May 17, 1947.
4
AMES LABORATORY - II
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
5
PERIODIC TABLE
Group
Period
1
18
1
2
1H
3
2 Li
11
3 Na
19
4
K
37
5 Rb
55
6 Cs
87
7 Fr
2
13
4
5
Be
13
3
Mg
20
Ca
38
Sr
56
Ba
6
B
12
21
Sc
39
Y
4
22
Ti
40
Zr
5
23
V
41
Nb
73
Ta
6
24
Cr
42
Mo
74
W
7
25
Mn
43
Tc
75
Re
8
26
Fe
44
Ru
76
Os
9
27
Co
45
Rh
77
Ir
10
28
Ni
46
Pd
78
Pt
11
29
Cu
47
Ag
79
Au
12
30
Zn
48
Cd
80
Hg
14
Al
31
Ga
49
In
81
Tl
15
7
C
14
Si
32
Ge
50
Sn
82
Pb
16
8
N
15
P
33
As
51
Sb
83
Bi
17
O
16
S
34
Se
52
Te
84
Po
He
10
9
F
17
Cl
35
Br
53
I
85
At
Ne
18
Ar
36
Kr
54
Xe
86
Rn
88
Ra
LANTHANIDES
RARE EARTHS (R): Sc, Y, + Lanthanides (Ln)
6
ELECTRONS DO IT ALL
Four 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
tungsten
second highest boiling point - 5725°C
f – do not move, sit by the nucleus; little if any bonding
lower melting point
lanthanides
no effect on boiling point
7
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
8
ELECTRON WAVE FUNCTIONS OF AN
ISOLATED GADOLINIUM ATOM
9
SHAPES OF THE 4f CHARGE DENSITIES
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
(f) Eu2+J = O M = O
Gd3+ M7/2; Lu3+
oblate (red shading)
prolate (green shading
spherical (blue shading)
10
ELECTRONIC STRUCTURES OF THE
RARE EARTH GROUND STATES
Element
Neutral Atom
Configuration
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
11
ELECTRONIC STRUCTURES OF THE
RARE EARTH IONS
4f Configuration of Known
Oxidation States
M+2
M+3
M+4
Sc
-
0
-
Y
-
0
-
La
-
0
-
Ce
-
1
0
Pr
-
2
1
Nd
-
3
-
Pm
-
4
-
Sm
6
5
-
Eu
7
6
-
Gd
-
7
-
Tb
-
8
7
Dy
-
9
-
Ho
-
10
-
Er
-
11
-
Tm
-
12
-
Yb
14
13
-
Lu
-
14
-
Element
12
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
13
USES THAT DEPEND UPON VALENCE
AND SIZE
Mixed rare earths
Petroleum cracking catalyst (also La, Ce)
Mischmetal
lighter flints
alloy additive
Individual rare earth elements
Nickel-metal(La)-hydride batteries
Alloying agent (La, Ce, Nd, Y)
14
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 lamps
optical displays (TV, etc.)
Fiber optics (very sensitive to impurities,  99.999%R;
i.e. 5 nines)
Er
15
USES THAT DEPEND ON THE ABSENCE OF
ELECTRONIC TRANSITIONS IN UV, OPTICAL
AND IR WAVE LENGTHS
High Purities Required  99.999%R
Optical lenses
La, Gd, Lu
Phosphor hosts
Y, Gd
Artificial gem stones
Y
___________________________
Y3+ and La3+ - no 4f electrons
Gd3+ lowest transition in the very short ultraviolet region
Lu3+ completely filled 4f level, all 4f electrons paired up
16
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
17
ABUNDANCES OF NATURALLY OCCURRING
ELEMENTS IN THE LITHOSPHERE
18
SELECTED RARE EARTH MINERALS
Name
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
19
RARE EARTH CONTENT IN SELECTED
MINERALS
Bastnasite
Rare
Mountain
earth
Pass,
element
USA
La
33.8
Ce
49.6
Pr
4.1
Nd
11.2
Sm
0.9
Bastnasite
Bayan
Obo,
China
23.0
50.0
6.2
18.5
0.8
Monazite
Mt.
Weld,
Australia
25.5
46.7
5.3
18.5
2.3
Xenotime
Lehat,
Malaysia
1.2
3.1
0.5
1.6
1.1
High Y
RE
laterite
Longnan,
China
1.8
0.4
0.7
3.0
2.8
Low Y
RE
laterite
Xunwu,
China
43.4
2.4
9.0
31.7
3.9
Loparite
Kola
Peninsula,
Russia
25.0
50.5
5.0
15.0
0.7
Eu
Gd
Tb
Dy
Ho
0.1
0.2
0.0
0.0
0.0
0.2
0.7
0.1
0.1
Trace
0.4
<0.1
<0.1
0.1
Trace
Trace
3.5
0.9
8.3
2.0
0.1
6.9
1.3
6.7
1.6
0.5
3.0
Trace
Trace
Trace
0.1
0.6
Trace
0.6
0.7
Er
Tm
Yb
Lu
Y
0.0
0.0
0.0
Trace
0.1
Trace
Trace
Trace
Trace
Trace
Trace
------<0.1
6.4
1.1
6.8
1.0
61.0
4.9
0.7
2.5
0.4
65.0
Trace
Trace
0.3
0.1
8.0
0.8
0.1
0.2
0.2
1.3
20
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
21
PROCESSING AND SEPARATION - II
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.
22
SCHEMATIC OF SEPARATING TWO RARE EARTHS
BY FRACTIONAL CRYSTALLIZATION
Original solution
Redissolve
and crystallize
Redissolve
and crystallize
Sol.
La3+
Sol.
Sol.
Liq.
Evaporate some
H2 O
Combine
Combine
Pr3+
Liq.
Combine
Nd3+
Evaporate some
H2 O
Liq.
Sm3+
23
SEPARATION OF RARE EARTH ELEMENTS
Chemically very similar
To get a given rare earth element
No specific chemical can pull out a desired RE
Must separate all of the elements before it in the series
Exceptions
Ce – 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
24
STABILITY CONSTANTS OF
COMPLEXING AGENTS
DTPA
EDTA
HEDTA
The slope is more important than the magnitude.
25
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
26
PROBLEM: SEPARATION SEQUENCE
Mountain Pass, California
R:
La
Ce
%
25
49
Pr
4
Nd
19
Sm
1
Eu
0.5
Gd
1
Tb
Dy
Ho
Er
Tm
Yb
Lu
0.1
0.1
--
0.1
--
--
--
Y
<0.1
Bokan Mountain, Alaska
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
--
--
--
Y
27
Mountain Pass, California
To get to Eu, Tb and Dy need to remove ~99% other RE
Remove Ce by oxidation process, cuts problem in half
Bokan Mountain, Alaska
To 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 half
After getting Eu  Y, can stop separation process and save Ho and Er
Still no simple solution to get the critical elements
27
ELECTROLYTIC PREPARATION OF RARE
EARTH METALS
For R with melting point less than 1050°C (La, Ce, Pr, Nd, MM)
Electrolytic Cell
(g) + some F (g)
+




4Nd
3O
2 Nd2O3 NdF
2
2
-LiF Flux
3
10,000 Amps
1100 C
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.
28
CALCIOTHERMIC PREPARATION OF
RARE EARTH METALS
For R with melting point greater than 1050°C (R = Sc, Y, Gd  Er, Lu)
R 2O3 + 3(NH 4F  HF) 
 2RF3 + 3H2O + 3NH3
700 C
2RF3 + 3Ca 1500

 2R + 3CaF2
C
OR
R 2O3 + 6HCl 

 2RCl3 + 3H2O
~600  C
Ta
CaF2
2RCl3 + 3Ca 

 2R + 3CaCl2
1200 C
Gd
29
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
Dy distillate
*Could also use Ce and MM (some Nd impurity) instead of La
30
SUBLIMATION/DISTILLATION OF
RARE EARTH METALS
To improve the metal purity
R + iheat
 Rg (i remains in residue)
R  rare earth metal
i  impurities (O, C, N, Ta, Mo)
Sublimation
Sc
Dy
Ho
Er
Distillation
Y
Gd
Tb
Lu
31
CLOSE-PACKED CRYSTAL STRUCTURES
OF THE RARE EARTH METALS
A
A
B
C
c
A
a
b
hcp: Sc, Y, Sm, Gd, Tb,
Dy, Ho, Er, Tm, Yb, Lu
[111]
A
A
C
C
B
A
C
B
A
c
A
C
a
b
dhcp: La, Ce, Pr, Nd, Pm
B
B
A
B
c
A
A
a
fcc: La, Ce, Yb
Eu is body-centered cubic (not shown)
b
Sm-type: Sm
32
METALLIC RADIUS OF THE
RARE EARTH METALS
33
THE MELTING AND TRANSFORMATION
TEMPERATURES OF THE RARE EARTH METALS
34
THE BOILING POINTS OF THE
RARE EARTH METALS
35
OXIDES
Normal 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
36
THE FREE ENERGY OF FORMATION AT 298 K OF
THE R2O3 PHASE
Eu2O3 and Yb2O3 are Anomalous
37
RARE EARTH HYDRIDES - I
Metals 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+ + 3H38
RARE EARTH HYDRIDES - II
Semiconductors
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
39
RARE EARTH HALIDES - I
X = 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
40
RARE EARTH HALIDES - II
Fluoride Melting Points
Halide Melting and Boiling Points
41
PHOSPHORS, LASERS
Critically depend on the 4f electrons
Sharp transitions
Each lanthanide (Ln) is unique
Must be 99.99+ pure
Y, La, Lu are hosts (no unpaired 4f electrons; also must be pure)
0.5 to 10% Ln in hosts
V BG
R
4f electron energy levels in the absence of a crystal field
42
MAGNETIC PROPERTIES - I
Curie 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)
43
MAGNETIC PROPERTIES - II
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+
44
MAGNETIC PROPERTIES - III
Magnetic Strength
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
Gyromagnetic Ratio, g
g  1+
J(J + 1) + S(S + 1) - L(L + 1)
2J(J + 1)
45
MAGNETIC PROPERTIES - IV
Magnetic Ordering Temperatures
DeGennes Factor: (g-1)2J(J+1)
46
MAGNETIC PROPERTIES - V
Magnetic Entropy: SM = Rln(2J+1)
Important in magnetic refrigeration,
the larger the entropy change the
larger the cooling power
47
PERMANENT MAGNET COMPOUNDS
Saturation Magnetic Moments = S3d  JR
For 3d = Co in RCo5+x
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
48
INTERESTING INTERMETALLIC
COMPOUNDS
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)
Magnetic Refrigeration/Heating
La(Fe1-xSix)13
49
TOXICITY
Low
Can 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
50