The Rare-Earth Hydrides

CHAPTER 9

The Rare-Earth Hydrides WILLIAM

M.

MUELLER*

There has been much interest in the rare-earth metals of the lanthanide series in the last few years for several reasons. Among the reasons are indications that these metals have relatively high hydrogen retention, relatively high melting points, and high thermalneutron absorption cross sections. The rare earths are those elements from atomic numbers 57 to 71 in the Periodic Table. Frequently, scandium (atomic number 21) and yttrium (atomic number 39) are treated along with the rareearth elements; however, hydrides of these two elements are discussed in Chap. 10 of this book. The characteristic property of the rare-earth elements is the electronic structure; the inner 4f shell is filled before the 5d shell, as shown in Table 9.1. The idealized 5c?1 6s 2 arrangement is not maintained, since there is a tendency for the 5d electrons to drop to the 4f level. Nevertheless, the energy difference between the two states is slight; in general, therefore the rare earths have essentially the same electronic structure and readily form the tri valent ion shown. Since half-filled and completely filled subshells are unusually stable, there is a tendency to maintain the Af1 and 4f14 configurations. Consequently the divalent states in europium and ytterbium are much more stable than in the other rare earths; this stability leads to differences between the properties of the hydrides of these two elements and those of the other rare-earth hydrides. The structural properties of the rare-earth metals are given in Table 9.2. The metals Gd, Tb, Dy, Ho, Er, Tm, and Lu all have the hexagonal close-packed structure. Lanthanum, praseodymium, and * American Society for Metals (formerly with the Denver Research Institute, University of Denver). 384

9

THE RARE-EARTH HYDRIDES

385

TABLE 9.1 E L E C T R O N I C S T R U C T U R E O F T H E R A R E - E A R T H A T O M S AND T R I P O S I T I V E I O N S 0

Neutral atoms

Element La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Atomic number 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

Idealized

Probable

3 Cation

5dW 6s2 6s2 6s2 6s2 6s2 6s2

5s25p6 ψ 5s25p6 ψ 5s25p6 ψ 5s25p6 ψ 5s25p6 4p 5s25p6 4f6 5s25jf 4f 5s25p6 ψ 5s25p6 ψ 5s25p6 4f1Q5s25p6 4fn5s25p6 4f125s25p6 4f135s25p6 4f45s25p6

2

ed^s

ψ 5dW ψ 5^6s2 ψ 5dW

ψ 5
ψ 5^652 ψ 5dW ψ 5dW

ψ SdïSs2 4f> 5 d W 4/ 1 0 5^6Α 2

4fn5d16s2 4f125d16s2 4fls5d16s2 4f145d16s2

Ψ ψ Ψ ψ ψ Ψ

ψ Sd^s2

6s2 6s2 4/11 6s2 4/12 6s2 4/13 6s2 4/14 6s2 4f145d16s2

ψ 4/10

a

From T. Moeller, The Chemistry of the Rare Earths, in The Rare Earths, F. H. Spedding and A. H. Daane (Eds.), Chap. 2, p.10, John Wiley & Sons, New York, 1961.

neodymium also have a hexagonal structure; however, the stacking sequence is ABAC, ABAC, etc., rather than the simple AB, AB, etc., of the hexagonal close-packed structure. Cerium, although it is facecentered cubic at room temperature, transforms to the A3' hexagonal structure if the temperature is lowered to — 10°C. The da ratio of the rare-earth metals exhibiting the A3' hexagonal structure is about 3.23 as compared to 1.58 for heavier metals having the hexagonal close-packed structure. Samarium has a unique rhombohedral structure which also can be indexed in the hexagonal system to give a da ratio of 7.25. The structures of europium and ytterbium would be expected to differ from those of the other rare-earth metals in view of the unusual stability of their Af7 and 4/ 1 4 electronic configurations as discussed previously. This is also reflected in their valence of two, as compared to a valence of three for the other rare earths, and in their anomalously high values for the metallic radius, as shown in Table 9.2. All the rare-earth metals, with the exception of cerium, have relatively high thermal-neutron absorption cross sections. This makes them, and in particular their hydrides, of interest as reactor shielding

386

9

T H E RARE-EARTH

HYDRIDES

TABLE 9.2 STRUCTURAL PROPERTIES O F THE RARE-EARTH

Element La Ce Pr Nd Pm Sm Eu Gd Tb

Dy

Ho Er Tm Yb Lu

Room-temperature structure Hex., A3' F.c.c., Al Hex., A3' Hex., A3'

—

Rhombohedral B.c.c, A2 H.c.p., A3 H.c.p., A3 H.c.p., A3 H.c.p., A3 H.c.p., A3 H.c.p., A3 F.c.c, Al H.c.p., A3

ELEMENTS"

Metallic radius (A)

Zachariasen* ionic radius (A) (+3)

Metallic valence

1.877 1.825 1.828 1.821 1.810 1.802 2.042 1.802 1.782 1.773 1.766 1.757 1.746 1.940 1.734

1.04 1.02 1.00 0.99 0.98 0.97., 1.11(4-2) 0.96 , 1.09(4-2) 0.94 0.92 0.91 0.89 0.87 0.86 0.85,, 0.93(+2) 0.84

3 3.1 3 3 3 2.9 2.1 3 3 3 3 3 3 2 3

a

From K. A. Gschneidner, Jr., Crystallography of the Rare Earth Metals, in The Rare Earths, F. H. Spedding and A. H. Daane (Eds.), Chap. 14, p. 192, John Wiley & Sons, New York, 1961. b W. H. Zachariasen, Crystal Chemistry of the 5f Elements, in The Actiniae Elements, Chap. 18, pp. 775-776, Division IV, Volume 14B, National Nuclear Energy Series, McGraw-Hill Book Company, Inc., New York, 1954.

and control materials. The high absorption cross sections would, of course, preclude the use of the metals or their hydrides for moderator or reflector application. Table 9.3 gives the thermal-neutron absorption cross sections of the rare-earth metals. The production of high-energy gamma radiation due to neutron capture would be a disadvantage in a material used for shielding application. Although several of the rare-earth isotopes producible from the naturally occurring elements by (η,γ) reactions do emit gamma radiation of fairly high energy, as shown in Table 9.4, all except the europium isotopes are so short lived that they are not troublesome.

9-1

GENERAL CHARACTERISTICS OF THE RARE-EARTH HYDRIDES

Studies of rare earth-hydrogen systems have been carried on more or less continuously since the first work by Winkler 1 in 1891. How-

9-1

GENERAL

387

CHARACTERISTICS

TABLE 9.3 THERMAL-NEUTRON

ABSORPTION

CROSS SECTION OF THE RARE-EARTH

Element

σα (th)

METALS3

(barns)

8.9 0.70 11.2 46 60 5500 4600 46000 44 1100 64 166 128 36 108

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

a Data from Harold Etherington (Ed.), Nuclear Engineering McGraw-Hill Book Company, Inc., New York, 1958.

Handbook,

pp. 2.19-2.20,

TABLE 9.4 HIGH-ENERGY

GAMMA-RAY-EMITTING

Isotope 140

La

40.2 hr

141

La

3.7 hr 19.3 hr 17.5 min

142p r i44pr

166

Ho

24.4 16 15.4 27.2

172

Tm

63

146p r 154

Eu

156Eu

a

Half-life

min years days hr hr

RARE-EARTH

ISOTOPES"

Energy (Mev) 1.60 2.5 3.0 -1.5 1.60 1.48 2.18 1.49 1.4 2.0 1.61 1.69 1.44 1.79

Data taken from Trilinear Chart of the Nuclides by William H. Sullivan, Superintendent of Documents, U. S. Government Printing Office, Washington, 1957.

388

9

T H E RARE-EARTH

HYDRIDES

ever, since the rare-earth metals are somewhat difficult to separate from one another, much of the early work was done on misch metal, a mixture of the rare-earth metals in varying amounts. Mixtures of the rare earths are described as lanthanum misch metal, cerium misch metal, etc., depending on the particular rare-earth metal that predominates in the composition. Thus work reported on supposedly pure rare-earth elements should be examined critically. Recently Mikheeva and Kost,2 Mikheeva, 3 Libowitz, 4 and Bos and Gayer 5 have surveyed the rare-earth hydrides. Because of their similarities, the rare-earth metals would be expected to have similar hydriding characteristics, with the possible exception of europium and ytterbium. This is actually the case. All the rare-earth metals form dihydrides (except for europium and ytterbium) that have the fluorite type structure and easily take up additional hydrogen to form trihydrides. In contrast, europium and ytterbium form orthorhombic-structured dihydrides. Additional hydrogen is taken up by YbH 2 with difficulty under high pressures of hydrogen, but attempts to prepare a higher hydride of europium have been unsuccessful. 6 There is also a difference between the hydrides of the light rare earths (lanthanum, cerium, praseodymium, and neodymium hydrides) 1 800

600

400

\

1

\ \

{

\

f

1

hex. hydride

\

f.c.c. \ hydride \ J

'/ T;

a + f.c.c. hydride

hydride |

+ "|

200

hex. 1 hydride

1

n 0

1

2

3

0

1

2

Solid Composition, H/M

Solid Composition, H/M

(a) La, Ce, Pr, Nd


F I G . 9.1 Schematic phase diagrams of the rare earth-hydrogen systems. 56

9-1

GENERAL

389

CHARACTERISTICS

and the heavy ones (Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu hydrides). The light rare earths form trihydrides with no change in structure by merely filling the octahedral sites in the fluorite structure with hydrogen atoms. 7 The octahedral sites of the heavy rare earths are also occupied initially, but, before the composition MH 3 is reached, the lattice transforms to a hexagonal structure. This difference is reflected in the schematic phase diagrams for the light and heavy rare-earthhydrogen systems in Fig. 9.1. TABLE 9.5 C R Y S T A L S T R U C T U R E S AND L A T T I C E P A R A M E T E R S O F R A R E - E A R T H

Dihyd ridefc Metal

Structure

HYDRIDES0

Trihydride 0

Parameters (A)

Structure

La Ce Pr Nd Sm

F.c.c. F.c.c. F.c.c. F.c.c. F.c.c.

a a a a a

= = = = =

5.663 5.575 5.518 5.464 5.374

Eu

Orthorhombic

Gd

F.c.c.

« b c a

= = = =

6.21 3.77 7.16 5.303

H.c.p.

Tb

F.c.c.

a = 5.246

H.c.p.

Dy

F.c.c.

a = 5.201

H.c.p.

Ho

F.c.c.

a = 5.165

H.c.p.

Er

F.c.c.

a = 5.123

H.c.p.

Tm

F.c.c.

a = 5.090

H.c.p.

Yb

Orthorhombic

F.c.c.

Lu

F.c.c.

a= b = c= a=

5.904 3.580 6.794 5.033

F.c.c. F.c.c. F.c.c. F.c.c. H.c.p.

H.c.p.

Parameters (A)

Ref.

= = = = = =

5.604 5.539 5.483 5.435 3.782 6.779

27 22, 26 51 51 51

= = = = = = = = = = = = =

3.73 6.71 3.700 6.658 3.671 6.615 3.642 6.560 3.621 6.526 3.599 6.489 5.192

61

a = 3.558 c = 6.443

51

a a a a a c

a c a c a c a c a c a c a

57

51 51 51 51 51 65

Modified from tabulation by Bos and Gayer. 5 Except for Eu (EuD 1 9 5 ), the parameters are given for the stoichiometric dihydrides. c The parameters given for the trihydrides are for compositions above MH 2 . 9 , with the exception of ytterbium (the highest composition of ytterbium trihydride for which data are available is YbH2.55). a

b

390

9

"i 5.70

THE RARE-EARTH HYDRIDES

1

1

1

1

1

' 1

i

1

r

\La O ^ ^

[·

-\

5.60 l·Ce 0

c ö

"55

^

^

_

^

^

Γ"

o 5.50 u

^

-

^ "1

0 - - C L ^ ^ ^ o

u

Pr o

^

Nd a —

^

—\

.

A

l· 5.40 h Sm O

-\

5.30 h

l·

1.8

j

1 2.0

i

1 2.2

i

1 2.4

i

1 2.6

i

' 2.8

■ 3.0

H/M Atom Ratio FlG. 9.2 Variation in lattice parameter of fluorite structure with hydrogen content. From C. E. Holley, R. N. R. Mulford, F. H. Ellinger, W. C. Koehler, and W. H. Zachariasen, The Crystal Structure of Some Rare Earth Hydrides, J. Phys. Chem., 59: 1227 (1955).

The crystal structures of the rare-earth hydrides are summarized in Table 9.5. The fluorite structure is based on a face-centered cubic lattice with four metal atoms per unit cell at the face-centered cubic positions (0 0 0), (0 i i), (i 0 i), and (i i 0), and hydrogen in the eight tetrahedral positions. 7 As indicated in Table 9.5 and illustrated in Fig. 9.2, the increase in hydrogen content above MH 2 results in a decrease in lattice parameter. This indicates a change in the bonding characteristics between the metal and hydrogen since the additional hydrogen occupies the octahedral interstices. Contraction has been observed to begin at a composition of about M H 1 8 ; this implies that some octahedral interstices are occupied while some of the tetrahedral sites are still vacant. The stoichiometric trihydride of the light rare-earth metals is obtained when all the octahedral interstices are filled with hydrogen, producing the BiF 3 type structure. Beck 8 proposed, on the basis of structural considerations, that the bonding of the dihydrides of the light rare-earth metals is predominantly metallic in nature and the bonding between the metal and the hydrogen occupying octahedral voids is predominantly ionic

9-1

G E N E R A L CHARACTERISTICS

391

in nature. This analysis is consistent with the decrease in lattice parameter when the dihydride changes to the trihydride. Additional evidence for this concept is given by the magnetic studies of Kubota and Wallace 9 and Wallace and associates 10 on Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm hydrides. These investigators observed that the tendency for magnetic alignment in these hydrides was suppressed with increase in hydrogen content beyond MH 2 . This was explained 9 by removal of conduction electrons (which act as intermediaries for magnetic interaction between 4f electrons) due to hydrogen anion formation. In contrast, Dialer 11 proposed, on the basis of heats of absorption of hydrogen, that the first two hydrogen atoms per metal atom are ionic and the third is covalent. The heat of absorption for the initial two hydrogen atoms is about 40 to 50 kcal per mole of H 2 , which is similar to that of the saline ionic hydrides (25 to 40 kcal per mole of H 2 ), and the heat of absorption of hydrogen beyond MH 2 is about 10 to 20 kcal per mole of H 2 . -4

-6

-8

£ o

I

o

-io

-12

u

$

-14

-16 -18

-20 900

1000

1100

1200

1300

Temperature, °K

F I G . 9.3 Standard free energies of formation of some face-centered cubic dihydrides as a function of temperature. 1 8

392

9

T H E RARE-EARTH

HYDRIDES

At present there is considerable uncertainty about the bonds between hydrogen and metal in the rare-earth-hydrogen systems. For more detailed discussions the reader is referred to the works of Libowitz and Gibb, 12 Mueller and Speiser, 13 Gibb and Schumacher, 14 Gibb, 15 Vainshtein et al.,16 and Libowitz. 4 A recent neutron-diffraction study 17 of holmium trideuteride demonstrated that the hydrogen (or deuterium) positions in the lattice are such that the hexagonal structure is not close-packed, as designated in Table 9.5, but that the a0 parameter is V 5 times that for the hexagonal close-packed structure, as calculated from X-ray diffraction. This makes the value of a0 equal to 6.308 A rather than 3.642 A as given in Table 9.5. The same situation can be expected to be true for the other hexagonal trihydrides listed in Table 9.5. Magee, 18 in his investigation of the relative stabilities of the dihydrides of the rare-earth metals, calculated the standard free energies of formation of the dihydrides from equilibrium dissociation pressures, as shown in Fig. 9.3. The degree of stability appears to increase with atomic number, with the exception of cerium and lanthanum.

1

o o l·-

Od

OGd

—

OD

Ol

—

5.8

1

1

1

5.6

5.4

5.2

5.0

Lattice Parameters, A

F I G . 9.4 Standard free energies of formation at 1100°K of some face-centered cubic dihydrides vs. lattice parameter. 18

9-2

SPECIFIC HYDRIDES AND DEUTERIDES

393

The parallelism between the trend shown in Fig. 9.3 and the decrease in atomic diameter of the rare-earth metals with increasing atomic number led Magee to investigate the relation between stability (as expressed by the free energy of formation at 1100°K) and the roomtemperature lattice parameters of the dihydrides. His data are shown in Fig. 9.4. The trend toward greater stability with decreasing lattice parameter is consistent, with the exception, again, of cerium and lanthanum. A high degree of pyrophoric behavior has been noted in some rareearth hydrides, particularly cerium. As expected, this behavior is pronounced in finely divided hydrides at high temperatures. However, spontaneous ignition has been observed in massive specimens (of the order of several grams) of cerium hydride at room temperature. Kost19 reports trouble in avoiding ignition even during the short exposure to air involved in transferring a specimen from a reaction tube to a desiccator. Maintaining an inert gas atmosphere around the specimen is very helpful.

9-2 9-2.1

SPECIFIC H Y D R I D E S AND D E U T E R I D E S LANTHANUM

(a) Hydriding Characteristics (a) Hydriding Characteristics The earliest work on the lanthanum-hydrogen system was done before techniques for separation of the rare earths from one another were well developed. Hence this work was done on lanthanum misch metal. Inasmuch as analytical techniques for the determination of the individual rare earths were not well developed, it is doubtful that the data are pertinent to actual lanthanum-hydrogen systems. Winkler 1 apparently was able to bring about absorption of hydrogen in misch metal to approach very closely the equivalence of the composition LaH 2 , i.e., about 66.7 at.% hydrogen. There was an indication that some reaction of hydrogen with misch metal occurred at temperatures as low as room temperature. Muthmann and Kraft20 achieved a hydrogen concentration of about 69 at.%, which yielded a material intermediate between LaH 2 and LaH 3 . Sieverts and Roell 21 recognized that the amount of hydrogen contained in the lanthanum d e p e n d e d upon the hydrogen pressure acting on the metal and that the pressure in equilibrium with a given composition increases with temperature.

394

9

THE RARE-EARTH HYDRIDES

More recent work by Korst22 using high-purity lanthanum showed variable behavior with respect to the temperature at which absorption of hydrogen begins. In general, reaction with hydrogen did not take place until the specimen had been heated to a temperature in the range of 260° to 365°C. In some instances, however, reaction took place at room temperature even though the specimen had not received any preliminary heating or outgassing. (b) Dissociation Pressures Pressures and (b) Dissociation and Thermodynamic Thermodynamic Properties Properties Pressure-composition-temperature studies of the lanthanumhydrogen system were carried out by Mulford and Holley 23 and Korst and Warf24 up to 1 atm of hydrogen. T h e results of Korst and Warf are tabulated in Tables 9.6 and 9.7 and illustrated in Figs. 9.5 and 9.6. Data were taken for both absorption and evolution of hydrogen. Within experimental error, there is no pressure-composition hysteresis as has been observed in other metal-hydrogen systems TABLE 9.6 DISSOCIATION PRESSURE DATA FOR T H E

LANTHANUM-HYDROGEN

SYSTEM IN T H E LOW-PRESSURE

Temperature (°C) 598

648 698

748 798

H/La 0.58 0.92 1.80 1.93 1.72 0.30 1.15 1.40 1.50 1.74 1.25 1.48 0.34 0.56 0.92 1.40 1.58 1.80

REGION0

Pressure (mm Hg) 0 0.0143 0.0143 0.0143 8.33 0.08 0.32 0.32 0.33 0.32 5.06 1.05 1.06 2.86 2.95 3.02 3.27 3.51 27.10

a a e a e a a a e a a e e e e e e e

Tabulated from the results of Korst and Warf.24 Abbreviations are (a) specimen absorbing hydrogen, (e) specimen evolving hydrogen. a

b

9-2

SPECIFIC HYDRIDES AND DEUTERIDES

395

TABLE 9.7 D I S S O C I A T I O N P R E S S U R E D A T A F O R T H E L A N T H A N U M - H Y D R O G E N SYSTEM IN T H E MEDIUM PRESSURE REGION0

606°C

H/La 1.97 1.99 2.01 2.06 2.06 2.09 2.14 2.15 2.18 2.20 2.20 2.23 2.23 2.27

656°C

Pressure (mm Hg) ft 6.0 8.5 12.5 29.0 31.0 49.0 87.0 108.0 159.0 196.0 223.0 273.0 315.0 452.0

e e e e a e e a e a e a e e

H/La 1.85 1.87 1.91 1.91 1.94 1.96 1.98 2.02 2.04 2.04 2.06 2.06 2.09 2.13 2.13 2.15 2.15 2.17 2.17 2.18 2.20 2.21

706°C

Pressure (mm Hg) ft 2.1 3.4 5.0 6.7 10.0 18.9 16.0 31.0 31.0 60.0 72.0 90.0 135.0 230.0 244.0 317.0 334.0 410.0 444.0 491.0 597.0 738.0

e e e a e a e a e a e a e a e a e a e a a a

H/La 1.81 1.81 1.83 1.85 1.89 1.91 1.94 1.98 1.98 1.99 2.05 2.05 2.07 2.08 2.11 2.11 2.12 2.13 2.14 2.15 2.16 2.17

756°C

Pressure (mm Hg) ft 1.8 2.0 2.4 4.2 6.3 9.4 18.1 25.5 29.4 46.0 73.0 95.0 168.0 185.0 257.0 301.0 348.0 478.0 419.0 515.0 672.0 765.0

e a e e e e e a e e a e e a e a e e a a e e

H/La

Pressure (mm Hg)ft

1.79 1.81 1.83 1.85 1.87 1.90 1.92 1.92 1.94 1.98 1.99 2.00 2.02 2.05 2.06 2.07 2.07 2.08 2.09 2.10 2.11 2.11 2.12 2.12

1.7 2.1 2.4 3.3 4.6 6.9 11.1 11.2 18.5 31.0 41.0 67.0 76.0 137.0 173.0 193.0 214.0 320.0 308.0 429.0 417.0 523.0 507.0 544.0

e e e e e e a e e a e a e e a a e e a e a e a a

Tabulated from the results of Korst and Warf.24 Abbreviations are (a) specimen absorbing hydrogen, (e) specimen evolving hydrogen. a

b

(see, for example, the discussion of the palladium-hydrogen system in Chap. 12). A similar study 24 for the lanthanum-deuterium system yielded the data shown in Table 9.8. The isotherms are of the same general shape as in the lanthanum-hydrogen system, although the equilibrium pressures are somewhat higher. This is usually the case for deuterium-hydrogen systems and is due chiefly to the larger absolute entropy of deuterium gas (see Ref. 4, p. 57). From the van't Hoff equation KP_AH dT KP

(9.1)

396

9

100 h

THE RARE-EARTH HYDRIDES

1

1

1

1

1

1

10 h

p

/ 1 798°C x

E E

*

1h h"

i

748°C

^ _

D

/

/

f

A

?

/ / / /

- — /

-x-

/

/

/

A

■A

/ /

«A IA

S 6 9 8C

°

D

0.1

L

'

A

648°C

1

l· 0.01

n □

n

5 9 8 C

0

°

1

1

0.3

0.6

A

J

0

1

1

1

1

0.9

1.2

1.5

1.8

H/La Atom Ratio

F I G . 9.5 Pressure-composition isotherms for the lanthanum-hydrogen system in hydrogen-lanthanum metal ratios less than two. Extracted from data from W. L. Korst and J. C. Warf, Rare Earth-Hydrogen Systems. I. Structural and Thermodynamic Properties, Inorg. Chem., 5(10): 1719-1726 (1966).

9-2

SPECIFIC HYDRIDES AND DEUTERIDES

397

the heat of reaction, AH, of the reaction 2 2-y-x

La(H x , saturated) + H 2

LaH 2

2-y-x

can be calculated, where Kp = pU2. If x and y are small, the calculated value of ΔΗ may be considered to be the heat of formation of lanthanum dihydride. As seen from Fig. 9.5, y is not small at elevated temperatures. Nevertheless, an approximate value for the heat of formation may be obtained by integrating Eq. (9.1) and plotting In pH2 vs. 1/Γ: AH In Kp = In pH2 = ~wf + constant

(9.2)

This is illustrated for lanthanum dihydride and dideuteride in Fig. 9.7. The pressures obtained by Mulford and Holley 23 are slightly 1

700

-

1

1

1 -\

Legend O Hydrogen absorption

•

Hydrogen desorption

600

uP/ £1 /

500

400

o°/

rJ

7

300

200

1° -

0 /

A

/

·/ / 100

^Λ?^6*Ώ n 1.8

2.0

1 2.1

l 2.3

H/La Atom Ratio

F I G . 9.6 Dissociation-pressure isotherms for the lanthanum-hydrogen system in the hydrogen-rich region. From W. L. Korst and J. C. Warf, Rare Earth-Hydrogen Systems. I. Structural and Thermodynamic Properties, Inorg. Chem., 5(10): 1722 (1966).

398

9

THE RARE-EARTH HYDRIDES TABLE 9.8

DISSOCIATION-PRESSURE

DATA FOR THE LANTHANUM-DEUTERIUM

D/La

Pressure (mm Hg) 0

585

1.02 1.02 1.02

0.0168 e 0.0173 a 0.0219 a

600

1.73 1.88 1.95 2.03 2.12

658

0.95

700

1.62 1.81 1.92 1.99

750

0.76 0.91 0.91

1.42 1.41 1.45

e e a

800

0.02 0.35 0.38 0.41 0.64 0.64 0.82 1.40 1.41 1.42 1.49 1.52 1.68

0.02 3.98 3.87 3.84 3.89 4.31 4.14 4.35 4.23 4.34 4.55 4.47 4.73

e a e a e a a e e a a e e

Temperature (°C)

0.6 5.2 50.2 159.0 586.0

SYSTEM0

e e e e e

0.165 a 1.4 10.5 127.0 513.0

e e e e

Tabulated from the results of Korst and Warf.24 Abbreviations are (a) specimen absorbing hydrogen; (e) specimen hydrogen. a

0

evolving

higher than those of Korst and Warf.24 The calculated equations for these isochores are as follows: LaH 2 _ x : log p H2 = (10.758 ± 0.001) -

1U

'°°° ~

M

( Re f. 23)

9-2

S P E C I F I C HYDRIDES AND D E U T E R I D E S

0.01

0.95

1.00

1.05

1.10

399

1.20

1.15

1/T°KX 103

FlG. 9.7 Van't Hoff isochores for plateau regions of the lanthanum-hydrogen and lanthanum-deuterium systems.

log p„ 2 = (10.644 ± 0.020) LaD 2 _ x : log

PD2

= (10.107 ± 0.016) -

10,84 10 17

'

^,±

19

(Ref. 24)

?, *

14

(Ref. 24)

The following heats of formation were calculated by comparing these experimental equations with Eq. (9.2): LaH 2 _ x (Ref. 23) LaH 2 _ x (Ref. 24) LaD2_.r (Ref. 24)

AH(kcal/mole H 2 ) -49.7 -49.6 -46.5

The agreement between the values of Mulford and Holley and those of Korst and Warf is excellent. A calorimetric determination of the heat of formation 25 of LaH 2 . 76 yielded a value of—40.1 kcal per mole of H 2 . T h e fact that this value

400

9

THE RARE-EARTH HYDRIDES

is lower per mole of hydrogen than the values given above for LaH 2 -^ illustrates the weaker bonding of the third hydrogen per metal atom. Hardcastle and Warf26 studied the lanthanum-hydrogen system at high hydrogen pressures (up to 30 atm) and obtained the isotherms shown in Fig. 9.8. From these data they calculated an average value of about 18 kcal per mole of H 2 for the partial molal heat of dissociation of LaH 2 . 7 to 2.8 to LaH 2 - x . (c) Physical

Properties

(1) Structure and Lattice Parameters. As mentioned previously, the dihydride has a fluorite structure which takes up additional hydrogen 40 30 20

10

5 E σ D

«A

2

1.0

0.5

0.1 2.0

2.5

3.0

H/La Atom Ratio

F I G . 9.8 High-pressure data for the lanthanum-hydrogen system. From K. I. Hardcastle and J. C. Warf, Rare Earth-Hydrogen Systems. III. High Pressure Investigations, Inorg. Chem., 5(10): 1731 (1966).

9-2

S P E C I F I C HYDRIDES AND D E U T E R I D E S

401

Legend O Korst (Ref. 22) • Holley et a I. (Ref. 7) D Goon (Ref. 27) x Stalinski (Ref. 28) Δ Ziegler a n d Young (Ref. 29)

1.8

2.0

2.2

2.4

2.6

2.8

3.0

H/La A t o m Ratio

FlG. 9.9 Variation of the lattice parameter of lanthanum hydride as a function of hydrogen content.

in the octahedral interstices of the lattice. Lattice parameter as a function of hydrogen content has been determined by several investigators, 7 ' 22 ' 27-29 and their results are summarized in Fig. 9.9. Korstand Warf24 proposed the following formula, which is represented as the straight line in Fig. 9.9, to calculate the room-temperature lattice parameter of lanthanum hydride at any given composition between LaHi.9 and LaH 3 : a0 (A) - 5.775 - 0.057r where r is the hydrogen-to-lanthanum atom ratio.

(9.3)

402

9

THE RARE-EARTH HYDRIDES

The lattice parameter of lanthanum deuteride of approximate composition LaD 1 8 5 was found 22 to b e 5.662 A, which is only about 0 . 1 % less than that of the corresponding hydride. (2) Density. Using ligroine as the fluid, Stalinski 28 measured the pycnometric densities of several compositions of lanthanum hydride. His results are given in Table 9.9 along with the theoretical X-ray densities calculated by using Eq. (9.3). Pycnometric densities frequently run lower than the theoretical X-ray values because of cracks or voids in the sample which cannot b e penetrated by the pycnometric fluid. In view of this, the agreement in Table 9.9 is fairly good. The only other pycnometric density reported in the literature 30 is a value of 5.83 g/cm 3 for LaH2.76· This value seems much too high when compared to the theoretical X-ray value of 5.306 g/cm 3 . (3) Thermal Expansion. By measuring the change in lattice parameter with temperature over the range 25° to 650°C, Goon 27 obtained a linear coefficient of expansion of 1.07 X 10 _5 /°C. (4) Electrical Properties. Stalinski 31 measured the electrical conductivity as a function of temperature of lanthanum hydrides of several compositions using compressed powders as samples. His results are shown in Fig. 9.10. T h e dihydride LaH 2 acts like a metal; LaH 2 . 7 also acts like a metal at lower temperatures but begins to show semiconducting behavior (increasing conductivity with temperature) at elevated temperatures. At H/La compositions of 2.86 and above, the hydride behaves like a semiconductor at all temperatures studied (80°K to room temperature). T h e reasons for the two separate activation energies of conduction at each composition and for the increase in activation energy with composition are not understood. Stalinski suggests that both electronic and ionic (protons or hydrogen anions) conduction may b e taking place. Libowitz 32 proposes that one

TABLE 9.9 DENSITIES O F LANTHANUM HYDRIDES"

Density (g/cm 3 )

La.rl2.00 LaH 2 .3o LaH 2 .63 a

Data taken from B. StalinskiJ

Pycnometric

X-Ray

5.12 5.20 5.26

5.157 5.215 5.280

9-2

SPECIFIC HYDRIDES AND DEUTERIDES

403

T LaH

75>o

3

4

5

6

o-o

7

1.98

o

8

9

o

10

11

o—o

12

13

1/T°K X 1 0 3

FIG. 9.10 Electrical conductivity of lanthanum hydrides. From B. Stalinski, Conductivity of Lanthanum Hydride LaH2-LaH3, Bull. Acad. Pol. Sei., Class III, 5: 1002 (1957).

activation energy represents the donor levels due to empty octahedral sites, or vacancies, in the band structure of LaH 3 and the other activation energy represents the band gap. The decrease in activation energy with decrease in hydrogen content (increasing vacancy concentration) may then be explained by the screening of ionized donors by conduction electrons and by decreasing band gap with vacancy concentration, respectively. Warf and Hardcastle 3 3 measured the room-temperature conductivity of a sample of LaH2.88 and obtained a value of 0.005 (ohm-cm) - 1 as compared to a value of about 0.06 (ohm-cm) - 1 interpolated from Stalinski's data. Warf and Hardcastle also found an increase in conductivity with temperature for the sample.

404

9

THE RARE-EARTH HYDRIDES

(5) Magnetic Properties. Magnetic susceptibilities of lanthanum hydrides were measured by Stalinski, 34 who found that the magnetic susceptibility of lanthanum dihydride LaH2.03 was 0.40 X 10~6 emu/g as compared to 0.68 X 10~6 emu/g for lanthanum metal. As more hydrogen was added to the dihydride, the magnetic susceptibility continued to decrease, so that at LaH2.7o the susceptibility was zero. At higher hydrogen contents the hydride became diamagnetic, reaching a susceptibility of —0.12 X 10~6 emu/g at LaH2.94. This behavior may be explained as the removal of unpaired electrons from the metal atoms to form hydrogen anions with paired electrons, or as the transfer of electrons from hydrogen atoms to the rf-band of the metal to form protons. The latter explanation requires a splitting of the d-s band of the metal such that the lower band is filled when the hydride is no longer paramagnetic. In a nuclear magnetic resonance study of some lanthanum hydrides, Schreiber and Cotts 35 observed no Knight shift, which would be caused by a nearly empty s-band, for hydrogen (a Knight shift is a shift in resonance frequency due to interaction of conduction electrons with the nuclei). They did, however, observe a shift of 0.2 to 0 . 1 % for lanthanum. They interpreted this behavior as due to transfer of electrons from hydrogen to metal to form protons. The occupancy of tetrahedral sites by hydrogen at compositions less than LaHi.94 was confirmed from measurements of the second moments (mean square line widths of the resonance lines). Occupation of the octahedral interstices commences at H/La > 1.94. Narrowing of the line widths indicated that the activation energy for diffusion of hydrogen is —25 kcal/mole at H/La < 1.95, decreases to about 10 kcal/mole at H/La = 1.95 to 2.36, and decreases still further at higher hydrogen contents. Diffusion was explained by formation of Frenkel defects (a Frenkel defect occurs when a hydrogen atom moves from one tetrahedral site to another through an octahedral site). At high hydrogen contents, when octahedral sites are already occupied, the activation energy is lowered because of Coulomb repulsion, making it easier for a tetrahedral hydrogen to be excited into the next octahedral site. 9-2.2

CERIUM

(a) Hydriding

Characteristics

Early studies of the cerium-hydrogen system were done under much the same conditions as those discussed for the lanthanumhydrogen system, i.e., on impure metal—largely on cerium misch

9-2

SPECIFIC HYDRIDES AND DEUTERIDES

405

metal. The more recent studies of Mulford and Holley, 23 Dialer and Rothe, 36 and Streck and Dialer, 37 among others, have established that cerium forms one hydride phase ranging from about C e H 1 8 1 0 1 9 to CeH 3 . Mikheeva and Kost2 reported one preparation having the formula CeH 3 1 5 . However, this appears to be inconsistent with structural considerations and implies that somehow additional hydrogen must enter the lattice after all the tetrahedral and octahedral sites are filled. Streck and Dialer 37 found that the conditions of preparation determined the properties of the hydride to some extent. Samples prepared at low temperatures (20° to 100°C) had higher dissociation pressures and lower heats of formation than those prepared at elevated temperatures (300° to 600°C) presumably due to a larger degree of disorder. Dialer and Rothe 36 reported that, of the two phases of cerium metal which coexist at room temperature, the beta phase does not react with hydrogen but that the gamma phase reacts readily to form the hydride phases. There is some confusion as to which phases they are referring to since they claim that both phases are face-centered cubic and that it is the denser one which reacts. T h e denser facecentered cubic phase, however, is usually designated as alpha and does not exist at room temperature since it cannot form38 until the temperature is below 100°K. Presumably, then, it is the face-centered cubic gamma phase (d = 6.77 g/cm 3 ) which reacts with hydrogen and the hexagonal beta phase (d = 6.66 g/cm 3 ) which does not react. A limited amount of work has been reported on the kinetics of the reaction between cerium and hydrogen. Dialer 11 studied the reaction rate at 20°C of hydrogen with cerium that had been pretreated at various temperatures. His results are shown in Fig. 9.11. The rate of reaction increases with the temperature of pretreatment of the cerium up to a pretreatment temperature of 650°C. If the pretreatment temperature is increased to 845°C, the rate of reaction decreases significantly. This decrease was explained by a decrease in the surface area of the cerium due to its fusion above 795°C. Mikheeva and Kost2'39 studied the reaction rates between cerium and hydrogen as a function of temperature. They found that at reaction temperatures above 200°C the induction periods shown in Fig. 9.11 disappeared. They also obtained the relations between reaction time (to saturation) and temperature given in Fig. 9.12 and suggested that the descending segment of the curve at low temperatures (~200°C) is associated with the formation of the trihydride, and the descending segment at high temperatures (700° to 800°C) is due to

406

9

THE RARE-EARTH HYDRIDES

250

200 Time, Minutes

F I G . 9.11 Time-concentration graphs for the cerium-hydrogen system at 20°C. Temperatures at which the cerium was pretreated are shown on the curves. Based on data from K. Dialer, Bonding in Rare Earth Hydrides, Monatsh. Chem., 79: 311 (1948).

formation of dihydride. The intermediate ascending segment corresponds to formation of nonstoichiometric compositions between CeH 2 and CeH 3 . Viallard 40 observed that the rate of reaction of hydrogen with metal is much more rapid than the rate of reaction of dihydride to form higher hydride. The induction period at low temperatures is due to formation of nuclei at surface defects and dislocations. The growth of the nuclei expands the structure leading to the formation of more active surfaces of metal and to an autocatalytic effect. The shape of the curve in Fig. 9.12 was explained by Viallard as follows: At tempera-

9-2

SPECIFIC HYDRIDES AND DEUTERIDES

407

tures below 200°C, the adsorption energy on the surface hydride that had initially formed is not large enough to cause dissociation of the hydrogen molecule, and the hydride surface is not penetrated to any appreciable degree. As the temperature approaches 200°C, hydrogen atoms are "evaporated" from the hydride phase and collide with hydrogen molecules. During the course of these collisions, the energetic requirements for dissociation of the hydrogen molecule and penetration of the hydride layer are met. At high temperatures (300° to 600°C) the evolving hydrogen begins to recombine at the surface, and also the hydrogen content of the hydride decreases; this leads to an overall decrease in rate of reaction. However, at still higher temperatures (>700°C) the thermal dissociation of the hydrogen molecules becomes sufficiently important that direct reaction between hydrogen atoms and the solid becomes appreciable and the rate increases. 60

50

40

0» +-

3 C g 30 Φ

E 20

10

0

0

200

400

600

800

1000

Temperature, °C

F I G . 9.12 Saturation time as a function of temperature for the cerium-hydrogen system. From V. I. Mikheeva and M. E. Kost, Interaction between Cerium and Hydrogen, Dokl. Akad. Nauk. SSSR, 115: 100 (1957).

408

9

(b) Dissociation

T H E RARE-EARTH

HYDRIDES

Pressures and Thermodynamic

Properties

Dissociation pressures in the cerium-hydrogen system have been obtained by Mulford and Holley, 23 Korst and Warf,24 and Streck and Dialer. 37 Expressions for the dissociation pressure in the plateau region are listed here and are plotted in Fig. 9.13; also included are the data obtained by Korst and Warf for cerium deuteride. Mulford and Holley 23 : log p H2 = 7.708 ± 0.226 -

7417 -+- 190 ^

Korst and Warf24: log pH2 = 10.630 ± 0.014 - ^ Streck and Dialer* 37 : log p H2 = 11.90 -

7 6

^

15

^jß

For C e - D (Ref. 24): log ρΌ2 = 10.205 ± 0.006 -

10 12 ±6

> ^

The results of Streck and Dialer and of Korst and Warf are in good agreement, but those of Mulford and Holley show unusually high dissociation pressures at low temperatures. Presumably, equilibrium was not reached in Mulford and Holley's investigation. The heats of formation (ΔΗ/5 kcal/mole) of CeH2-x obtained from the data of these investigators are - 4 9 . 2 (Ref. 24), - 5 4 . 9 (Ref. 37), a n d - 3 3 . 9 (Ref. 23), and the ΔΗ, of CeD 2 _ x is - 4 6 . 3 (Ref. 24). The low value of AHf obtained from the data of Mulford and Holley reflects the discrepancy shown in Fig. 9.13. The isotherms at compositions of H/Ce > 1.9 as determined by Korst and Warfare shown in Fig. 9.14. Streck and Dialer and Mulford and Holley show similar-shaped isotherms, although the equilibrium hydrogen pressures of Streck and Dialer are slightly higher than those of Korst and Warf, and the pressures of Mulford and Holley are considerably higher. The partial molal heat of solution of hydrogen in cerium hydride can be calculated from these isotherms from the slopes of In PH2 V S · 1/Γ curves at constant compositions. Values obtained by Streck and Dialer as a function of composition are shown in Fig. 9.15. The sharp drop in partial molal heat of solution at H/Ce ratios greater than two is indicative of the weaker bonding of the third hydrogen atom per cerium atom. From the data of Korst and Warf, the partial molal heat of solution in the range CeH 2 .i to CeH 2 . 2 was calculated to be 21 kcal per mole of H 2 , which is in good agreement with the values shown in Fig. 9.15 at the corresponding compositions. * Calculated from the data given in Ref. 37.

9-2 10

π~τ*

P k r

i ΓΓ t-

\

\

S P E C I F I C HYDRIDES AND D E U T E R I D E S

1

1

1

1

1

3

j

H

-|

^•N

[■

x E E

1

409

H -q

XX

^*

V

H

\

Xv >

x

x

·

u

-J

o.i b

\\

h

\ \ \

Γ~

\ \\

\ X

\

Γ~

L

Legend

L

O · a x

\~ .90

-\

%

t X

o.oi br F

o.ooi

w

Korst and W a r f (Ref. 24) Mulford and Holley (Ref. 23) Streck and Dialer (Ref. 37) C e - D system (Ref. 24)

\

\\ \\

~\

\

\\

\ \

1

1

1

1

1

1

.95

1.00

1.05

1.10

1.15

1.20

"H

\

~j \ 1

-\ 1.25

1/T°KX 10 3

F I G . 9.13 Van't HofF isochores for the plateau region of the cerium-hydrogen and cerium-deuterium systems.

The data of Hardcastle and Warf on high-pressure studies (to about 40 atm) of the cerium-hydrogen system, as shown in Fig. 9.16, are not in good agreement with the low-pressure data of other investigators. These discrepancies may be due to thermal gradients or to the presence of oxygen. The partial molal heats of solution of hydrogen in cerium hydride have also been calculated from these data and are shown in Fig. 9.15. The value at H/Ce = 2.5 is considerably higher than the value of Streck and Dialer, but the other points are reasonably close to an extrapolation of their data. A calorimetric determination 2 5 of the heat of formation of CeH2.69 yielded a value of —56.8 kcal per mole of hydride (or 42.3 kcal per

410

9

T H E RARE-EARTH

HYDRIDES

F I G . 9.14 Dissociation-pressure isotherms for the C e - H system in the hydrogenrich region. From W. L. Korst and J. C. Warf, Rare Earth-Hydrogen Systems. I. Structural and Thermodynamic Properties, Inorg. Chem., 5(10): 1722 (1966).

mole of H 2 ). Using the average value (between the data of Korst and Warfand of Streck and Dialer) of—52 kcal per mole of H 2 for the heat of formation of CeHi. 9 and —20 kcal per mole of H 2 for the average partial molal enthalpy of solution between C e H 1 9 and CeH 2 6 9 , we can calculate a value of —57 kcal/mole for the heat of formation of CeH 2 6 9 ; this value is in excellent agreement with the calorimetric value. The specific heat of CeH 2 was measured from 20° to 360°K by Stalinski and Bieganski. 41 Their values are shown in Table 9.10 along with the absolute thermodynamic functions calculated therefrom. There were no anomalies indicative of a phase transformation apparent in the Cp vs. T curves.

9-2

(c) Physical

SPECIFIC HYDRIDES AND DEUTERIDES

411

Properties

(1) Structure and Lattice Parameters. The structure of the cerium hydrides is the same as that of the lanthanum hydrides—fluorite structure at CeH 2 with all the octahedral sites filled at CeH 3 . Lattice parameters as a function of hydrogen content are shown in Fig. 9.17. The straight line in the figure represents the formula a0 = 5.654 — 0.040 (H/Ce), given by Korst and Warf.24 Ayphassorho 42 reports a CeH 2 phase (prepared by a 30-hr anneal at 350°C) which has a lattice parameter of 5.645 A. He explains this new phase as being created by partial redistribution of hydrogen atoms into octahedral sites. Ayphassorho 43 claimed to have prepared, by rapid desorption of normal C e H 2 4 , a hydride CeH 0 . 7 of lattice parameter equal to 5.04 A in which all the hydrogen atoms are in octahedral sites. The lattice parameter of cerium dideuteride as measured by Korst and Warf24 is 5.662 ± 0.003 A. -38

—

1

1

1

1

1

1

1

1

1

1

1

-36 -34 -32

■f

-'Μ)

Φ

o

^

-28

Jtf

-26

σ u

>>

Q. O

c 111 O

Streck and Dialer (Ref. 37)

h-

\

Hardcastle and Warf (Ref. 26)

|

-24 -22

$

-20

"5 t

-1R

£

l·

-16

\-

sp

-14 -12

I □ΙΊ

LJ 1.9

1

1

2.0

2.1

1

1

1

1

1

1

1

2.2 2.3

1

2.4

2.5

2.6

2.7

2.8

2.9

I

H/Ce Atom Ratio FlG. 9.15 Partial molal heats of solution of hydrogen in cerium hydride.

412

9

THE RARE-EARTH HYDRIDES

~i—i—i—

Legend —O— Hardcastle and - O - S t r e c k & Dialer - ^ ^ - H o i ley et al. --•--Korst & Warf

40 30

20

10

£ o

5

2

D

I i 7 η i< ,-/ i 2

1.0

0.5

0.2

ι 2.0

M i Γι

/

/

f

£Α L^L

st I

s:

i

ΛΊ

2.5

3.0

H/Ce Atom Ratio

F I G . 9.16 Isotherms for the cerium-hydrogen system. From K. I. Hardcastle and J. C. Warf, Rare Earth-Hydrogen Systems. III. High Pressure Investigations, Inorg. Chem., 5(10): 1731 (1966).

(2) Density. Sieverts and Gotta 25 measured the density of CeH2.69 using petroleum ether as the pycnometer fluid and obtained a value of 5.55 g/cm 3 . The X-ray value for this composition, calculated from the data in Fig. 9.17, is 5.559 g/cm 3 , which is excellent agreement with the experimental value. Korst and Gol'der 4 4 measured the densities of a series of samples from CeH0.2 to CeH 3 using argon as the pycnometric medium. A minimum density (5.4 g/cm3) was observed near the composition CeH 2 . At CeH 3 the density had increased monotonically to 5.5 g/cm 3 . The X-ray densities at these two compositions are 5.45 and 5.61 g/cm 3 , respectively.

9-2

S P E C I F I C HYDRIDES AND D E U T E R I D E S

413

TABLE 9.10 H E A T C A P A C I T Y AND T H E R M O D Y N A M I C F U N C T I O N S O F C E R I U M

Temperature (°K)

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 273.15 298.15 a

DIHYDRIDE"

Cp (cal/°K/mole)

H° - H°0 cal/mole

S° (cal/°K/mole)

-(F°-H°0)/T (cal/°K/mole)

0.85 1.76 2.63 3.45 4.23 4.75 5.11 5.38 5.59 5.75 5.90 6.05 6.20 6.35 6.50 6.68 6.86 7.05 7.26 7.46 7.68 7.90 8.11 8.38 8.64 8.95 9.25 9.55 9.83 10.10 10.35 10.58 10.79 10.99 11.17 9.05 9.78

6.65 19.79 41.63 72.06 110.4 155.3 204.6 257.1 311.9 368.6 426.8 486.6 547.8 610.6 674.8 740.7 808.3 877.9 949.4 1023 1099 1177 1257 1339 1424 1512 1603 1697 1794 1894 1996 2100 2207 2316 2427 1540 1776

0.59 1.11 1.73 2.41 3.10 3.79 4.45 5.07 5.65 6.19 6.70 7.18 7.63 8.06 8.48 8.87 9.26 9.64 10.01 10.37 10.72 11.06 11.41 11.74 12.08 12.41 12.74 13.07 13.40 13.72 14.05 14.37 14.69 15.00 15.31 12.51 13.33

0.26 0.45 0.69 0.96 1.26 1.57 1.89 2.22 2.53 2.84 3.14 3.43 3.72 3.99 4.26 4.51 4.77 5.02 5.26 5.49 5.72 5.95 6.17 6.39 6.60 6.81 7.01 7.21 7.42 7.61 7.81 8.00 8.20 8.39 8.57 6.87 7.38

From B. Stalinski and Z. Bieganski, Low Temperature Heat Capacity of Cerium Dihydride. Crystal Field Effects, Bull. Acad. Pol. Sei., Ser. Sei. Chim., 7: 332 (1964).

414

9

T H E R A R E - E A R T H HYDRIDES

(3) Electrical Properties. Daou 45 measured the resistances of cerium hydride samples between cerium metal and CeH 2 at 500°C. After a slight initial increase in resistance due to the solubility of hydrogen in the metal phase, there was a linear decrease in resistance (through the two-phase metal-plus-hydride region) until a composition of approximately C e H 1 7 was reached. The resistance at this composition was about 70% that of the pure metal (p ~ 0.5 X 10~4 ohm-cm). At hydrogen contents greater than H/Ce = 1 . 7 , there was a sharp rise in resistance. The results of Stalinski's 46 measurement of the conductivities of compressed powders of cerium hydrides ranging in composition from CeHi.98 to CeH2.86 and in the temperature range 83° to 298°K are shown in Fig. 9.18. At compositions up to CeH 2 . 5 , the hydride has metallic characteristics. At CeH 2 . 56 there is a slight increase in conductivity with temperature near room temperature which is indicative of semiconducting behavior. At CeH 2 8 6 , as in lanthanum hydride, there is complete semiconducting behavior with two activation energies. Daou's 4 7 measurement of resistivity as a function of Ί

1

Γ

O • X —

Legend Korst (Ref. 22) Holley et al. (Ref. 7) Hardcastle and Warf (Ref. 26) Korst and Warf (Ref. 24)

< Φ Φ

E S σ

5.532

1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3.0

H/Ce Atom Ratio

F I G . 9.17 Lattice parameters of cerium hydride as a function of hydrogen content.

9-2

SPECIFIC HYDRIDES AND DEUTERIDES

415

1/T°K X 1 0 3

FlG. 9.18 The temperature dependence of electric conductivity for the ceriumhydride phase. From B. Stalinski, Magnetic Properties of Cerium and of the C e r i u m Hydrogen System. Electric Conductivity of Cerium Hydride, Bull. Acad. Pol. Sei., Ser. Sei. Chim., 7: 273 (1959). 7000

Γ

J

x

k

kxK

Legend D 750°C A 700°C x 650°C Δ 600°C o 550°C • 23°C

6000 h-

5000

E

.>

l· ^ L 3000 hu 4000 l·-

V\

H

\ \

L 1000 u \2.0

J

-\

2000 \—

\ x

1

A j

h U

-\

1

2.2

i

1

2.4

^ \

. X 2.6

-J -\ 2.8

H/Ce Atom Ratio

F I G . 9.19 Conductivity of cerium hydride as a function of composition. From R. C. Heckman, Electrical Properties of the C e r i u m - and Gadolinum-Hydrogen Systems, J. Chem. Phys., 40: 2958 (1964).

416

9

THE RARE-EARTH HYDRIDES

temperature in the range —200° to + 500°C for a sample of C e H 1 9 showed that, as expected, the resistivity was metallic. However, the room-temperature resistivity of 4 X 10" 6 ohm-cm was several orders of magnitude less than Stalinski's value of 0.1 ohm-cm. More recently, Heckman 4 8 has measured the conductivities of hydrided bars of cerium in the composition range H/Ce = 0 to 2.5 and the temperature range room temperature to 750°C. His results, like Daou's results, showed an approximate linear increase in conductivity from cerium metal (p = 8 X 10" 5 ohm-cm) to CeHi. 6 (p = 4 X 10~5 ohm-cm). At compositions greater than CeH 2 , the conductivity increased as shown in Fig. 9.19. Heckman's room-temperature value near CeH 2 . 0 falls closer to Daou's than to Stalinski's value, and his conductivities at higher compositions run about two orders of magnitude higher than those of Stalinski. This is probably due to intergranular resistances in Stalinski's samples. Heckman also measured Seebeck coefficients and obtained an average value of—17 μν/Χ! over the range CeH 2 .i to CeH 2 . 37 . The negative value indicates that the current carriers were electrons. (4) Magnetic Properties. The room-temperature magnetic susceptibility 46 of cerium hydride decreases only slightly with hydrogen content, varying from 18.2 X 10 - 6 emu/g at C e H 1 9 8 to 16.8 X 10~6 emu/g at CeH 2 9 2 . Cerium metal has a room-temperature magnetic susceptibility of about 18 emu/g. The temperature dependence of the magnetic susceptibility followed the Curie-Weiss law X

T

C _0

where the constants C and Θ varied with composition. At C e H 1 9 8 the Weiss constant, 0, was 16°K and the Curie constant, C, was 5.46 X 10" 3 emu/g. 9-2.3

PRASEODYMIUM

The early work on the praseodymium-hydrogen system done by Muthmann and Beck 49 and Sieverts and Roell 50 and summarized by Mikheeva and Kost2 is subject to some question because of probable impurity contamination. (a) (a) Dissociation Dissociation Pressures Pressures and and Thermodynamic Thermodynamic Properties Properties Pressure-composition-temperature data on this system have been obtained by Mulford and Holley 23 and Korst and Warf.24 Their isotherms are given in Figs. 9.20 and 9.21. The agreement in the plateau

9-2

S P E C I F I C H Y D R I D E S AND D E U T E R I D E S

10

Ί 800°C(1)

Ty^g^fto.

n

Q g

800°C (2) n

417

Γ

o-o-

750°C (1) 750°C (2)

$-5

0.1

700°C(1) °σ—OTT -e—Û-

700°C (2)

O To

650°C 1 r> Τ3Ό

_8r

Ü ft O

Legend (1) Data of Mulford and Holley (Ref. 23) (2) Data of Warf and Korst (Ref. 24) 600°C (1)

0.01

o —o u o o600°C (2)

f 0.2

0.4

-L

0.6

0.8

_L 1.0

1.2

1.4

y

.ΔΑ.

1.6

J

1.8

2.0

H/Pr Atom Ratio

FlG. 9.20 Isotherms of the praseodymium-hydrogen system in the range 0 < (H/Pr) < 2.

region is better than in the single-phase hydride region. Graphs of log pH2 v s · 1/^ of the plateau region are shown in Fig. 9.22, and the corresponding equations are as follows: Korst and WarP 4 : log p H2 = (10.526 ± 0.005) Mulford and Holley 23 : log p H2 = (10.229 ± 0.048) -

10 87Q ±

'

5

T 10,446 : 46

The heats of formation as calculated from these equations are —49.7 and —47.8 kcal per mole of H 2 , respectively. A calorimetric value of —39.5 kcal per mole of H 2 has been reported by Sieverts and Gotta 25 for PrH2.84· (h) Physical

Properties

(1) Structure and Lattice Parameters. The lattice parameters of praseodymium hydrides as given by Korst,22 Holley et al.,7 and

418

9 600

THE RARE-EARTH HYDRIDES

Ί

Γ

S

500

1

I

I

I

I

2.4

2.5

2.6

2.7

2.8

Legend > - Mulford and Holley (Ref. 23) ►- Korst and Warf (Ref. 24)

550

450 400 σ> X E 350 E i 300 D

I

250 200 150 100 50 0 1.8

1.9

2.0

2.1

2.2

2.3

2.9

H/Pr Atom Ratiosystem at high hydrogen contents. F I G . 9.21 Isotherms of the praseodymium-hydrogen FIG. 9.21 Isotherms of the praseodymium-hydrogen system at high hydrogen contents.

Pebler and Wallace 51 are shown in Fig. 9.23. In this case the equation of Korst and Warf24 for lattice parameter as a function of hydrogen content (a0 = 5.566 — 0.27r), which is represented by the dashed line in Fig. 9.23, does not give good agreement with the bulk of the data. A more accurate expression, represented by the solid line in the figure, would be a0 = 5.613 — 0.048r, which gives a value of 5.469 A for PrH 3 , rather than the value shown in Table 9.5. (2) Density. The pycnometric density of PrH2.84 was found 25 to be 5.56 g/cm 3 ; this is considerably lower than the corresponding X-ray density of 5.813 g/cm 3 . The X-ray densities at PrH 2 and PrH 3 are 5.651 and 5.842 g/cm 3 , respectively. (3) Electrical Properties. Resistivity measurements by Daou 47 ' 52 in the composition range praseodymium metal to PrH 2 indicated that the electrical properties of this hydride are similar to those of lanthanum and cerium hydrides. As hydrogen is added to praseodymium, the resistance decreases linearly, after a slight initial increase, to about 62% of the resistance of pure metal (i.e., to about 0.4 X 10~4 ohm-cm) at PrH 1 8 6 , after which it rises sharply. The increase in resistivity with temperature indicated metallic behavior.

9-2

SPECIFIC HYDRIDES AND DEUTERIDES 10

419

H III1 II1 I| 1 II I| III I1 I II I| III H

0.1

i i i i I i i i i I i i i i I i i i i I i i i nJ i i i i

0.01 0.90

0.95

1.00

1.05

1.10

1.15

1.20

1/T°K X 10 3 F I G . 9.22 Van't Hoff isochore for the praseodymium-hydrogen system. 5.520

1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

H/Pr Atom Ratio

F I G . 9.23 Lattice parameters of praseodymium hydride.

3.0

420

9

THE RARE-EARTH HYDRIDES

3.00

2.00 ho

'■C

σ

Ct

E o

< ■D

z

1.00

h

200

400

600

800

1000

Time, sec

FlG. 9.24 Rate of absorption of hydrogen by neodymium ~29°C and a pressure of — 680 torr. From W. L. Korst, Studies of the Rare Earth Hydrides, Ph.D. Thesis, p. 69, University of Southern California, 1956.

(4) Magnetic Properties. Magnetic studies by Kubota and Wallace 9 show that the dihydride of praseodymium, PrH 2 . 0 3, has a much higher susceptibility (1.43 X 10~4 emu/g at room temperature) than praseodymium metal (0.345 X 10~4 emu/g). The room-temperature susceptibility decreases with further addition of hydrogen, reaching a value of 0.331 emu/g at PrH 2 . 72 . The dihydride did not obey the C u r i e Weiss law, and the trihydride obeyed this law only at temperatures above 100°K. Below this temperature the variation of susceptibility with temperature is anomalous. The Curie and Weiss constants for PrH 2 . 72 are 11.1 X 10~3 emu/g and —34°K, respectively. No magnetic ordering was observed down to 4°K. 9-2.4

NEODYMIUM

(a) Hydriding

Characteristics

Early work on the neodymium-hydrogen system by Kellenberger and Kraft,53 Muthmann and Beck, 49 and Sieverts and Roell 50 indicated that it bears a strong similarity to the lanthanum-, cerium-, and

9-2

S P E C I F I C HYDRIDES AND D E U T E R I D E S

421

praseodymium-hydrogen systems. The lower limit of the dihydride phase is about N d H 1 9 1 as determined by X-ray studies. 51 The only reported data on the kinetics of hydriding of neodymium are those of Korst22 shown in Fig. 9.24.

(b) (b) Dissociation Dissociation Pressures Pressures and and Thermodynamic Thermodynamic Properties Properties The pressure-composition isotherms of Mulford and Holley 23 and Korst and Warf24 are shown in Figs. 9.25 and 9.26. The two 700°C isotherms of Korst and Warf were on two different samples. Again the agreement between the two investigations was good in the plateau region but not in the single-phase hydride region.

839°C (2) 800°C (2)y

-o

\-

u o-

J

- O - * 800°C(1) 798°C (2)

<*> 750°C (2)

0.2

J_ 0.4

_L 0.6

_L 0.8

Legend (1) Data of Mulford and Holley (Ref. 23) (2) Data of Warf and Korst (Ref. 24)

J 1.0

I 1.2

I 1.4

I 1.6

I 1.8

2.0

H/Nd Atom Ratio FIG.

9.25 Low-pressure isotherms of the neodymium-hydrogen system.

422

9

T H E RARE-EARTH

HYDRIDES

750

Legend

~~\

Mulford and Holley (Ref. 23) Korst and Warf (Ref. 24H

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

H/Nd Atom Ratio

F I G . 9.26 Isotherms for the neodymium-hydrogen system at high hydrogen contents.

Dissociation pressures in the plateau region are given by the following two equations which were derived from the data in Fig. 9.27: Korst and Warf24: log p H2 = (10.482 ± 0.028) Mulford and Holley 23 : log pm = (9.370 ± 0.179) -

11,031 ± 28 T 9796 ± 171

The heats of formation derived from these equations are —50.5 and —44.8 kcal per mole of H 2 , respectively. (c) Physical Properties (1) Structure and Lattice Parameters. The lattice parameters as a function of hydrogen content are given in Fig. 9.28. As in the praseodymium-hydrogen system, the equation for lattice parameter as a

9-2

SPECIFIC HYDRIDES AND DEUTERIDES

423

function of hydrogen content given by Korst and Warf24 (a0 = 5.52 — 0.033r), which is represented by the dashed line in Fig. 9.28, does not fit the bulk of the data. The solid line in the figure, which corresponds to the equation a0 = 5.551 — 0.044r, is a more nearly accurate expression. According to this relation the lattice parameters of NdH 2 and NdH 3 are 5.463 and 5.419 A, respectively. (2) Density. The X-ray densities of NdH 2 and N d H 3 as computed from the data in Fig. 9.28 are 5.96 and 6.14 g/cm 3 , respectively. (3) Electrical and Magnetic Properties. Electrical conductivities of neodymium hydride have b e e n reported by Heckman and Hills 54 in the range neodymium metal to NdH 2 . As in the previously mentioned hydrides, the conductivity increased in the two-phase (metal hydride) region, reaching a maximum value of 31,000 o h m - 1 c m - 1 (p = 32 jLtohm-cm) at N d H 1 9 . Magnetic susceptibilities of neodymium hydride as measured by Kubota and Wallace 9 are given in Table 9.11. The Curie-Weiss law is obeyed above 70°K, and the Weiss constants are also shown in

10

I I I I I I I I I I M I 1I M I I 1I I I1 I I I I I

S^Mulford and Holley (Ref. 23)

0.1 F

0.01

Korst and Warf (Ref. 2 4 ) \

\

I I I I I I I I I I 1 I I I I I I I I 1 I I I I hl I I

0.90

0.95

1.00

1.05

1/T°K X 10

1.10

1.15

1.20

3

FIG. 9.27 Van't Hoff isochores for the neodymium-hydrogen system.

9

424

T

Ί

D.H / Z

5.468

<

\

5.464

-

5.460

-

I

1

1

1

1

1

1

Γ

Legend O Pebler and Wallace (Ref. 51) D Korst (Ref. 22) Δ Holley et al. (Ref. 7)

D

N/f

OA

XV

5 452 —

\

X°V

^

5.448 —

Φ

Γ

-\

H -\

5.456

J \

°\

π

—

·£ 5.444 s 5.440

1

o\

1E ·

o_

T H E RARE-EARTH HYDRIDES

Ν

—\ -\

\

\ \ \

—

5.436

-j

5.432

-\

5.428

— 1.8

J

1.9

L_ J

2.0

2.1

1 2.2

L_ J 2.3

2.4

1

1

1

2.5

2.6

2.7

1 M 2.8

2.9

3.0

H/Nd Atom Ratio

F I G . 9.28 Lattice parameters of neodymium hydride.

Table 9.11. The dihydride becomes ferromagnetic at temperatures below 9.5°K, but the trihydride (NdH 2 . 6 ) does not. 9-2.5

SAMARIUM

(a) Hydriding

Characteristics

There have been only few studies of the samarium-hydrogen system. The early work of Muthmann and Bauer 55 must be considered as qualitative. Samarium is the first of the rare-earth metals which does not form a trihydride by filling of the octahedral sites in the fluorite structure. Instead, at an overall H/Sm ratio of 2.55, a hexagonal phase is formed 51 which has a homogeneity range of SmH2.59 to SmH 3 . The homogeneity range of the dihydride is S u i H ^ to SmH2.55. (b) Dissociation

Pressures and Thermodynamic

56

Properties

Mulford reported that the dissociation pressure of samarium dihydride can be represented by the equation log p H2 = 11.4

11,700

9-2

SPECIFIC HYDRIDES AND DEUTERIDES

425

TABLE 9.11 MAGNETIC SUSCEPTIBILITIES O F NEODYMIUM HYDRIDES0

Magnetic susceptibility X 104 emu/g

Nd metal NdH 2 . 02 NdH 2 . 03 NdH 2 . eo NdH275

78°K

300°K

Weiss constant (°K)

1.22 0.833 0.877 0.812 0.847

0.303 0.345 0.296 0.311 0.304

-30 -36 -39 -39

a From Y. Kubota and W. E. Wallace, Magnetic Characteristics of Praseodymium, Neodymium, and Samarium H y d r i d e s , / . Appl. Phys., 34: 1348 (1963).

which gives a value of —53.3 kcal per mole of H 2 for the heat of formation. Korst22 obtained one isotherm at 600°C for H/Sm values greater than 2. This is shown in Fig. 9.29. (c) Physical

Properties

(1) Structure and Lattice Parameters. The lattice parameters of the cubic dihydride phase of samarium are shown in Fig. 9.30. As in 500

1.80

1.90

2.00 D/Sm Atom Ratio

2.10

2.20

F I G . 9.29 Isotherm for the samarium-deuterium system at 600°C. Data taken from W. L. Korst and J. C. Warf, Rare Earth-Hydrogen Systems. I. Structural and Thermodynamic Properties, Inorg. Chem., 5(10): 1723 (1966).

426

9

THE R A R E - E A R T H HYDRIDES

5.380

5.376

1

I

I

I

I

I

1

Γ

I

L

Legend D Holley et al. (Ref. 7) O Pebler and Wallace (Ref. 51)

h

5.372 h-

5.368

h

5.364

3

5.360

5.356

5.352

J

I

I

I

I

ILJ

I

1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0

3.1

H/Sm Atom Ratio

FlG. 9.30 Lattice parameters of cubic samarium hydride.

previous cases, the lattice parameter decreases with increasing hydrogen content. However, at H/Sm = 2.55 the new hexagonal phase appears. The lattice parameters of the hexagonal phase 5 1 are a0 = 3.782 A and c0 = 6.779 A on the basis of hexagonal close packing. If the structure is the same as hexagonal holmium trihydride, discussed earlier, then a0 is 6.551 A TABLE 9.12 M A G N E T I C S U S C E P T I B I L I T I E S O F SAMARIUM H Y D R I D E S "

Magnetic susceptibility X 104 emu/g

S m metal SmH1>99 SmH 2 .oi SmH2.28 SmH 2 .9i

4.2°K

78°K

300°K

0.100 0.236 0.493 0.414 0.492

0.090 0.092 0.096 0.082 0.096

0.074 0.068 0.072 0.060 0.060

a Y. Kubota and W. E. Wallace, Magnetic Characteristics of Praseodymium, Neodymium, and Samarium H y d r i d e s , / . Appl. Phys., 34: 1348 (1963).

9-2

SPECIFIC HYDRIDES AND DEUTERIDES

427

(2) Densities. The X-ray density of SmH 2 is 6.51 g/cm 3 . This increases to 6.64 g/cm 3 at SmH 2 . 55 . The density of the trihydride is 6.06 g/cm 3 . (3) Magnetic Properties. Magnetic susceptibilities of samarium hydrides are shown in Table 9.12. The Curie-Weiss Law was not obeyed for any of the hydride compositions, nor was any magnetic ordering observed. 9-2.6 9-2.6

EUROPIUM EUROPIUM

There have been no dissociation pressure or thermodynamic studies of the europium-hydrogen system. A europium deuteride, EuD L95 ^ was prepared by Korst and Warf,57 and a composition EuHi. 8 6 was prepared by Zanowick and Wallace. 58 Attempts to prepare a higher hydride of europium at pressures of 33 to 61 atm of hydrogen and temperatures ranging from 300° to 500°C were unsuccessful. 26 The structure of the dihydride is orthorhombic with lattice parameters as given in Table 9.5. T h e structure is similar to that of the alkaline-earth dihydrides. The magnetic properties of EuHi. 8 6 were studied by Zanowick and Wallace. 58 The susceptibility of the hydride is 1.56 X 10" 4 emu/g. The Curie-Weiss law is obeyed at temperatures above 30°K with a Weiss constant of 25°K and a Curie constant of 42.9 X 10" 3 emu/g. The hydride becomes ferromagnetic at temperatures below 24°K. The magnetic moment of the hydride is approximately 6 Bohr magnetons, which is close to the value for Eu 2 + ions. This is to be expected on the basis of the divalent characteristics of europium and ytterbium hydrides as discussed in the introductory sections of this chapter. 9-2.7

GADOLINIUM

(a) Hydriding (a) Hydriding Characteristics Characteristics The earliest work reported on the gadolinium-hydrogen system is that of Viallard, 59,60 who claimed that Gd 2 H 3 is formed when gadolinium is heated in hydrogen to about 220°C. When this compound is cooled, it transforms to GdH 2 . Viallard's work is seriously questioned by Sturdy and Mulford, 61 who obtained the cubic fluorite type dihydride with a homogeneity range of about G d H 1 8 to GdH 2 . 3 and a hexagonal hydride, GdH2.85 to GdH 3 , similar to the samarium-hydrogen system. It is likely that the metal and the hydrogen used by Sturdy and Mulford were of higher purity than those used by Viallard. Moreover, the data on other hydride systems do not indicate the likelihood of a Gd 2 H 3 phase as reported by Viallard.

428

9

T H E R A R E - E A R T H HYDRIDES

10

800°C

-c—o—

1b
750°C

σ-ο—Θ-

700°C

-σ—ö^ 0.1

75

650°C °Π3—O-

Q

600°C -o I o

0.01

0

n

l.O H/Gd Atom Ratio

F I G . 9.31 Pressure-composition isotherms of the gadolinium-hydrogen system at H/Gd < 2. From G. E. Sturdy and R. N. R. Mulford, The Gadolinium-Hydrogen System,/. Amer. Chem. Soc, 78: 1084 (1956).

(b) Dissociation

Pressures and Thermodynamic

Properties

Since this system forms two hydrides, there are two constant pressure plateaus in the isotherms—one for the Gd H- GdH 2 -s two-phase region, illustrated in Fig. 9.31, and the other for the GdH2+ô + GdH 3 _ ô two-phase region, illustrated in Fig. 9.32. The van't Hoff isochore for the plateau regions in Fig. 9.31 is shown in Fig. 9.33. The equation for the isochore is log PH2=

(9.72 ± 0 . 1 5 )

10,250 ± 140

which yields a value of —46.9 kcal per mole of H 2 for the heat of formation of nonstoichiometric gadolinium dihydride. (c) Physical

Properties

(1) Structure, Lattice Parameters, and Densities. The lattice parameter of the cubic dihydride (fluorite structure) is 5.303 A, which gives an X-ray density of 7.08 g/cm 3 . This is in good agreement with a more

9-2

SPECIFIC HYDRIDES AND DEUTERIDES

429

recent value of 5.297 A for a0.62 The hexagonal hydride has the lattice parameter a0 = 3.73 A and c0 = 6.71 A on the basis of hexagonal close packing. For the holmium trihydride structure, a0 would be 6.46 A. The X-ray density of the trihydride is 6.58 g/cm 3 . (2) Electrical Properties. The electrical conductivity of the gadolinium-hydrogen system from gadolinium metal to GdH 2 . 3 was measured by Heckman. 4 8 As for the other rare-earth metals, the conductivity increased linearly in the two-phase metal-plus-dihydride region reaching a maximum value of about 27,500 ohm _ 1 cm _ 1 (p = 36 μοηιη-cm) at GdHi. 65 . Increase of hydrogen content beyond this point led to a sharp decrease in conductivity. Heckman's results at compositions greater than GdH 2 are shown in Fig. 9.34. The conductivity was metallic over all compositions measured; however, extrapolation of the curves indicates that the metallic conduction vanishes at about H/Gd = 2.3. Since this is the composition at which the hexagonal trihydride appears, Heckman postulated that the ap300

250

200

σ>

x E E 3

«Λ «Λ

2

Q.

100

50

0

1.8

2.0

2.2

2.4

2.6

2.8

3.0

H/Gd Atom Ratio

FlG. 9.32 Pressure-composition isotherms of the gadolinium-hydrogen system at H/Gd > 2. From G. E. Sturdy and R. N. R. Mulford, T h e Gadolinium-Hydrogen System,/. Amer. Chem. Soc, 78: 1087 (1956).

430

9

THE RARE-EARTH HYDRIDES

1.05 1/T°K X 1 0

1.10

1.15

1.20

3

F I G . 9.33 Van't Hoff isochore for gadolinium dihydride. From G. E. Sturdy and R. N. R. Mulford, The Gadolinium-Hydrogen System, / . Amer. Chem. Soc, 78: 1085 (1956).

pearance of the new phase is associated with the absence of free electrons. Seebeck coefficients were negative in this system, changing from - 9 /xv/°C at GdH 2 . 0 to - 6 μν/°ϋ at GdH 2 . 12 . (3) Magnetic Properties. The magnetic susceptibilities of gadolinium dihydride and trihydride were found 10 to be 1.56 X 10~4 and 1.36 X 10~4 emu/g, respectively. Both hydrides followed the Curie-Weiss law, the trihydride down to 9°K and the dihydride down to only 55°K since it becomes antiferromagnetic at 21°K. Wallace et al.10 obtained a Weiss constant, 0, of 3°K and a Curie constant, C, of 0.0463 emu/g for the dihydride. Trombe 6 3 found the slightly different values of Θ = 11°K and C = 0.480 emu/g for these constants. For the dideuteride he obtained Θ = -6.5°K and C = 0.0470 emu/g. For the trihydride Wallace et al. found the values Θ = — 3°K and C = 0.0412 emu/g. The effective magnetic moments of the dihydride and the trihy-

9-2

S P E C I F I C HYDRIDES AND D E U T E R I D E S

431

dride were 7.7 and 7.3 Bohr magnetons, respectively. This value is in agreement with seven unpaired electrons in the 4 / shell, corroborating that it is only the 5d and 6s electrons which take part in bonding. 9-2.8

T TERBIUM ERBIUM

Terbium forms two hydrides with homogeneity ranges TbH 1 9 o to TbH 2 .i 5 and TbH 2 . 8 to TbH 3 . Lattice parameters are given in Table 9.5. On the basis of the HoH 3 structure, a0 should be 6.409 A. The X-ray densities for dihydride and trihydride are 7.40 and 6.81 g/cm 3 , respectively. The only other available data on terbium hydrides are the magnetic studies of Wallace et al.10 All values measured fpr the magnetic susceptibilities of ten compositions between T b H 1 9 5 and TbH 2 . 97 fell between 2.15 and 2.20 X 10" 4 emu/g, except for the composition TbH2.2i which exhibited a susceptibility of 2.07 emu/g. The hydrides also obeyed the Curie-Weiss law over most of the temperature range studied. Weiss constants were generally negative (—13° to —2°K) 8000

7000

6000 I 5000 I

E £ 4000

*>

"g 3000 o υ 2000

1000

0 2.0

2.1

2.2

2.3

H/Gd Atom Ratio F I G . 9.34 Conductivity of gadolinium dihydride vs. composition as measured by Heckman. 48

9

432

THE RARE-EARTH HYDRIDES

over the composition range investigated, except for TbH2.o8> which had a Weiss constant of+7°K. Curie constants were about 0.067 emu/g. For compositions TbHi. 98 to TbH 2 .i 0 , the samples became antiferromagnetic with a Néel temperature of about 45°K. No magnetic ordering was observed at higher hydrogen contents. A neutron investigation 64 of TbD 2 showed that the antiferromagnetic ordering below 40°K was due to antiparallel coupling of adjacent ferromagnetic (0 0 1) layers. From the shape of the l/χ vs. temperature curve, there was an indication that at the composition TbH2.08 the ordering changed from antiferromagnetic to ferromagnetic below 20°K. The positive Weiss constant was taken as additional evidence for this change. 9-2.9 9-2.9

DYSPROSIUM

From the X-ray studies of Pebler and Wallace, 51 it has been established that dysprosium forms a cubic dihydride ranging from DyHi.94 to DyH 2 0 8 and a hexagonal trihydride ranging from D y H 2 6 8 to DyH 3 . The lattice parameter of the dihydride is 5.201 A, and the trihydride lattice parameters are a0 = 3.671 Â (6.358 A for the HoH 3 structure) and c0 = 6.560 A. The X-ray densities are 7.76 g/cm 3 for the dihydride and 7.12 g/cm 3 for the trihydride. The room-temperature magnetic susceptibility 9 of both the dihydride and trihydride of dysprosium is about 2.8 X 10~4 emu/g. The Curie-Weiss law is obeyed down to 10°K; the constants were Θ = -16°K and C = 0.0897 emu/g for the dihydride DyH 2 0 o and θ = - 7 ° K and C = 0.0871 for the trihydride DyH 2 . 92 . The dihydride becomes antiferromagnetic below about 8°K. 9-2.10

HOLMIUM

(a) Hydriding

Characteristics

The X-ray work of Pebler and Wallace 51 established that holmium forms a cubic dihydride ranging from HoH 1 9 5 to HoH 2 . 24 and a hexagonal trihydride from HoH 2 6 4 to HoH 3 . (b) Structures,

Lattice Parameters, and

Densities

The lattice parameter of the fluorite type cubic dihydride phase is given in Table 9.5. In a recent neutron-diffraction study of the hexagonal trideuteride of holmium, 17 it was shown that, although the holmium atoms form a hexagonal close-packed structure, the deuterium atoms are situated such that the unit cell is three times larger than the hexagonal close-packed unit cell. The space group is

9-2

SPECIFIC HYDRIDES AND DEUTERIDES

433

P3cl(Djd) with 6 holmium and 18 deuterium atoms per unit cell as follows: 6 holmium at ± ( t 0 i, 0 H , H i) 12 deuterium at (g) : ±(x yz, yx — yz, y — xxz, y x i+ z, x x — y i+ z, y — x y i + z) where x0 = 0.356, y9 = 0.028, and za = 0.096 4 deuterium at (d) : ± ( H z , H i + z ) with %d = 0.167 2 deuterium a t ± ( 0 0 i ) The structure may be looked upon as a hexagonal close-packed metal structure with hydrogen atoms situated in positions which are slightly displaced from the normal tetrahedral and octahedral interstices in the hexagonal close-packed structure. The c0 parameter remains the same as in the hexagonal close-packed structure, but the a0 parameter is larger by a factor of VS. This makes a0 equal to 6.308 A rather than the value shown in Table 9.5. It is probable that the other hexagonal trihydrides have the same structure. The X-ray densities are 8.04 g/cm 3 for the dihydride and 7.40 g/cm 3 for the trihydride. (c) Electrical

Properties

As in the other rare-earth hydrides, the room-temperature conductivity 54 of holmium hydride increases from H/M = 0 to a maximum value of 40,000 ohm _ 1 cm _ 1 (p = 25 /xohm-cm) at H o H 1 9 . (d) Magnetic

Properties

Room-temperature magnetic susceptibilities of holmium hydrides 9 are reasonably constant at about 2.4 ± 0.1 X 10~4 emu/g in the composition range HoH 1>60 to HoH 3 . The Curie-Weiss law is followed down to temperatures of 12°K with Θ = - 5 ° K and C = 0.0725 emu/g for the dihydride and Θ = - 8 ° K and C = 0.0754 emu/g for the trihydride. The effective magnetic moment remains constant at about 10 Bohr magnetons over the whole composition range including pure metal. The dihydride becomes antiferromagnetic at 8°K. 9-2.11

ERBIUM

The homogeneity ranges of the erbium hydrides are ErHi. 95 to ErH2.3i and ErH2.82 to ErH 3 . Lattice parameters are given in Table 9.5. On the basis of the HoH 3 structure, a0 should be 6.272 A. The X-ray densities of dihydride and trihydride are 8.36 and 7.63 g/cm 3 , respectively. Mulford 56 reported the following relation for the dissociation of pressure of nonstoichiometric erbium dihydride:

434

9

THE RARE-EARTH HYDRIDES

log

PH2

= 11.0 -

11,900

which gives a heat of formation of—54.2 kcal per mole of H 2 . The maximum in electrical conductivity 54 occurs at E r H 1 8 with a value of 38,000 ohm _ 1 cm _ 1 (p = 26 μοηηι-cm). Kubota and Wallace 9 reported that the conductivity falls by about five orders of magnitude as the trihydride composition is approached. Room-temperature magnetic susceptibilities 9 are 2.14 X 10" 4 emu/g for the dihydride and 2.05 X 10" 4 emu/g for the trihydride. The Curie-Weiss law is obeyed down to about 25°K with constants Θ = -18°K and C = 0.0677 emu/g for the dihydride, and Θ = -19°K and C = 0.0655 emu/g for the trihydride. The effective magnetic moments are 9.74 and 9.54 Bohr magnetons, respectively, as compared to 9.85 for the metal. There was no magnetic ordering down to 4°K. 9-2.12

THULIUM

The homogeneity ranges 51 of the thulium hydrides are TmH L 9 9 to TmH2.4i and TmH 2 . 76 to TmH 3 . Lattice parameters are given in Table 9.5. On the basis of the HoH 3 structure, aQ should be 6.234 A. The X-ray densities are 8.61 and 7.84 g/cm 3 , respectively. Magnetic susceptibilities 9 of the thulium hydrides average about 1.26 X 10" 4 emu/g over the composition range T m H L 9 to TmH 3 . The Curie-Weiss law is followed down to about 120°K with Θ = —40°K and C = 0.435 emu/g for the dihydride, and Θ = - 3 4 ° K and C - 0.0410 emu/g for the trihydride TmH 2 .9 5 . The effective magnetic moment is 2.00

1.60

\-

σ 1.20

ÛÈ

E o

<

0.80 h-

0.40

H

Time, Minutes

F I G . 9.35 Rate of absorption of deuterium by ytterbium at 400°C and 350 to 450 torr deuterium pressure. From W. L. Korst, Studies of the Rare-earth Hydrides, Ph.D. Thesis, p. 69, University of Southern California, 1956.

9-2

SPECIFIC HYDRIDES AND DEUTERIDES

435

H/Yb Atom Ratio

F I G . 9.36 Isotherms for the ytterbium-hydrogen system. From K. I. Hardcastle and J. C. Warf, Rare Earth-Hydrogen Systems. III. High Pressure Investigations, Inorg. Chem., 5(10): 1734 (1966).

about 7.5 Bohr magnetons, essentially the same as pure metal. There is no magnetic ordering down to 4°K. 9-2.13

YTTERBIUM

(a) Hydriding

Characteristics

Ytterbium forms a stable dihydride 5 7 similar to europium dihydride, a high-temperature dihydride (200° to 400°C) which is cubic and metastable at room temperature, 6 5 and at high pressures a nonstoichiometric trihydride (maximum composition attained was YbH2.55) which is also cubic. 9 Under normal pressures and temperatures, the orthorhombic dihydride will form. For example, at 600°C and a starting hydrogen

436

9

T H E RARE-EARTH

HYDRIDES

pressure of 450 torr, the rate of deuterium uptake to form YbD 2 is shown 22 in Fig. 9.35. The nonstoichiometric trihydride is prepared by heating the dihydride under 20 atm of hydrogen. The high-temperature cubic dihydride phase is prepared by decomposing the higher hydride and quenching to room temperature. (b) Dissociation Pressures Pressures and Properties (b) Dissociation and Thermodynamic Thermodynamic Properties Pressure-composition isotherms for the ytterbium-hydrogen system 9 are shown in Fig. 9.36. The plateaus are for the two-phase region consisting of the high-temperatue cubic dihydride and the higher cubic hydride YbH2>5. Warf and Hardcastle 65 suggest that the additional breaks in the 280°, 300°, and 320°C isotherms may be due to a hexagonal hydride coexisting with the higher cubic hydride. The van't Hoff isochores for the plateau regions of Fig. 9.36 are given in Fig. 9.37. The equation for this curve is !

D

1440

Q KA

log Patm = 3.54 -

~γ-

30 20

! •2 3

-

irt «A Φ k.

Q.

5

0 1.60

1.70

1.80

1.90

1/T°K X 10

2.00

2.10

2.20

3

FlG. 9.37 Log P vs. reciprocal temperature graph for the region YbH2 to YbH2.6. From K. I. Hardcastle and J. C. Warf, Rare Earth-Hydrogen Systems. III. High Pressure Investigations, Inorg. Chem., 5(10): 1734 (1966).

9-2

SPECIFIC HYDRIDES AND DEUTERIDES

437

which gives a value of 6.6 kcal per mole of H 2 for the heat of dissociation of the higher hydride to the cubic dihydride. (c) Structures,

Lattice Parameters, and

Densities

The structure of the normal dihydride phase is orthorhombic, 56 as in europium, and similar to the dihydrides of the alkaline-earth metals, 66 which again emphasizes the divalent nature of ytterbium and europium as compared to the other rare earths. The lattice parameters of the orthorhombic phase are shown in Table 9.5. The X-ray density of the dihydride is 8.09 g/cm 3 . As is usually the case for the metal d e u t e n d e s , the deuteride 5 6 has slightly smaller lattice parameters, a0 = 5.871 A, b0 = 3.561 A, and c0 = 6.763 A. Its density is 8.31 g/cm 3 . The high-temperature dihydride phase has metal atoms in the face-centered cubic positions and presumably has the fluorite type structure of the other rare-earth dihydrides. The lattice parameter is 5.253 A, and the density (assuming a fluorite structure) is 8.02 g/cm 3 . The higher hydride 6 5 also appears to have the fluorite structure with approximately 5 5 % of the octahedral sites occupied by hydrogen atoms; the lattice parameter is a0 = 5.192 A. The X-ray density for YbH 2 5 5 is 8.33 g/cm 3 . (d) Electrical

Properties

The electrical conductivity of the ytterbium-hydrogen system 54 decreases with hydrogen content in the range H/Yb = 0 to 2, as opposed to the other rare-earth hydrides. This, again, reflects the divalent character of ytterbium. The resistivity approaches 107 ohm-cm at compositions near stoichiometric YbH 2 . (e) Magnetic

Properties

Wallace et al.10 report that orthorhombic YbH 2 is weakly paramagnetic. This was confirmed by Warf and Hardcastle, 65 who found values of about 0.02 X 10~4 emu/g. This is two orders of magnitude lower than europium dihydride. The higher hydride (cubic form) has a much higher susceptibility, however, with a value of 0.194 X 10" 4 emu/g at YbH2.42 and 0.236 X 10" 4 emu/g at YbH 2 . 55 . 9-2.14

LUTETIUM

Lutetium forms a fluorite type dihydride ranging from L u H 1 8 5 to LuH 2 . 23 and a hexagonal hydride from LuH 2 . 78 to LuH 3 . Lattice parameters are given in Table 9.5. On the basis of the HoH 3 structure, a0 should be 6.163 A. The X-ray densities of the dihydride and trihydride are 9.22 and 8.36 g/cm 3 , respectively.

438

9

THE RARE-EARTH HYDRIDES REFERENCES

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