Characteristics of some binary transition metal hydrides

Journal of the Less-Common Metals Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

CHARACTERISTICS

OF SOME

BINARY

315

TRANSITION

METAL

HYDRIDES

L. C. BEAVIS

San&

Laboratories, Albuquerque,

New Mexico 87115 (U.S.4 .)

(Received July r4th, 1969)

SUMMARY

The purpose of this report is to review some of the recent results obtained the

study

previously

of binary

transition

unpublished

data

covered are the distinct and their kinetic,

metal

from

hydrides.

the

Sandia

addition

Laboratories

and unusual properties

thermodynamic,

In

to published is included.

of these hydrides,

and structural

in

work,

Subjects

their preparation,

properties.

INTRODUCTION

The principal

theoretical

reason

for studying

hydrides

is the fact that

the

hydrogen atom possesses only one electron, which gives hope of obtaining a simple, theoretical, and calculable picture of the bond between hydrogen and other elements. This wish for simplicity is ambivalent,

the nature

not been determined. Hydrogen

has borne fruit in some cases; of the hydrogen,

The transition

element

bond exists between

Alternatively,

may

hydrogen

acquire

an anion, or the hydrogen

been completely

hydrides

removed

an extra

hydrogen

positive or negative

the hydrogen

may become

because

with its compounds,

has

are a case in point.

may appear as an atom with a slightly

that is, when a covalent forming

in others,

when combined

electron

charge;

and the other element.

to fill the

IS

orbit,

a positive ion when its electron

and has passed to the other element

thus has

in the compound.

In the alkali metal hydrides, the hydrogen appears as an anion; it acquires the outer electron from the alkali metal. However, in hydrogen fluoride, it is apparent that

the hydrogen

has lost, or nearly

lost, the electron

necessary

to complete

the

stable shell of the halogen. For elements which appear at the center of the Periodic Table, it is not clear in what manner the combination with hydrogen takes place. In some combinations, it is easiest to explain the nature of the hydrogen bond in terms of more than one model. For the transition metal hydrides, the rapid diffusion of hydrogen through the hydride or through the metal can be explained by assuming that the hydrogen is present as a proton. On the other hand, interatomic spacing in the crystal lattice can be explained best on the basis that the hydrogen is present as an anion. In studies of the electronic configurations of the elements, subtle differences can be probed

by observing

the nature

of the interaction J. Less-Common

of the elements

with

Metals, 19 (1969) 315-323

L. C. BEAVIS

316

For example, the difference in the behavior of the 4f and 5f electrons is observed in a study of the lanthanide and actinide hydrogen compounds. Because of the rather large mass ratio between the three hydrogen isotopes, isotopic effects in compounds of hydrogen are more pronounced than they are in compounds of other multi-isotope elements. The practical reasons for studying the interaction of hydrogen with other elements are several. Hydrogen, because of its low mass, is an excellent moderator of neutron energy. A neutron, in colliding with hydrogen, rapidly loses its energy because the masses are nearly equal. In order to have a compact moderator, it is desirable to have a rather high hydrogen density. Hydrides are especially useful because they retain a high density of hydrogen, even at elevated temperatures where the containment of elemental hydrogen becomes a problem. Hydrogen isotopes are also used to produce controlled numbers of neutrons. In the DD reaction [zH(d,n)3He], a neutron and a helium 3 nucleus are formed with emission of 3.26 MeV energy. In the TD reaction, [3H(d,n)JHe], a neutron and a helium 4 nucleus are formed with emission of 17.6 MeV energy. The TH reaction, [3H(p,n)3He], in which a neutron and a helium 3 nucleus are formed, is endothermic. An energy of 0.76 MeV must be supplied to either the triton or proton so that the reaction will take place. These are a few of the reactions with hydrogen which will produce neutrons. These reactions will take place in the gas, but it is more convenient in many cases to retain the target hydrogen isotope as a hydride because there is more versatility in the use of a stable, solid material than a gas. Hydrogen may also be used as a neutron detector. For example, a fast neutron, colliding with a hydrogen atom, may ionise the atom, the relaxation of the ionisation, i.e., the recombination, being observed by the detection of the emitted photons by a photomultiplier. Alternatively, a slower neutron may be captured by a proton to give an excited deuteron, the photo-decay of which may likewise be observed with a photomultiplier. Hydrides are also used as a supply of high-purity hydrogen. Two of the most commonly used in this connection are uranium trihydride and titanium dihydride. Gases, such as oxygen and nitrogen, which may be contaminants in gaseous hydrogen are removed because they react readily with uranium and titanium to form thermally stable compounds. The hydrides of uranium and titanium, however, decompose at relatively low temperature. Hydrogen may be purified by passing it at a high temperature over either uranium or titanium. In this procedure, the hydride does not have an opportunity to form, but the oxygen and nitrogen react chemically and form thermally stable compounds. hydrogen.

DISTINCT AND UNUSUAL

PROPERTIES

OF TRANSITION

HYDRIDES

The transition elements, of which the hydrides are discussed in the present report, include those of SC, Y, La and the Lanthanides, Ti, Zr, Hf, V, Nb and Ta. This report is concerned only with binary hydrides, which are a combination of a single metal and a single hydrogen isotope. A characteristic property of these hydrides is their large departure from stoichiometric compositioni-12. The ratio of hydrogen to metal atoms can be 3 to I, 2 to I, or r to I, but a stable hydride phase forms when the hydrogen-to-metal ratio is IO J. Less-Common Metals,

19 (1969) 315-328

CHARACTERISTICS OF SOME BINARY TR,~NsITIoK ~~ETAL HYDRIDES

317

or more percent less than these integral ratios. It may be noted from Table I that the departure from stoichiometry is greater at high than at low temperatures. This temperature effect can be explained on the basis of thermally activated defects. It may be also noted that ytterbium hydride does not depart as much from stoichiometry as the hydrides listed above it ; it behaves more like the non-transition saline element hydrides which are more stoichiometric 7. It should be pointed out that the existence range of these hydrides is for a maximum pressure of 760 torr. TABLE

I

SINGLEPHASE HYDRIDE

EXISTENCE

RANGE

Metal

High temp. (600°C)

Room

SC Y

ScHl,s-ScHz.o YHu-YH1.9 *Y&.~-YH~.o LaHl,s-LaHz.6 ErHl,e-ErHz.0 *ErH2.9-ErH3.o TiH,.a-TiHz.0

ScH,.&cHz.o YH1,vYHa.z YHz.a-YH3.o I,aHz.o-LaHs.0 ErH1.~pErH2.1 ErH3.o TiHl,s-TiHe.0 VH.vVH.9 YbHl.os-YbHz.0

La Er Ti v Yb

YbHl.s-Yb&.o

temp

* Exists at 3oo’C Pressure 760 torr.

b x

0

8. ’

SC

I

Ti

Fig. I, Normalized

I

Y

I

Zr

I

La

b Ce

x I

Gd

0

x I

Er

1

Yb

Hf

density of metal hydrides.

The normalized density of the transition metal hydrides is Fig. I. The horizontal line at a ratio of I represents the density of a to the density of the metal. The density of the hydride is usually density of the metal, except in the case of ytterbium. It may be noted

illustrated in hydride equal less than the also that the

J. Less-Common Metals, 19 (1969)

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L. C. BEAVIS

318

Group IV elements-titanium, zirconium, and hafnium-tend to show the greatest change in density upon hydridin~ 13. Yttrium, gado~nium, erbium and other heavy rare-earth metals form trihydrides as well as dihydridesl*. The density decreases in these cases when the trihydrides are formed, but when lanthanum or cerium form trihydrides the density increases 6~15-18. In the case of yttrium, gadolinium, and erbium, the trihydride forms a separate and new phase, whereas in the case of lanthanum and cerium the trihydride is formed by the addition of hydrogen to the dihydride structure. The structure contracts somewhat upon combination with the additional hydrogen. Deuterium as well as protium can be used to form a hydride. The deuterides are denser than the hydrides for two reasons. Deuterium is heavier than protium by a factor of 2, and the spacing in the metallic sublattice is less when deuterium is added than when protium is added. The difference in spacing is small but nonetheless detectable.

TABLE

II

COM’PARATIVE

DENSITlES

OF FILM

HYDRIDES

AND

BULK

HYDRIDES

PM-eMHz film ___.__ ..~ @PI-@WHZ bulk SC Ti Y Gd Er

1.32 1.17 I.11 I.12 1.08

TABLE

III

DENSITY

OF HYDROGEN

IN HYDRIDES

NH (H atamslcm3) S.T.P. hydrogen gas zo°K liquid hydrogen 4’K solid hydrogen 15°C water SCH2

TiKz VH0.s YHa YH3 ZrH2 -I-& LaHz GdHz GdH, ErHa ErHs

x

10-02

5.4 x 10-S 4.2 5.3 6.7 7.3 9.1 5.1 5.7 7.8 7.3 4.4 6.9 5.4 7.4 5.9 8.1

Table II shows a comparison between the densities of the thin-film hydrides and the bulk hydrides (Sandia data). It may be noticed that the films are denser than the bulk material. The films are about I p thick. Films of this thickness expand J. Less-Comma

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(1~69)

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CHARACTERISTICS

OF SOME BINARY

TRANSITION

METAL

HYDRIDES

319

they do not expand in directions only in the free direction, i.e., upon hydriding parallel to the substrate. Another unique property of the transition metal hydrides is their high atomic density of hydrogen. This property is shown for a number of hydrides in Table III; the number of hydrogen atoms per cubic centimeter, NH, is multiplied by 10-22. It may be noted that the hydrides have values of NH equal to, or greater than, the hydrogen density in solid hydrogen at 4°K and are many thousand times the density of standard temperature and pressure hydrogen gas. These high values of NH are retained for many of these hydrides to rather high temperatures. This property makes them useful as reactor moderators. If deuterium or tritium instead of hydrogen is used for the preparation of the hydride, it is found that NT is greater than Nn which is greater than NH (Sandia data). The differences are small but still detectable. The optical properties of these hydrides have not been studied thoroughly. The dihydrides of erbium, yttrium, and gadolinium are a robin’s-egg-blue color. The dihydrides of titanium, zirconium, and hafnium are grey. Going from the dihydride to the trihydride of erbium, yttrium, and gadolinium, the material becomes very dark or black in color (Sandia data). The reason for these differences is not known. PREPARATION

AND

KINETIC

PROPERTIES

OF TRANSITION

HYDRIDES

When hydrogen is brought into contact with a transition element, it dissociates at the surface into atomic hydrogen which then dissolves into the metal lattice to a limited amount as shown in Table IV. The solubility at room temperature is rather low for most transition elementsl2~13~19. H owever, the solubility increases TABLE SOLUBILITY

IV LIMITS

OF

HYDROGEN

IN

METALS

AT

ROOM

TEMPERATURE

as the temperature is increased and amounts to IO or more at.% at a few hundred degrees Centigrade. Extrapolating to room temperature the high-temperature data on solubility of hydrogen in the metal lattice, in comparison with the direct measurement of the room-temperature solubility, gives too high a ratio of H/M. This is particularly true for the Group III and rare-earth elements. LUNDIN20 has attempted to determine the exact location of the solubility boundary in the erbium hydrideierbium system. His results are given in Fig. 2. The plateau which is apparent at about 300°C would normally be indicative of an additional phase. X-ray crystallographic examination does not indicate the formation J, Less-Common

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19 (1969)

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I,. C. BEAVIS

320

of a new phase, but gives information only concerning the metal lattice and nothing concerning the hydrogen structure. The plateau is probably due to an ordering of the hydrogen at certain preferred sites. The hydrogen is present in the metal lattice and essentially forms a new phase, whereas the metal lattice does not undergo a phase change. Similar work on the scandium hydride/scandium system indicates that the hydrogen solubility limit shows no inflection down to temperatures well below room temperature. Thus, scandium is unique in having a high (40 at.?;) solubility for hydrogen in the metal phase at room temperature. No other metallic element is known to have this property. ml 0 SINGLE PHASE l TWOPHASE

700

Erss

0

/ / /

0 ,--/

/*

5'

2.

01

'.

l

l

E'**tEiti* ,

/

I

/

0.1

0.2

0.3

0.4

HYDROGEN-TO-

Fig.

e/ / /.

ERBIUMRATIO

Plot of hydrogensolubility in erbium.

After exceeding the hydrogen solubility limit in the metal, the deficient monohydride for Group Vr3,21 or the dihydride for Groups III and IVr-3,14 precipitates with continued addition of hydrogen at a pressure of 760 torr or less. The Group 1IW3~3 elements, yttrium, lutetium, and the rare earths (except europiumand ytterbium) will form trihydrides at these pressures. It should be pointed out that these hydrides are chemical compounds. The reaction between hydrogen and the transition element hydride-formers takes place at room temperature or below at pressures as low as 10-9 atm (Sandia data). If the reaction is not observed to take place at room temperature, it is believed that a layer of contaminant exists on the metal; that is, there is no energy barrier or activation energy for the transition metal-hydrogen reaction to take place. At low pressures and temperatures the rate of reaction with the metal is governed by the rate at which hydrogen can diffuse through the metal and/or hydride. For large departures from stoichiometry diffusion probably takes place by the movement of vacancies in the structure. A number of vacant sites are available in the non-stoichiometric compound for hydrogen to migrate through. As stoichiometry is approached, the number of vacant sites decreases. Probably the diffusion process then takes place through interstitial migration, possibly through the octahedral sites in the fluorite structure which are available particularly in the dihydrides. As the hydrogen pressure is raised from very low values, 10-3 torr or less, and depending upon the geometry, the reaction of the metal with hydrogen will achieve a rate at which the energy due to compound formation cannot be conducted or radiated away and the temperature of the sample being hydrided rises. This enhances diffusion and causes microcracks J. Less-Common

Metals,

19 (1969) 315-328

CHARACTERISTICS

OF SOME BINARY

TRANSITION

META

321

HYDRIDES

which increase the surface area considerably. If hydrogen is continually supplied the reaction soon becomes runaway. Table V gives the minimum pressures required for a runaway reaction to start for a I ,u thick metal film on a I cmz, suspended, 4 mm thick molybdenum foil (Sandia data). These values of pressure are for this particular geometry and a clean film. Other geometries will doubtless have different threshold pressures for the uncontrolled reaction. TABLE MINIMUM

Metal SC Ti Y DY Ho Er TIII Yb LU

V PRESSURE

REQUIRED

FOR

RAPID

HYDRIDING

OF METAL

FILMS

Pressure (tow) 04 0.4 0.43 0.21 0.8 0.28 0.23 >0.6 0.2I

EXPOSURE INTORR SEC

X103WloCl

Fig. 3. Effect of water vapor exposure

hydride

Upon contact with the atmosphere, hydrides exist for only a few seconds. It is believed that a layer of oxide forms on the transition hydride when it contacts the atmosphere. The use of an argon glove-box does not stop the formation of this oxide layer because even the best of glove boxes contain about I p.p.m. of water in the argon. This passive oxide layer is 30-50 w thick or a layer of about IO atoms (Sandia data). If hydrides at slightly elevated temperatures are exposed to air or water vapor, the oxide layer may be as much as IOOO A thick (Sandia data). Figure 3 shows the effect of exposing erbium hydride to water vapor at 400°C (Sandia data). The passive surface layer which forms on metals as well as J. Less-Common

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xg (1969)

315-328

322

L. C. BEAVIS

hydrides greatly inhibits the flow of hydrogen through the material. Thus, reports of difficulties in forming hydrides are not imaginary. The effect of the passive layer is quite dependent upon surface topography; the effect is greater on smooth than on rough surfaces (Sandia data). In making measurements of the effect of water-vapor exposure, the water vapor is introduced at a pressure of 10-5 or 10-6 torr for exposure times of about IOOO sec. Then the sample is placed in an accelerator where the DD proton reaction may be observed. Formation of a surface layer will result in a reduced number of protons being formed at any given energy. From this information the thickness of the passive layer can be inferred (Sandia data). Raising the temperature of the passivated metal improves the flow of hydrogen. If the temperature is raised high enough in a clean atmosphere the passive layer fails structurally, either by dissolution of oxide or by thermal expansion mismatch between the hydride or metal and the oxide overlay. For low-melting-point metals, the passive layer fails structurally by metallic migration around the oxide or passive layer, which permits the hydrogen to come in contact with the metal; the reaction then proceeds more rapidly (Sandia data). This evidence is an indication that the diffusion coefficient of hydrogen in the passive layer of oxide is much slower than in the hydride or metal. A difference of several orders of magnitude can be inferred from these studies. THERMODYNAMIC

PROPERTIES OF TRANSITION

HYDRIDES

Thermodynamic properties are usually obtained through pressure, composition, and temperature (PTC) studies, A weighed sample of the metal is placed in a small chamber in which the temperature can be controlled accurately, known quantities of gas are added to the sample at a given temperature, and the equilibrium pressures are measured. These pressures are plotted as a function of composition at a given temperature to give an isotherm. An isobar may be obtained by maintaining the pressure constant, varying the composition, and determining the temperature required to maintain any given equilibrium composition. As hydrogen is added to the metal, the pressure rises isothermally until the hydride phase begins to form. At this point, the pressure no longer rises, although the composition changes, until all the material becomes a non-stoichiometric hydride, after which the pressure rises again as more hydrogen is added. The transition metal hydrides are fairly stable compounds. This is apparent from the Van’t Hoff plots which may be derived from isothermal plots of plateau pressure vs. temperature. Such plots of decomposition pressure of hydrogen over the hydride vs. 103/T are shown in Fig. 4. Notice that the Group III and rare-earth dihydrides are the most stable 8, followed by the dihydrides of Group IV-titanium, zirconium, and hafniumll~24. The Group V monohydrides (not plotted in Fig. 4) 12p13,‘J1,25,are the least stable of the transition metal hydrides discussed in this report. The middle member of each group seems to form the most stable hydride. Yttrium dihydride is more stable than lutetium dihydride or scandium dihydridea, zirconium dihydride is more stable than hafnium dihydride or titanium dihydridelr.24, and niobium dihydride is more stable than tantalum dihydride or vanadium dihyJ.

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Metals,

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CHARACTERISTICS

OF SOME BINARY

TRANSITION

METAL HYDRIDES

323

dridei3. It may be noticed also that the trihydrides, of which only erbium trihydride is plotted in Fig. 4, are much less stable than the dihydrides. The enthalpies and entropies of formation can be derived from the Van’t Hoff plots. The slope of the line gives the enthalpy of formation and the intercept gives the entropy of formation. The enthalpies and entropies of formation of some of the hydrides of interest are given in Table VI. The enthalpy is given in kcal/mole of hydrogen and the entropy is given in terms of entropy units per mole of hydrogen.

Fig. 4. Van’t Hoff plots of hydrides.

TABLE

VI

ENTHALPY AND ENTROPY OF FORMATION OF HYDRIDES

ScHz TiHz.0 VHo.5 YHz YH3 ZrHa NbHo.67 LaHz LaH3 ErHz ErH3 HfH1.7 TaHo.5

- AHf (kc&)

-AS,

47.8 29.6 8.2 54.’

34.9

38.9 10 49.6

40.1 52.6

19.8 37.7 9.0

PER

MOLE

OF

HYDROGEN

(e.u.)

30.3 11.4 33.9 31.9 32.1 12.3 35.6 35.2 30.’ 24.7 11.6 J. Less-Commo+z Metals, ‘9

(1969) 315-328

324

L. C. BEAVIS

It may be noted that the enthalpies of formation are generally quite high except for the monohydrides of vanadium, niobium, and tantalum, which are IO kcal or less per mole. In addition, the dihydrides have much higher enthalpies of formation than the trihydrides; thus, the trihydrides are less stable than the dihydrides. The entropies are rather large and negative, as would be expected. If deuterium instead of protium is used to form hydrides, a slight difference in the enthalpies and entropies of formation occur+10,26,27. A comparison is given in Table VII. The difference between the enthalpy of formation of the hydride and enthalpy of formation of the deuteride is given in the second column. A negative value indicates that the hydride is more stable than the deuteride, and a positive value indicates that the deuteride is more stable than the hydride. The third column gives the difference between the entropy of formation of the hydride and entropy of formation of the deuteride. Again, a negative value indicates that the entropy of formation of the hydride has a larger negative value than that of the deuteride. TABLE

VII

COMPARISON

Y Zr La Ce Ho Er

OF ENTHALPY

AND

ENTROPY

OF FORMATION

(AHfri - AHro) (kcallmol)

(AS,, - A.%) (e.u./mol)

-2.5 1.32 -3.0

-0.8 2.6 -2.1

-2.9

-1.9

-2.2

-1.1

OF HYDRIDES

AND

DEUTERIDES

0.6

0.3

There does not seem to be any universal consistency in the data of Table VIIboth zirconium deuteride27 and erbium deuteridela appear to have higher enthalpies of formation than the corresponding hydrides. In most instances, of course, the enthalpy of formation of the hydride is greater than that of the deuteride. In all instances, the difference is not large, typically 5% or less. For erbium it is less than 1%. PHASE DIAGRAMS AND STRUCTURES

OF TRANSITION

HYDRIDES

The phases formed by the hydrides are generally studied by X-ray diffraction. The interaction of X-rays with multi-electron elements is fairly high, whereas this interaction with hydrogen, which has only one electron associated with it, is small. X-ray studies give a picture of the metal lattice, but they provide no information as to the location of the hydrogen with respect to its metal sublattice. It is possible to determine the position of the hydrogen by using neutrons; neutrons interact strongly with protium and more strongly with deuterium. Figure 5 gives the generalized phase diagram for the transition Group III and rare-earth hydride-formers which can form a separate trihydride phase16917P22S2*. (Although scandium is included, it should be pointed out that it does not form a trihydride.) The elements and their pure metal structures are given; h.c.p. signifies hexagonal close-packed and rh signifies rhombohedral, which occurs only in the J. Less-Common

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Ig (1969)

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CHARACTERISTICS

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TRANSITION

METAL HYDRIDES

325

case of samarium. The dihydride phase for all the elements is face-centered cubic (f.c.c.) of the fluorite (CaF2) structure; that is, the hydrogen resides at the tetrahedral sites in the f.c.c. metal sublattice. On further addition of hydrogen, a new h.c.p. phase of trihydride forms. The trihydride phase is generally limited to fairly low temperatures, less than 500°C. Figure 6 shows the generalized phase diagram for the light rare-earth hydrides6,14,29. Again, the dihydride is f.c.c. with the fluorite structure. However, on further addition of hydrogen, the hydrogen apparently enters the octahedral sites of the fluorite structure rather than forming a new phase. The generalized phase diagram for europium and ytterbium hydrides is given in Fig. 77933933.These hydrides have the structure and character of the saline hydrides, such as calcium hydride; the orthorhombic is exactly the same as the structure for

/

SC. Y. Sm. Gd. lb. Oy. Ho. Er. HCP HCP RH HCP HCPHCP HCP HIP &Ol&

.5

1

1.5

Tm. HCP

E \ \

HCP

SCANDIUM DOES NOT FORMTRIHYDRIDE

2

2.5

3

La. H.C.P.

Ce. f.c.c.

1

Pr. H.C.P.

Nd H.C.P.

2

3

WM

Fig. 5. Generalized phase diagram formers.

for Group III

transition-element

and rarc-earth

hydride

Fig. 6. Generalized phase diagram for light rare-earth hydrides.

-

I Eu b.c.c.

Yb f.c.c.

I

a ANDPMETAL PHASES YAND 6 HYDRIDE PHASES

aTI. Zr. HI H.C.P. 6Ti. I.c.c. 8Zr. Hf. f.c.t.

wKQ-

M

loo -

1

!$

Fig. 7. Generalized phase diagram for saline type rare-earth hydrides. Fig. 8. Generalized phase diagram for Group IV transition-element

hydrides

J. Less-Common

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calcium hydride. The europium parent metal is body-centered cubic (b.c.c.), whereas the ytterbium metal is f.c.c. The generalized phase diagram for the Group IV transition hydrides is given in Fig. 825. This diagram is more complex in that the Group IV element hydrides exist as two phases, depending upon composition, and the metal goes through a rather low-temperature, solid-phase transformation. The transition temperature from a to p is different for titanium, zirconium, or hafnium. To use the phase diagram, the ordinate is adjusted to match this temperature. Other aspects are nearly the same for the three metals. The 01and /3phases are metal phases, whereas the y and S phases are hydride phases. The y phase is always f.c.c. and possesses the fluorite structure. In the case of zirconium and hafnium, however, as more hydrogen is added the y phase becomes distorted enough for the f.c.t. 6 phase to be formed. For titanium, the S phase is f.c.c., but with slightly different lattice parameters than the y phase. Figure 9 presents the generalized phase diagram for the Group V transition element hydrides of vanadium, niobium, and tantalum25. Again, the phase diagram is complex, compared with that for the Group III elements. No temperatures are given on the ordinate, and the diagram can be applied to the different elements by merely shifting it up and down, depending upon which metal-hydrogen system is being considered. I-

I

WV.

Nb, Ta

fib,

ed”‘i.c.

b.c.c. ortharhombic

I

H zi

Fig. 9. Generalized phase diagram for Group V transition-element

hydrides

If isotopes other than protium are used the phase diagrams will be much the same. The metallic structures in these phases are the same, and there is reason to believe that deuterium and tritium behave the same as hydrogen, the only differences being that the lattice parameters are slightly less for deuterides than for hydrides and less still for tritides compared with deuteridesl’. REFERENCES I J. F. STAMPLER, JR., The scandium-hydrogen system, LA-3473, Alamos, New Mexico, January, 1966. 2 M. L. LIEBERMAN AND P. G. WAHLBECK, The thermodynamics system, J. Phys. Chem., 69 (1965) 3514-3519. J.

LeSS-Common

Metals,

19 (1969)

315-328

Los Alamos

Sci. Lab., Los

of the scandium-hydrogen

CHARACTERISTICS

7

8

9 10 ,I I2

13 ‘4 ‘5 IG ‘7 18 19 20 2r 22 23 24 25 26

27

28 29

OF SOME BINARY

TRANSITIOX

METAL

HYDRIDES

327

R. L. BECK, Research and development of metal hydrides summary report, L4 R-ro, Denver Res. Inst., Denver, Colo., November, 1960. L. N. YANNOPOULOS, K. K.. EDWARDS AND P. G. WAALBECK, The thermodynamics of the yttrium-hydrogen system, J. Phys. C&m., 69 (‘965) ZjIO--2515. C. E. LUNDIN AND J, F’. RLACKLEDGE, Pressure-temperature-composition relationship of the yttrium--hydrogen system, J. Electrocham. Sot., rag (1962) 838. P. T. PETERSON AND J. A. STRAATMAN, Lanthanum-lanthanum hydride phase system, J. Ph.ys. Chem., 70 (1966) zgBo-2984. C. E. MBSER, T. Y. Cno AND T. R. f’. BIGG, JR., Dissociation pressures in the system ytterbium--ytterbium dihydride; a comparison with calcium-calcillnl dihydride, J. Less-Cosnmon il/letu$s, 12 (1967) 411-418. P. M. S. JONES, J. SOUTHALL AND K. GOODHEAD, The thermal stability of metal hydrides. Part I.-Rare earth and yttrium hydrides and deuterides, .-3WRE o-22/64, Atomic Weapons Res. &tab., Aldermaston, Berkshire, England, June, 1964. C. E. LUNDIN AND F. C. PERKINS, The holmium-hydrogen system, DRI Rep. KG. 238.1, Denver Res. Inst., Deuver, Colo., 1967. K. W. SULLIVAN _Q+II C. E. LC‘NDIN, The erbium-hydrogen-deuterium svstem, DRI R&t. IXU. 2379, Denver Res. Inst., Denver, Cola., 1967. . .~ V. V. SOFINA AND N. G. PAVLORSKAYA, Equilibria in the titanium-hydrogen and zirconiumhydrogen systems at low pressures, Russ. J. Phys. Chem., 34 (1960) 1104-1109. I’. K~PSTAD AND W. E. WALLACE, Vapor pressure studies of the vanadium-hydrogen system and thermodynamics of formation of vanadium-hydrogen solid solutions, J. Am. Chem. Sac., 81(1959) jOI+5022. 1%'.&I. MUELLER, J. P. BLACKLEDGE AND G. C. LIBOWITZ, dle?al Hydrides, Academic Press, J.ondon and New York, 1968. Ii. N. Ii. MULFORD AND C. E. HOLLBY, Pressure-temperature-composition studies of solne rare-earth-hydrogen systems, J. Phys. Chem., 59 (I955j 1222-1226. _ A. PEHLER AND W. E. WALLACE, Crystal structure of some lanthanide hydrides, .[. Phys. Chem., 66 (1962) 148-151. C. E. HOLLEY, R. N. R. MULFORD, F. H. ELLINGER, W. C. KOEKLER AND W. H. ZACKARIASF,N, The crystal structure of some rare-earth hydrides, J. Phys. Chem., 59 (1955) 1226-1228. F. C. PERKINS, R. W. SULLIVAN AND C. E. LUNDIN, S-Kay diffraction study of erbium hydride, deuteride and tritide, RRI Rep. No. 2446, Denver Res.“Inst., Denver, C&o.. 1968. _ G. C. LIROWITZ, Solid St&e Chemistry of Binary Metal Hvdkdes, Benjamin, New York, 196.5. C. E. LUNDIN, Structural studv of-the erbium-erbium”dihydride and scandium-scandium dihydride systems, DRT Rep AFo. 2459, Denver Rcs. Inst., Denver, Cola., 1968. C. E. LUNDIN, DRI Rept. :Vo. 2489, Denver Res. Inst., Denver, Coio., 1969. 1). W. RUDIJ, D. W. ‘1’0s~ AND S. JOHNSON, The permeability of niobium to hydrogen, f. Ph_y~. G&m., 66 (1962) 35r-35.3. W. J. WORST AND J. C. WARF, The rare earth-hydrogen systems. I-Structural and thermodynamic properties, Inovg. Chem., 5 (1966) 17191725. 1. C. WARF. Studies of rare earth-hvdroeen svstems, Final Rcut.. Contract NONR 228/r5j. “, >>arts r and II, Univ. Southern Calif:, r9(,1. ’ l<. I<. EDWARDS AND E. VELECKIS, Thermodynamic properties and phase relations in the system hydrogen-hafniun~, f. P&z. Chem., 66 (1962) 1657-1661. K. M. MACKAY, ~~ld~oge~~~rn~o~i~~ds the ~~etu~~ic Ekments, Spon, London, England, 19%. G. BREYNAT, M. DUBUS, R. BERBIER AND J. F. MAURIN, Stabilite des composes tritium-metal utilises comme cibles sur les generateurs de neutrons rapides, Bull. Irzform. Sci. Tech., y8 (1965) XT-92. D. Ii. FREDRICKSON, R. I,. MUTTALL, II. E. FLOTO\I. ANU W. N. HUBBARD, The enthalpies of formation of zirconium dihydride and zirconium dideutcride, J. Phys. Chem., 67 (1963) IjOG-1509. % XI. &ARK AND V. I-I.Funrx, X-Ray structural phase analysis of the s~andium~hydrogen system, Sovrst Phys. Cvyst., 10 (1965) 21.-23. L. J,YNDS, Melting in the systems cerium-hydrogen and ceriunr-deuterium, SCPP-64-34. North American Science Center, Los Angeles, Calif., 1964.

of

BJ 3LIOGRAPHY H. E. FLOTOW, D. W. OSBORNE AND K. OTTO, Heat capacities and thermodynamic functions YH2 and YD2 from 5°K to 350’K and the hydrogen vibration frequencies, J. Chem. Phys., (1962) 866-872. J. Less-Common

Metals,

of 36

~9 (1969) 315 328

L. C. BEAVIS

328

W. E. WALLACE AND K. H. MADER, Magnetic and crystallographic characteristics of praeseodymium hydrides, J. Chem. Phys., 48 (1968) 84-89. M. L. LIEBERMAN AND P. G. WAHLBECK, The scandium-yttrium-hydrogen system, J. Phys

Chem., 69 (1965) 3973-3980. J. C. MCGUIRE AND C. P. KEMPTER, Preparation

and properties of scandium

dihydride, J. Chem.

Phys., 33 (1960) 1584-1585. G. E. STURDY AND R. N. R. MULFORD, The gadolinium-hydrogen

system, J. Am. Chem. Sot., 78 (1956) 1083-1087. R. N. R. MULFORD, A review of rare earth hydrides, AECU-3813, Los Alamos Sci. Lab., Los Alamos, New Mexico. K. DIALER, The question of bonding in the rare earth hydrides, Monthly Bull. Chem., 79 (1948) 311-315. F. RICCA AND T. A. GIONGI, Equilibrium pressures of hydrogen dissolved in zirconium, J. Plays.

Chem., 71 (1967) 3627-3637. J. E. JAFFE, Metallographic

AIME,

identification

and crystal

symmetry

of titanium

hydride,

Trans.

206 (1956) 861, (J. Metals, 8).

L. ESPAGNO, P. Azou AND P. BASTIEN, Contributions to the study of the hafnium-hydrogen system, Argonne Natl. Lab., Trans. No. 166 (March 1965), translated from Corn@. Rend., 250, April-June

1960, 4352-4354. A. J. MAELAND, Investigation of the vanadium-hydrogen J, Phys. Chem., 68 (1964) 2197-2200.

system by X-ray diffraction

techniques,

R. L. BECK, Bonding mechanisms in transition metal hydrides, TID 12507. Denver Res. Inst., Denver, Cola., 1960. T. R. P. GIBB, JR., Progress in Inorganic Chemistry, Vol. 3 : Primary Solid Hydrides, Interscience, New York, 1962. R. F. GOULD, (ed.), Non-Stoichiometric Compounds; Advances in Chemistry, Series No. 39, Am. Chem. Sot., 1963, Washington, D.C. L. MANDELCORN, (ed.), Non-Stoichiometric Compounds, Academic Press, New York, 1964. W. G. Bos AND K. H. GAYER, The rare earth hydrides, J. Nucl. Mater. 18 (1966) 1-30.

J. Less-Common Metals, 19 (1969) 315-328