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Encyclopedia of Physical Science and Technology EN009A-426 July 6, 2001 20:44

Metal Hydrides 451

2. Ternary Transition Metal Hydrides favored over the formation of a ternary transition metal
hydride AM m H 2h . However, this reaction often has a high
Many ternary transition metal hydrides A a M m H h are activationenergy,andthusatmoderate p–T conditionsthe
based on intermetallic compounds A a M m (A, M = tran- ternary hydrides are formed as metastable compounds. A
sition metal). On hydrogenation the crystal structure purely geometrical approach to hydride formation is given
of the intermetallic is expanded and possibly distorted. by the Westlake criterion, according to which the minimal
Frequently the degree of distortion increases with increas- hole radius in a metal matrix to accommodate a hydrogen
ing hydrogen content because of the growing importance atom is r min = 33 pm. In real systems, however, only voids
of H–H repulsion and with decreasing temperature. A with r min > 40 pm are being occupied. Together with the
complete reconstruction of the intermetallic structure on further condition of a minimum H–H distance of 210 pm
hydrogenation happens less often. In many cases the hy- and the assumption that the largest holes are occupied first,
drogen uptake is reversible, i.e., the intermetallic can be this model predicts preferred site occupation of hydrogen
recovered by decomposing the hydride. In the ternary tran- in an intermetallic compound. Besides these geometri-
sition metal hydride, the hydrogen atoms occupy inter- cal factors, the enthalpy of metal hydride formation H
stices of the intermetallic substructure, often octahedrally also depends on electronic structure as accounted for by
or tetrahedrally surrounded by metal atoms, analogous to Griessen and Driessen’s band structure model. According
the situation in the binary interstitial hydrides (III.B.1) to this semiempirical approach, H depends on the dif-
such as PdH x . These hydrides show pronounced com- ference between the energies of the Fermi level and the
positional ranges according to a variable occupancy of lowest conduction band of the intermetallic compound.
crystallographical positions and generally exhibit metal- The band structure model and the criteria just mentioned
lic conductivity. Good hydrogen absorbers A a M m are apply for main group metals, as well, e.g., for the ionic
based on at least one transition metal that forms a sta- hydrides of the alkaline metals.
ble binary hydride (see III.B.1 and Table II) and they The important application for the ternary transition
often show multiplateau behavior on hydrogen absorp- metalhydrides(reversiblehydrogenstorage),theirrelative
tion as described in Section II.A. According to an em- insensitivity toward air, and the huge combinatorial poten-
pirical model introduced by Miedema, Buschow, and tial of intermetallic compounds make them the by far most
van Mal, the stability of ternary transition metal hy- investigated subclass within the metal hydrides. From the
drides AM m H 2h (A = Sc, Y, La, Ti, Zr, Hf, Th, U, Pu; vast number of A a M m –H systems, only some prominent
M = any transition metal) can be predicted. The estimated representatives can be discussed in detail. Table III gives
value for the enthalpy of formation H(AM m H 2h ) = an overview of some hydride phases of important sub-
H(AH h ) + H(M m H h ) − H(AM m ) should be less classes of A a M m intermetallic compounds.
An important and numerous class of intermetallic com-
than −38 kJ/mol to form a stable ternary hydride at a H 2
5
pressure around 10 Pa and room temperature, i.e., at least pounds AM 2 are the cubic Laves phases (MgCu 2 type,
one of the metals A and M should form a stable hydride C15). Many representatives with A or M being one of
(large negative H) and the thermodynamic stability of the transition metals forming stable binary hydrides (see
the intermetallic compound AM m should not be too high III.B.1) absorb considerable amounts of hydrogen up to
(rule of reversed stability). For many systems the decom- compositions AM 2 H 7 . Hydrogen occupies tetrahedral in-
position into the binary hydrides is thermodynamically terstices of the crystal structure, thereby expanding it.

TABLE III Composition and Space Group of Some Intermetallic Compounds and
Their Hydrides a
ZrV 2 (Fd ¯ 3m, MgCu 2 type) ZrV 2 H 4.5 (Fd ¯ 3m), ZrV 2 H 1<x <4 (Fd ¯ 3m, 360 K),
ZrV 2 H 3.6 (I4 1 /a 230 K), ZrV 2 H 3 (P2 1 /c, 77 K),
ZrV 2 H 1.9 (C2/c, 100 K)
ZrCr 2 (Fd ¯ 3m, MgCu 2 type) ZrCr 2 H 3.8 (Fd ¯ 3m, 298 K), ZrCr 2 H 3.8 (C2/c, 1.6 K)
ZrCr 2 (P6 3 /mmc, MgZn 2 type) ZrCr 2 H 3.8 (P6 3 /mmc, 298 K), ZrCr 2 H 3.8 (R ¯ 3c, 100 K)
FeTi (Pm ¯ 3m, CsCl type) FeTiH (P222 1 ), FeTiH 2 (Cmmm)
LaNi 5 (P6 3 /mmm, CaCu 5 type) LaNi 5 H 3 (P6 3 /mmm), LaNi 5 H 6 (P31m),
LaNi 5 H 6.7 (P6 3 mc)
Y 6 Mn 23 (Fm ¯ 3m, Th 6 Mn 23 type) Y 6 Mn 23 H 30 (Fm ¯ 3m, 295 K), Y 6 Mn 23 H 30 (P4/mmm,4 K)
a All hydride phases (right column) represent hydrogen filled, some of them distorted, variants
of the intermetallic structure (left column).