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

452 Metal Hydrides

According to a purely geometrical approach, [A 2 M 2 ] forms a hydride EuPdH 3 with the EuPd substructure of the
type interstices (Fig. 3, right) are preferred for low H CsCl type.
concentrations if the lattice parameter a < 800 pm and The vast majority of metal hydrides are crystalline
[AM 3 ] type interstices are preferred for a > 800 pm. For phases. Sometimes, however, hydrogenation induces an
high H concentrations usually both [A 2 M 2 ] and [AM 3 ] amorphization, e.g., in the cubic Laves phase CeFe 2 ,
are occupied. Because of the high crystallographic multi- GdFe 2 , SmNi 2 whose hydrides are amorphous materials.
plicity of these positions and the closeness of neighboring the reverse process is known as well, i.e., the formation of
equivalent positions their occupancy is generally low, i.e., a crystalline metal hydride from an amorphous material,
H is statistically disordered over the tetrahedral interstices. e.g., the hydrogenation of amorphous Zr 0.33 Ti 0.67 yielding
Temperature-dependent structural transitions from these the crystalline spinel type ZrTi 2 H 4 .
cubic phases with disordered H distribution to a lower
symmetric low-temperature phase with an at least partially
3. Hydrogen as a Lattice Gas
ordered H distribution frequently occur. Some examples
and Order–Disorder Transitions
are summarized in Table III. In all cases crystallographic
group–subgroup relationships prove the structural rela- Hydrogen disturbs the crystal structure of a metal or an
tionship between the ordered (low temperature) and the intermetallic compound much less than other nonmetals
disordered (high temperature) modification, suggesting (low defect power), most of which do not form inter-
the possibility of second-order (displacive) phase transi- stitial compounds, but in a reconstructive reaction form
tions. Unlike the Laves phases discussed so far, those con- compounds completely distinct in structure and proper-
taining a nontransition metal, such as AMg 2 (A = La, Ce, ties from the former metal. Further elements capable of
Sm), transform into stoichiometric, nonmetallic phases forming interstitial compounds are, for instance C, N, O.
such as ternary ionic hydrides (III.A.2), but like the Some of the interstitial carbides, nitrides, and oxides take
other Laves phases and unlike typical ionic hydrides they uphydrogentoformmixedcompoundssuchasZr 3 V 3 OH x
show a pronounced structural similarity to the parent or ZrC 1−x H y . Asa first approximation interstitial hydrides
intermetallic. can be described as host–guest systems in which hydro-
Another example of hydrogen-induced structural dis- gen can be treated by a lattice gas model. Hydrogen can
tortion is the class of H-absorbing AM compounds with migrate in the metal hydride nearly freely, and the H–H
the cubic CsCl type structure, such as the technologically interactions are mainly long-range attractive forces and
relevant FeTi used for reversible hydrogen storage. FeTi short-range repulsion as in the pair potential for gas par-
forms a solid solution phase (called α) of approximate ticles. Self-diffusion constants at room temperature are
2
composition FeTiH 0.06 . On increasing the hydrogen pres- high, e.g., D = 4 × 10 −4 mm /s for hydrogen in PdH 0.7 ,
sure a so-called β-phase (see Fig. 1), orthorhombic FeTiH, which is comparable to that of protons in water. Hydro-
crystallizes in which H occupies octahedral positions in gen concentration can be changed continuously, i.e., a
a deformed CsCl type FeTi substructure. A stronger dis- solid solution MH x is formed with x covering a broad
tortion of the metal atoms structure is found in the higher stoichiometric range. Below a critical temperature T c two
hydride FeTiH 2 (called γ ), clearly showing the depen- distinct phases of different density are in equilibrium: a
dence of the degree of structural distortions on the H gas and a liquid in the model, or two phases MH x and
concentration. MH y with nonoverlapping solid solution regions x and y
To date the most important class of intermetallics for in the real hydride systems. As an example, Fig. 5 shows
reversible hydrogen storage is that based on the hexag- the phase diagrams of the systems Pd–H and ZrCr 2 –D.
onal CaCu 5 type structure. LaNi 5 forms a solid solution Below T c = 570 K the uniform Pd–H phase dispropor-
phase (α) LaNi 5 H 0.3 and several higher hydrides, hexag- tionates into two phases, α and β, with a miscibility gap
onal LaNi 5 H 3 , trigonal LaNi 5 H 6 , hexagonal LaNi 5 H 6.7 in between. The same behavior is found for ZrCr 2 –D with
(Table III). All hydrides have LaNi 5 arrangements that T c = 350 K. This lattice gas–liquid transition is driven by
are distorted CaCu 5 type structures with H occupying dis- short-range order effects. In phases of the lattice liquid or
torted tetrahedral and distorted octahedral positions. gas type, low occupancies of hydrogen in interstices are
The metal atom substructure may also differ completely often observed, i.e., hydrogen is disordered statistically.
fromtheintermetallicstructure,i.e.,theintermetalliccom- However, the interstices are not filled in a totally random
pound may suffer a reconstruction during formation of manner. A short-range order is introduced by the H–H re-
the metal hydride. On hydrogenation of ZrCo (CsCl type pulsion, which blocks nearest neighbor sites around each
structure), a hydride ZrCoH 3 with a completely rearrange H atom within a radius of 210 pm. This is evident by a
ZrCo substructure (CrB type) is formed. The reverse re- “liquid-like,” very broad peak at d = 210 pm in neutron
construction is found for EuPd (CrB type structure), which diffraction patterns on disordered metal hydrides. Such