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Metal Hydrides 457

hydrogen fuel cell buses (CITARO) to be delivered by TABLE IV Characteristic Features of Binary Main Group
the end of 2002. In Zollbr¨uck, Switzerland, the house and and Transition Metal Hydrides a
a hydrogen driven van of M. Friedli is powered exclusively
Main group Transition
by photovoltaically produced hydrogen that is stored in a metal hydride metal hydride
metal hydride tank. Iceland with its abundant geothermal
and water power sources recently launched an initiative to Composition Stoichiometric Nonstoichiometric
become a hydrogen economy by 2025. Crystal structure Ordered Disordered, order–
(H distribution) disorder transitions
H content Metal valence Geometrical and
B. Other Applications
determined by electronic factors
Besides reversible hydrogen storage, metal hydrides have H diffusivity Low High
found other important fields of application. The enthalpy Electrical Insulating or Metallic or
of formation of a metal hydride is rather a problem for the conductivity semiconducting semiconducting
storage of hydrogen as a fuel, as it has to be removed con- Appearance Nonmetallic Metallic
−
tinuously during charging with hydrogen. However, it may Air sensitivity High (H as base) Low
also be utilized in heat pumps in which the heat of decom- Density Higher than metal Lower than metal
position of a metal hydride is recovered at a different place a Not all listed properties may apply for a given compound. Some
by recombination of the produced hydrogen in a second exceptions are discussed in the text.
reservoir with a hydride-forming material. In TRIGA-type
reactors, ZrH 2 is used as moderator with a negative tem-
not for main group metals. This phenomenological ap-
perature dependence of moderating efficiency, i.e., those
proach to a classification of binary metal hydrides does
reactor types have an inherent safety. In chemical syn-
not make any statement about the specific type of chemical
thesis hydride formation is of use for the purification of
bonding, thus avoiding difficulties of assigning chemical
reaction gases (H 2 getter) and metal hydrides as precur-
bonding in borderline cases such as BeH 2 .
sors for the preparation of finely divided metal powders,
The major exceptions to this simplified picture are prob-
e.g., uranium by thermal decomposition of UH 3 , and in
ably CuH and ZnH 2 , which are reported to be stoichiomet-
the HDDR process (hydrogenation–disproportionation–
ric, nonmetallic solids like the main group metal hydrides,
desorption–recombination) for the synthesis of magnetic
but their crystal structures and physical properties are not
materials such as Nd 2 Fe 14 B and SmCo 5 . Vanadium is be-
well characterized. Borderline cases are the rare earth hy-
ing used for isotope separation of H and D via formation
drides, which show a hydrogen concentration-dependent
of the hydrides/deuterides. The temperature dependence
semiconductor–metal transition. EuH 2 and YbH 2 behave
of the desorption pressure of metal hydrides is the basis
like alkaline earth dihydrides because of the special elec-
for their use in thermal compressors and fire detectors. 2+ 7 2+ 14
tron configuration of Eu (4 f ) and Yb (4 f ).
Ternary hydrides containing main group and transition
V. CONCLUSION AND OUTLOOK metals in the same compound show an interesting com-
bination of features of the two groups. In complex transi-
Thepronouncedvarietyincrystalstructuresandproperties tion metal hydrides A a [M m H h ] (A = Li–Cs, Mg–Ba, Eu,
+ 2+
of metal hydrides does not justify a strict categorization Yb; M = transition metal) ionic (between A or A and
x −
according to chemical bonding types, i.e., ionic, covalent, [M m H h ] ) and covalent bonding (M–H) predominates
and metallic (interstitial) hydrides. However, within the as in main group metal hydrides. Furthemore, they are
frameworkofthedifferentiationinmain-group,transition- nonmetallic, stoichiometric compounds with an H con-
metal, and complex hydrides, some trends become appar- tent determined by metal valencies (18-electron rule). On
ent. For binary hydrides there is a clear distinction between the other hand, in some cases order–disorder transitions,
the hydrides of main group and transition metals includ- structural relationships to the underlying intermetallic or
ing the rare earths. Characteristic general features as sum- border cases with metallic character, are observed, re-
marized in Table IV show main group metal hydrides as sembling the typical features of transition metal hydrides
stoichiometric, nonmetallic valence compounds with an (Table IV). Thus, complex hydrides combine features of
ordered hydrogen distribution in the crystal structure and both groups.
a low hydrogen mobility. In contrast, transition-metal hy- For future research, those metal hydrides representing
drides may be regarded as nonstoichiometric, often metal- border line cases in view of chemical bonding and the
lic compounds with a variable content of hydrogen that is classification just given are particularly interesting. An
disordered and very mobile in the crystal structure. Struc- enhanced use of theoretical calculations to account for
tural relationships between the metal and the hydride crys- the electronic structure of metal hydrides, limiting hy-
tal structure are obtained for some transition metals, but drogen concentrations, and preferred site occupancies in