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

442 Metal Hydrides

TABLE I Binary Metal Hydrides a

a Integer stoichiometric indices are used for fully ordered structures. Transition metal hydrides have broad nonstoichiometry ranges, but precise
formulae are only given where necessary for the distinction of different phases. Greek symbols are used only for the distinction of different phases
of the same stoichiometry, i.e., they do not represent α and β phases as presented in Fig. 1. Bold:Crystal structure fully characterized (see II.C), in
general by neutron diffraction on the deuteride. (In parentheses): Existence not yet clear. Italics:High pressure phase (prepared under high H 2 or
hydrostatic pressures). LT:Low temperature modification.

Ever since many more metal–hydrogen systems have in PdH 0.7 . However, gradual rather than abrupt transi-
been studied, and today we know that most elements tions occur and many hydrides combine features of differ-
form chemical compounds with hydrogen, called hydrides ent bonding types. The orthodox classification as ionic,
(Table I). The focus of this article are compounds con- covalent, and metallic (interstitial) hydrides is therefore
sisting of metals and hydrogen only. Hydrides containing abandoned here, and a more natural sequence is adopted
other nonmetals will be mentioned only briefly. according to the metal’s position in the periodic table.
Hydrogen is the lightest of all elements containing only After a brief introduction in preparation and characteriza-
1
one proton and one electron ( H). Because the electron tion of metal hydrides (II) we will discuss first structures
1
configuration 1s lies in between the more stable ones, and properties of main group metal hydrides (III.A), then
0
2
1s and 1s , hydrogen shows Janus-head-like chemical turn to the transition metal hydrides including lanthanides
properties with parallels to both the alkaline metals and and actinides (III.B), and finally consider a group of hy-
the halogens. This unique feature and its medium elec- drides containing both main group and transition metals,
tronegativity (χ = 2.2) largely determine its behavior in the complex hydrides (III.C).
metal hydrides. A fascinating characteristic of metal– Metal hydrides find a wide range of applications of
hydrogen systems is the existence of a vast number of which the most prominent is reversible hydrogen storage
phases of different stoichiometry, crystal and electronic (IV.A). This is of great importance in view of a possible fu-
structures, properties, and applications. In terms of the ture hydrogen economy in which hydrogen would replace
classical concepts of chemical bonding, metal hydrides fossil fuels as an energy carrier. Other fields of application
can be typical ionic compounds, such as NaH (NaCl type includeheatpumps,hydrogenpurification,isotopesepara-
structure), covalent, such as SnH 4 (discrete tetrahedral tion,andmoderationinnuclearreactors(IV.B).Becauseof
SnH 4 molecules), or metallic compounds with hydrogen this multiformity and its impact on both basic and applied
in interstices of a closest metal atom packing, such as research, metal hydrides have always been the subject of