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Metal Hydrides 447

FIGURE 2 View of the crystal structures of the ternary ionic hydrides EuLiH 3 (two unit cells shown) and EuMg 2 H 6 .
Large spheres represent Eu, middle-sized spheres Li and Mg, respectively, and small spheres H. The structure
of EuMg 2 H 6 (right) is related to that of EuLiH 3 (left, cubic perovskite type structure) by doubling the c-axis and
omitting every other Eu layer. [Reprinted from Physica B 276–278, H. Kohlmann, Crystal structure solution of hydrides
containing nat Eu from neutron powder diffraction data., 288–289, 2000, with permission from Elsevier Science.]

hydride (H ) and a cluster anion Ga 8− which satisfies the 1. Binary Transition Metal Hydrides
−
6
Wade–Mingos rules. A similar situation seemingly oc-
The thermodynamic stability of binary transition metal
curs in Na 15 K 6 Tl 18 H, and for both compounds the bond-
hydrides decreases with increasing group number in the
ing situation is in full agreement with valence electron
periodic table of the elements (see H values in Table II).
rules after the hydrogen content is correctly assigned. All
Metals of groups 6b–8b (with the notable exception of
these compounds are stoichiometric and have semicon- Pd) form hydrides only under extreme high hydrogen
ducting or insulating properties. Ternary hydrides with
pressures, e.g., FeH, which is likely to play an impor-
a nonmetal, such as hydride halides AHX (A = Ca, Sr,
tant role in the geochemistry of the earth’s core. Binary
Ba, Eu, Yb; X = Cl, Br, I) or Th 6 Br 15 H 7 , oxide hydrides
transition metal hydrides are in general nonstoichiomet-
(Ba 21 Ge 2 O 5 H 24 ), and hydride nitrides (Ba 2 HN, Sr 2 HN,
ric compounds with a disordered H distribution, high H
Li 4 NH), will not be discussed.
diffusivity, and metallic properties.
The lanthanides (except Eu, Yb) and group 3b metals
B. Transition Metal Hydrides
(Y, Sc, La, Ac) show a rather complex behavior on hydro-
Transition metals and intermetallic compounds consisting genation. They form nonstoichiometric dihydrides MH 2
of transition metals form only hydrides, which often derive in which the metal atoms adopt a Cu type arrangement, in
from closest packing of the metal atoms with hydrogen contrast to the parent metals, most of which crystallize in
filling tetrahedral and/or octahedral voids. Therefore, they an Mg-type structure. All tetrahedral voids of the Cu-type
are often called interstitial hydrides. Structures and prop- structure of M are occupied by hydrogen, i.e., the dihy-
erties of transition metal hydrides (including lanthanides drides MH 2 crystallize in a fluorite type structure (Fig. 3,
and actinides) range from nonstoichiometric compounds left).
with broad composition ranges and metallic properties to The dihydrides easily take up further hydrogen, which
valence compounds with a more saltlike or covalent char- is accommodated successively in octahedral voids. For
acter. Ternary hydrides including nontransition metals are M = La, Ce, Pr the cubic structure is retained until all
excluded here. The most important group among them, octahedral voids have been filled according to a limiting
the complex transition metal hydrides, will be discussed stoichiometry MH 3 (CeD 3 type structure). For the heav-
in section III.C. ier rare earths (Nd, Sm, Gd–Tm, Lu), the cubic dihydride