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Metal Hydrides 449

FIGURE 4 Change of optical properties on hydrogenation of yttrium. A 500-nm-thick yttrium film covered with a
5
20-nm-thick palladium protection layer in a hydrogen atmosphere (p H2 = 10 Pa). (a) The dihydride phase YH 2−x
(x ≈ 0.2) is formed after 1 min hydrogen exposure. Its metallic-like optical reflectivity can be seen by the mirror image
of the knight in front. (b) After formation of the trihydride YH 3−x the film becomes transparent, as now the chessboard
pattern behind is visible. The faint mirror image of the knight is caused by reflection from the Pd layer. (Photographs
courtesy of Prof. R. Griessen, University of Amsterdam.) [Reprinted from Huiberts, J. N., Griessen, R., Rector, J. H.,
Wijngaarden, R. J., Dekker, J. P., de Groot, D. G., and Koeman, N. J. (1996). Yttrium and lanthanum hydride films
with switchable optical properties. Nature 380, 231–234, with permission from Nature.]

The elements of group 4b and Th also form dihydrides sure. They crystallize with a metal atom substructure of
MH 2−x with broad compositional ranges (0 ≤ x ≤ 0.5). the Cu type (Mn) or of the Mg type (Fe, Co) with hydrogen
Those with a composition close to that of the dihydride occupying octahedral interstices.
MH 2 adopt a tetragonally distorted fluorite-type structure. The best-investigated metal–hydrogen system is Pd–H
The distortion is driven by electronic factors, i.e., lower- (Fig. 5a). The Cu type structure of Pd is retained on hydro-
ing the density of states (DOS) at the Fermi level E F by genation and H enters octahedral cavities. As a Cu type
lifting a degeneracy, as shown by quantum mechanical arrangement of n atoms accommodates n octahedral in-
calculations. terstices (Fig. 3, left), the limiting stoichiometry is PdH
Thebinaryhydridesofgroup5barebasedontheW-type with a NaCl type crystal structure. The phase boundary at
arrangements of the parent metals by filling tetrahedral T = 50 K (50 K anomaly, Fig. 5) is considered to belong
voids with hydrogen. The phase diagrams are rather com- to an order–disorder transition. Pd is remarkable insofar
plicated and show several superstructures with a distorted as it is the only metal of groups 6b–8b readily forming a
W-type structure and an ordered hydrogen distribution hydride.
with stoichiometries M 2 H, M 4 H 3 , and MH, but only those The red-brown CuH and the white ZnH 2 cannot be pre-
of β-NbH (Cccm) and β 1 -Ta 2 H (C222) are fully struc- pared by direct hydrogenation of the metals, but only
turally characterized. by solution methods. They are very reactive, presum-
The crystal structures of the two modifications of ably nonmetallic, and stoichiometric solids with a pro-
CrH that may be prepared by electrochemical and high- nounced covalent character. CuH probably crystallizes in
pressure methods are based on Cu- and Mg-type struc- a wurtzite-type structure.
tures of Cr, respectively, but the H positions have not yet Crystal structures and properties of the binary transition
been determined. Manganese, iron, and cobalt hydrides metal hydrides underscore the difficulty of a strict cate-
can only be synthesized under very high hydrogen pres- gorization of metal hydrides according to their chemical