Hydrogen and deuterium in transition metal-p element compounds: Crystal chemical aspects of interstitial solid solubility and hydride phase formation

Compounds of transition metals with p elements of groups IIIb–VIIb which have been reported to exhibit hydrogen-absorbing properties are surveyed. Hydrogen is taken up only by those materials that have metallic properties or contain metal clusters with delocalized bonding. Almost all p elements can be constituents of these materials, and at least one transition metal component which is itself capable of dissolving hydrogen or forming hydride phases must be present. The crystallography of the hydride phases is presented and discussed with particular emphasis on results obtained by neutron diffraction methods. The hydrogen atoms are accommodated either in interstitial solid solutions with only minor changes in the host lattices or in new phases formed by major rearrangements of both transition metal and p element atoms. The distribution of the hydrogen atoms on the interstitial sites in the structures appears to be governed by two empirical rules: the hydrogen atoms are always more than 2 Å apart and they preferentially occupy those sites that are most distant from the p element atoms.