these metal hydrides or interstitial hydrides may have variable
composition (for example TiHx 7), depending on the uptake of
hydrogen, i.e. they are non-stoichiometric. One further property in
particular distinguishes these metal hydrides from the ionic hydrides;
in the latter, uptake of hydrogen is not only quantitative but causes
a contraction, i.e. the centres of the metal atoms (which become
Figure 5.1. Interstitial positions between layers of metal atoms
cations) move closer together—the metal lattice is, as it were, drawn
together. In the metal hydrides, there is no such contraction, and,
indeed, the metal atoms may move apart slightly. Hence formation
of an ionic hydride leads to an increase in density, but formation of
a metal hydride causes a decrease in density.
REACTIONS WITH