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