What do you understand by the term ‘non-stoichiometic hydrides’? Do you expect this type of hydrides to be formed by alkali metals?

‘Non-stoichiometric’ are those hydrides in which the ratio of the metal to hydrogen is fractional. This fractional ratio is not fixed but varies with temperature and pressure. Many of the d-block elements and f-block elements (lanthanoids and actinoids) react with hydrogen at elevated temperature to give metallic or non-stoichiometric hydrides. For example LaH_2.87, TiH_1.8.,Zr H_1.9, VH_1.6 etc. Since in these hydrides, hydrogen is present in the interstices (holes or voids) existing in between the atoms, therefore these hydrides are also called interstitial hydrides.

Alkali metals do not form non-stoichiometric hydrides and form only stoichiometric hydrides. Alkali metals hydrides are ionic in which H^- ions are present in holes existing in between the atoms in the lattice. Since hydride ion (H⁺) is formed by complete transfer of lone electron of alkali metal to hydrogen, the, therefore, ratio of metal to hydrogen is always fixed. As a result, these alkali metals do not form non-stoichiometric hydrides.