Non-Stoichiometric hydrides are hydrogen-deficient compounds formed by the reaction of dihydrogen with d-block and f-block elements.
These hydrides do not follow the law of constant composition. For example: LaH2.87, YbH2.55, TiH1.5 – 1.8 etc. Alkali metals form stoichiometric hydrides.
These hydrides are ionic in nature. Hydride ions have comparable sizes (208 pm) with alkali metal ions. Hence, strong binding forces exist between the constituting metal and hydride ion. As a result, stoichiometric hydrides are formed.
Alkali metals will not form non-stoichiometric hydrides.