Why do titanium, zirconium, and hafnium form hydrides with variable compositions?

Titanium, zirconium, and hafnium form hydrides with variable compositions because they are classified as metallic hydrides (formerly known as interstitial hydrides). These compounds are intermediate in nature between salts and alloys, consisting of metal atoms and hydrogen within an electron sea.1

Unlike stoichiometric compounds, which have a fixed ratio of elements, these nonstoichiometric systems do not possess a definite composition. When these transition metals absorb hydrogen, they form these alloylike structures that retain metallic characteristics, such as electrical conductivity and lustre, while exhibiting variable physical properties. These hydrides are typically prepared by heating the metals or their alloys with hydrogen gas, a process during which the metals absorb the hydrogen and release heat.1