Q.What are interstitial (metallic) hydrides? Explain why they have a non-stoichiometric composition and why the parent metal retains its metallic character in them.

Many transition metals of the $d$- and $f$-block -- palladium, titanium and the lanthanides among the most commonly cited examples -- can absorb large quantities of hydrogen gas to form what are called interstitial or metallic hydrides.

Why the composition is non-stoichiometric. Unlike an ionic or covalent hydride, where hydrogen forms a definite chemical bond to the other element in a fixed, whole-number ratio, an interstitial hydride forms when small hydrogen atoms simply diffuse into and occupy the empty spaces -- the interstitial voids -- that naturally exist between the atoms of the metal's crystal lattice. Because these voids can be filled to any degree depending on how much hydrogen gas the metal is exposed to (and at what pressure and temperature), there is no fixed ratio of hydrogen atoms to metal atoms; the formula is written with a variable, non-integer subscript, such as $\text{PdH}_x$ with $x$ typically around $0.6$-$0.8$, or $\text{TiH}_{1.5\text{-}1.8}$, and this ratio can be adjusted continuously (within limits) simply by changing the hydrogen pressure.

Why the metal retains its metallic character. Since hydrogen atoms are simply slotting into pre-existing gaps in the metal's own crystal structure, rather than reacting to form a new compound with its own distinct crystal structure, the metal lattice itself remains essentially intact. As a result, the interstitial hydride keeps the parent metal's characteristic metallic lustre and its ability to conduct electricity and heat, unlike ionic hydrides (which are ionic solids) or covalent hydrides (which are molecular substances).

This combination -- the metal absorbing and later releasing large amounts of hydrogen simply by changing pressure/temperature, while keeping its solid, metallic form throughout -- is exactly what makes interstitial hydrides practically useful as a compact, relatively safe way of storing hydrogen gas.

Explain the structural mechanism (H atoms occupying lattice voids, not forming a discrete bond) and connect it to both consequences asked for: non-stoichiometry (voids filled to a variable degree) and retained metallic character (host lattice undisturbed).

Describing interstitial hydrides as simply 'impure' or 'defective' compounds rather than explaining the void-occupation mechanism; forgetting to explain why metallic properties are retained.