Non-stoichiometric hydrides are compounds that do not have a fixed ratio of hydrogen to the other element. Among the given options, metallic hydrides are non-stoichiometric hydrides. Let's understand why metallic hydrides are non-stoichiometric in detail.

Metallic Hydrides:
Metallic hydrides are formed when hydrogen reacts with metals. These hydrides are characterized by the presence of metallic bonding between the metal atoms. Metallic bonding involves the delocalization of electrons in a sea of positive metal ions.

Non-Stoichiometric Nature:
The non-stoichiometric nature of metallic hydrides arises due to the ability of hydrogen to occupy interstitial sites within the metal lattice. The hydrogen atoms can occupy these interstitial sites in different ratios, leading to a variable composition.

Key Points:
- Metallic hydrides have a variable composition, which means they do not have a fixed ratio of hydrogen to the metal.
- The hydrogen atoms occupy interstitial sites within the metal lattice, leading to a non-stoichiometric nature.
- The ability of hydrogen to occupy different interstitial sites and form different types of bonds with the metal atoms contributes to the non-stoichiometric behavior.

Example:
An example of a metallic hydride is palladium hydride (PdHx), where x can vary between 0.02 and 0.7. This indicates that the ratio of hydrogen to palladium can vary within this range, highlighting the non-stoichiometric nature of the compound.

In conclusion, metallic hydrides are non-stoichiometric hydrides because they do not have a fixed ratio of hydrogen to the metal. The ability of hydrogen to occupy interstitial sites within the metal lattice and form different types of bonds contributes to their non-stoichiometric behavior.