How does hydrogen fit into the crystal lattice of an interstitial hydride?

In metallic hydrides—formerly referred to as interstitial hydrides—hydrogen atoms are incorporated into the crystal lattice of the host metal. In many instances, the metal lattice simply expands to accommodate the dissolved hydrogen without undergoing any other structural changes.1

These compounds are considered intermediate between alloys and salts. They essentially consist of protons and metal atoms situated within an electron sea, which accounts for their metallic characteristics, such as lustre and electrical conductivity. Because these hydrides often exhibit variable compositions, they are frequently classified as nonstoichiometric systems.2