How do some metals absorb hydrogen to form interstitial hydrides?

Metallic hydrides, formerly known as interstitial hydrides, are formed when certain transition metals, lanthanoids, and actinoids absorb hydrogen gas, typically at elevated temperatures.12

The process occurs as follows:

• Absorption and Lattice Expansion: In many cases, the metal's crystal lattice expands to accommodate the dissolved hydrogen atoms within its structure without undergoing other fundamental changes.2
• Heating: These compounds are generally produced by heating the metal or its alloy in the presence of hydrogen gas.1
• Nature of the Bond: These materials 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 properties, such as lustre and electrical conductivity.1

Because these hydrides often have variable compositions, they are frequently classified as nonstoichiometric systems.13