Step 1: Crystal structure of transition metals.
Transition metals have closely packed crystal lattices (FCC or HCP arrangements) where large metal atoms are arranged in a regular pattern.
Step 2: Presence of interstitial voids.
Between the large metal atoms, small empty spaces called interstices (or voids) exist. These voids are of two types: tetrahedral and octahedral.
Step 3: Small non-metal atoms fill the voids.
Atoms of small non-metals such as hydrogen (H), carbon (C), and nitrogen (N) have very small atomic radii. They can fit snugly into these interstitial voids without disrupting the metallic lattice.
Step 4: Result.
The trapped non-metal atoms form non-stoichiometric compounds (e.g., $TiC$, $Mn_4N$) called interstitial compounds. These compounds are harder and have higher melting points than the parent metals. Transition metal lattices have interstitial voids that can accommodate small atoms like H, C, and N, leading to the formation of interstitial compounds.