What defines an interstitial solid solution?

An interstitial solid solution is defined by the phenomenon where solute atoms occupy the voids or interstitial spaces in the crystal lattice of the solvent material. In solid solutions, the primary structure remains that of the solvent, but smaller solute atoms can fit into the gaps between the larger atoms of the solvent's crystal lattice without significantly disrupting the overall structure.

This concept is crucial for understanding the behavior of many alloys and ceramic materials, where different atoms combine to form a single homogeneous phase at the atomic level. The size of the solute atoms is a key factor; they must be considerably smaller than the solvent atoms to fit into the interstitial sites without causing strain in the lattice structure.

The other options do not accurately depict the nature of interstitial solid solutions. For instance, substitutional solid solutions involve solute atoms replacing solvent atoms in the lattice, not occupying interstitial spaces. The choices related to gases and non-variable composition do not apply to the definition of solid solutions in the context of interstitial structures.