Body face
- compound whose element ratios deviate from the whole-number ratios of its ideal formula, and can vary with conditions
- caused by crystal defects: vacant sites or extra (interstitial) atoms, with charge balanced by mixed oxidation states
- e.g. wüstite Fe₁₋ₓO (iron vacancies, some Fe³⁺), oxygen-poor TiO₂₋ₓ
- clathrate hydrates: fixed number of cages, but a variable fraction of them occupied by guest molecules