Whereas a pure molecular species has a definite stoichiometry, this is not often true for solids. Defects in crystals will include vacancies (atoms missing from their expected sites) and interstitials (extra atoms in sites normally open in the unit cell). An imbalance of defects involving different elements shall introduce nonstoichiometry. This is common in compounds of transition metals, where variable oxidation states are found. As describe, the sodium tungsten bronzes are formulated as NaxWO3, where x can have any value in the range 0-0.9.
Fig. 1. Alternative views of the CsCl structure