Chemical Tips Sharing – Variable-Composition Compounds

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Compounds that do not obey the law of definite proportions are called non-stoichiometric compounds, or compounds with variable proportions. In such compounds, the ratio of the constituent elements is not an integer ratio, and it can vary continuously within a certain range of ratios. Although the ratio of the constituent elements in common solid solutions in metals, mixed crystals in ionic crystals, etc., is also not an integer ratio and can vary continuously within a certain range, they are generally not regarded as non-stoichiometric compounds but rather as solid-state melts. Amorphous compounds lie between ordinary compounds that obey the law of definite proportions and solid solutions; of course, there is no clear boundary between them and these two extremes as well. Typical non-stoichiometric compounds are those with lattice defects; for example, in metal oxides of valence II that belong to the NaCl crystal structure, non-stoichiometric compounds often form as a result of some M2+ ions being absent from their octahedrally coordinated sites or some M2+ ions occupying the tetrahedrally coordinated voids. In these oxides, for example, Fe0.944O and the like suffer from metal defects, in which some of the metals are oxidized to higher oxidation states. In the well-known variable-stoichiometry compound tungsten bronze NaxWO3, 0.3