Abstract:

"Nonstoichiometric compound" is a widely accepted concept in solid-state chemistry and materials chemistry. However, in the literatures, the meaning of this concept is ambiguous. In this paper, based on the composition analyses for selected nonstoichiometric compounds, the relation among the defect, solid solution and the nonstoichiometric compound is built. According to the structure analyses for some examples, the essential difference between Daltonid and Berthollid, and the relation between the Berthollid and its parent compound are clarified. As a result, a comprehensive description for "nonstoichiometric compound" is given. Under certain conditions, crystalline inorganic materials (metals or compounds) may contain lattice defects (mainly vacancies, interstitials, substitutional defects, valence changed cation defects and so on) with high concentrations (quantitatively detectable by routine chemical methods), which can be considered as solid solutions. For the solid solutions with the compositions of their parent compounds at the boundaries of the solid solution ranges, the parent compounds are termed as Daltonids and the solid solutions are termed as "Daltonid solid solutions"; while no parent compounds can be found at the boundaries of the solid solution ranges, the solid solutions are named as Berthollids, their parent compounds (virtual compounds) may be located outside of the solid solution ranges. Since their compositions vary and deviate from the ratio of small natural number, as a broad concept, both Daltonid solid solutions and Berthollids are all considered as "nonstoichiometric compounds". Since real parent compounds do not exist, as a narrow concept, only Berthollids are regarded as "nonstoichiometric compounds".

Key words: Nonstoichiometric compound, Defect, Solid solution, Solid solution mechanism