NONSTOICHIOMETRY IN OXIDES

Nonstoichiometry phenomena have been extensively studied in oxide systems. It is proposed that some models developed for oxides could be usefully extended for understanding nonstoichiometry in other systems. Many aspects of nonstoichiometry are closely connected with order-disorder phenomena. The broadfield of nonstoichiometric compounds extends from variable composition phases with randomly distributed point defects (entropy controlled systems) to sets of discrete intermediate compounds, each corresponding with a fully ordered state resulting from assimilation or annihilation of defects (enthalpy controlled systems). Between these two extreme cases, one finds intermediate situations such as clusters with a short range order, shear structures with possibly swinging shear planes, coherent intergrowths and infinitly adaptative structures in which any composition, even irrational, corresponds with an unique ordered structure.