STOICHIOMETRY, DEFECTS AND ORDERING

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Abstract
The constitution of phases of variable composition — nonstoichiometric compounds and heterotype solid solutions — cannot be understood solely in terms of point defects. Extended defects are important, involving some local modification of the crystal structure ; they include isolated defect clusters, planar faults such as crystallographic shear (CS) planes and chemical twinning, and superlattice ordering which transforms point defects into essential structure elements of a new crystalline order. The completely coherent intergrowth between topologically compatible structures is an important principle. The ordering of extended defects can generate a succession of crystallographically defined structures, closely spaced in their compositions, and closely related in structure to each other and to the host structure. A disordered array of such extended defects is equivalent to an intergrowth between different members of the homologous series and simulates the properties of a phase of variable composition. There is increasing evidence for infinitely adaptive structures : truly nonstoichiometric, yet mono-phasic and fully ordered for every composition. Two types are the 'Vernier structures', with two incommensurable partial lattices, and the adaptive layer stackings, exemplified in systems with changing orientation of CS planes. The behaviour of particular systems is determined by the balance between the net enthalpy of formation of (usually extended) structural defects and the configurational entropy of the defects. Systems may be enthalpy-controlled, their equilibrium state corresponding to a succession of intermediate, stoichiometric compounds, or entropy-controlled, leading to phases of variable composition

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