The volume of formation of Schottky defects in ionic crystals

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Abstract
The usual lattice-statics expressions for the volume change due to the formation of Schottky defects in ionic crystals lead to results which differ from those obtained experimentally. In particular, the volume of lattice relaxation is predicted to be negative but is observed instead to be positive. In the present paper a correction to these usual expressions is derived and then evaluated in the Mott-Littleton approximation. The numerical predictions are now in much better agreement with experiment and in close agreement with volumes of formation of Schottky defects calculated directly by a defect simulation program (Gillan 1981, following paper). The need for the correction arises because the usual lattice-statics expressions are derived by assuming that the distorted lattice is strictly harmonic (i.e. force constants independent of volume) whereas only a quasi-harmonic assumption is required.

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