Comment on the effect of pressure on the structure of Ge III and Si III

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Abstract
Calculations of the bulk modulus of Si III and Ge III are presented. They are based on Keating's expression for the elastic energy. A numerical procedure is used to relax the structures to equilibrium at various volumes. The results indicate that, for reasonable force constants, the Ge III (or ST-12) structure is about twice as compressible as the diamond cubic and Si III (or BC-8) structure. This is due to the contribution from large internal displacements, which must consequently be carefully calculated in any analysis of pressure-dependent properties of Ge III.