The relation of inner compressibilities to the pressure dependence of atomic position parameters in simple crystal structures

The application of hydrostatic pressure to crystals in which some atoms occupy sites with polar point-group symmetry causes the position coordinates of these atoms to change. This phenomenon is here related to inner compressibility tensors. These arise naturally in the theory of inner elasticity, where they are linear combinations of the macroscopic linear compressibilities weighted by components of the internal strain tensors, and they indicate the ease with which the separation between pairs of sublattices may change under pressure. The form of the inner compressibility tensors is presented for eleven simple crystal structures involving up to four atoms in the basis. Finally, the inner compressibilities and con- straining equations for components of the internal strain are obtained from the pressure dependence of the structure of the elements As, Sb, Bi, Se, Te and I.