THE BODY-CENTERED TETRAGONAL HIGH-PRESSURE PHASE OF TIN

White tin (beta-Sn) transforms to a body-centered tetragonal structure when an external pressure above 95 kbar is applied. The axial c/a ratio increases slightly from 0.91 with increasing pressure until it is around 500 kbar jumps to 1.0 (b.c.c. structure). This peculiarity, i.e. the fact that Sn does not transform directly into the highly coordinated b.c.c. structure is a result of the b.c.t. structure being stabilised by the tetragonal splitting of the H15 p-level close to the Fermi level. A simple model allowing semiquantitative calculations is applied to predict the pressure range over which the b.c.t. structure is stable.