Printed in Great Britain. Pergamon Press Ltd THE BODY-CENTERED TETRAGONAL HIGH-PRESSURE PHASE OF TIN

White tin (P-S,) transforms to a body-centered tetragonal structure when an external pressure above 95 kbar is applied. The axial c/a ratio increases lightly from 0.91 with increasing pressure until it is around 500 kbar jumps to 1.O (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 His 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. TIN ASSUMES the diamond structure, but only at low pressures and low temperatures. At atmospheric pressure this o form is stable below 13°C (grey tin), whereas, the usual metallic “white tin ” is the modification observed at higher temperatures and pressures. Its crystal structure, P-Sri, is derived from