PRESSURE-INDUCED CHANGE OF THE STEREOCHEMICAL ACTIVITY OF A LONE ELECTRON PAIR. Björn Winkler, Mineralogisch-Petrographisches Institut der Christian Albrechts Universität, Olshausenstr. 40, D-24098 Kiel, Germany; and V. Milman, BIOSYM/MSI 240/250 The Quorum, Barnwell Road, Cambridge CB5 8RE, UK.

Ab initio total energy calculations based on density functional theory and the generalized gradient approximation in conjunction with a constant pressure minimisation algorithm have been used to investigate the pressure induced phase transition in CsGeCl₃. The results of the calculations are in good agreement with experimental data, in that the calulated zero pressure structure is rhombohedral, while the high pressure (5 GPa) structure is cubic. The calculations show that the transition from the rhombohedral to the cubic structure involves a fundamental change in the stereochemical activity of the `lone electron' pair of Ge<sup>2+</sup> which has partially p-character in the low pressure phase and becomes an `inert pair' with s-character in the high pressure phase.