PHASE-TRANSFORMATIONS AND VOLUME OF THE IV-VI GETE SEMICONDUCTOR UNDER HIGH-PRESSURE

The properties of the ferroelectric IV-VI semiconductor GeTe have been studied as a function of pressure up to 25 GPa at room temperature, by powder X-ray diffraction in a diamond anvil cell. In a truly hydrostatic medium, the transition from the rhombohedral to cubic NaCl-type structure was not observed up to 8 GPa while it occurred below 5 GPa in a solid pressure transmitting medium. In both cases, no discontinuity could be observed in the lattice parameter. However, the transformation probably remains of the first order, as at normal pressure, but with a very small volume discontinuity. The presence of small anisotropic stress components has a large effect on the equilibrium pressure of the transition and possibly on its mechanism. In addition, a transition to an orthorhombic structure takes place in the same pressure range; no cubic CsCl-type structure was evident below 25 GPa.