PRESSURE-INDUCED PHASE-TRANSITIONS IN SPINEL AND WURTZITE PHASES OF ZNAL2S4 COMPOUND

ZnAl2S4 single crystals with spinel (alpha-phase) and wurtzite (w-phas e) structures have been studied by Raman spectroscopy under hydrostati c pressures of up to 300 kbar. Significant changes in the phonon spect rum of the ct-phase have been observed at the critical pressure of 230 kbar, which are attributed to a reversible phase transition to a dens er high-pressure phase, having a similar structure to that of calcium ferrite. In the pressure interval of 180 to 230 kbar, the two phases c oexist. The irreversible disappearance of the Raman signal of w-ZnAl2S 4 doped by Cd at pressures above 90 kbar was attributed to a phase tra nsition to a rocksalt-type structure. This critical pressure is 40kbar lower than that in undoped w-ZnAl2S4 and is explained on the basis of crystal structure quality. Different structures were realized upon re moving the pressure, depending on the highest pressure previously reac hed, such as a mixture of wurtzite and spinel phases, a spinel quasi-c rystalline structure, or a pressure-induced amorphous phase. The behav ior of the quasi-crystalline spinel structure upon repeating the press ure cycle was found to be different from that of the ct-phase single c rystal.