STRUCTURAL PHASE-TRANSFORMATIONS AND THE EQUATIONS OF STATE OF CALCIUM CHALCOGENIDES AT HIGH-PRESSURE

The crystal structures of three calcium chalcogenides CaS, CaSe, and CaTe were investigated by energy-dispersive x-ray-diffraction techniques using a synchrotron source to a static compression up to 52 GPa. For CaS and CaSe, a first-order phase transformation from the NaCl phase to the CsCl phase was observed at 40 and 38 GPa, respectively. For CaTe, it first transforms from the NaCl phase to an intermediate state possibly with a mixture of NaCl and MnP phases at 25 GPa, and then to the CsCl phase above 33 GPa. The phase transitions were reversible with small hysteresis. The equations of state and bulk moduli were obtained for all three compounds. The systematic behavior among the calcium chalcogenides under compression and the unusual transition sequence of CaTe are discussed. © 1994 The American Physical Society.