B1-B2 transition in CaO and possibility of CaSiO3-perovskite decomposition under high pressure

Abstract

X-ray powder diffraction measurements of CaO at high pressure and temperature have been performed using a lever-spring type diamond anvil cell equipped with an external ring heater. The B1 structure transformed into B2 at about 61.2-63.2GPa at room temperature during the compression, and the back transformation from the B2 to B1 structure was found at 59.8GPa during the depression. The equation of states of both the B1 and B2 structures are obtained by Birch-Murnaghan equation. Because B2 structure of CaO is unstable at ambient conditions, the bulk modulus of the B2 structure at high temperature was firstly determined from the P-V-T curve. The B1-B2 transition pressure slightly lowers from 58.8GPa at 295K to 53.1GPa at 685K, resulting in dP/dT<0. A back transformation from the B2 to B1 structure by depression shows a large hysteresis. The B1-B2 transition highly depends on pressure rather than temperature. Only from the volumes of the CaSiO3 components, CaSiO3 perovskite possibly decomposes to SiO2 (CaCl2 type) and CaO (B2) in the pressure and temperature range of the lower mantle.