Equation of state of Ca2AlSiO5.5 oxygen defect perovskite

Abstract

The elastic properties of synthetic low-pressure and high-pressure Ca2AlSiO5.5 oxygen defect perovskites were investigated by in situ X-ray diffraction in a large-volume high-pressure apparatus. The P–V–T data were collected up to 22.75 GPa at room temperature, up to 12.88 GPa and 1,300 K for low-pressure phase and up to 25.76 GPa at room temperature for high-pressure phase. The P–V data at room temperature were fitted using a third-order Birch–Murnaghan equation of state to obtain K0 = 146.1(9) GPa and \(K_{0}^{\prime }\) = 3.64(9) for the monoclinic low-pressure phase, and K0 = 150.4(19) GPa and \(K_{0}^{\prime }\) = 3.16(23) for the rhombohedral high-pressure phase. If \(K_{0}^{\prime }\) was fixed at 4.0, the isothermal bulk moduli were obtained as 142.6(3) and 144.0(8) GPa for low-pressure and high-pressure phase, respectively. Both the low-pressure and high-pressure Ca2AlSiO5.5 oxygen defect perovskites are much softer than pure CaSiO3 perovskite. The P–V–T data of low-pressure phase were fitted by the high-temperature Birch–Murnaghan equation of state to get thermoelastic properties as V0 = 827.0(3) A3, KT = 146.8(22) GPa, \(K_{T}^{\prime }\) = 3.55(21), (∂KT/∂T)P = −0.037(2) GPa/K and αT = 7.67(20) × 10−5 − 3.20(30) × 10−8 T. Based on the results, the density profiles of low-pressure and high-pressure Ca2AlSiO5.5 oxygen defect perovskites were calculated and compared with those of some mantle silicate minerals to discuss the potential occurrence of Ca2AlSiO5.5 oxygen defect perovskite in the Earth’s interior.