High-pressure phase relations in Ca2AlSiO5.5 and energetics of perovskite-related compounds with oxygen defects in the Ca2Si2O6–Ca2Al2O5 join

High-pressure high-temperature experiments of Ca2AlSiO5.5 were performed at 7.5–23 GPa and 1473–1873 K to clarify the high-pressure phase relations. Rhombohedral perovskite phase was recovered in runs at pressure higher than about 14 GPa. Samples synthesized below 14 GPa showed a powder X-ray diffraction pattern different from that of the rhombohedral perovskite phase. A phase boundary between the rhombohedral perovskite phase and the low-pressure phase was determined tightly to be at 13–14 GPa over the experimental temperatures. By measuring drop-solution enthalpies of the Ca2AlSiO5.5 rhombohedral perovskite, Ca2AlSiO5.5 low-pressure phase and brownmillerite-type Ca2Al2O5, their formation enthalpies from constituent oxides were obtained to be −67.98 ± 5.88, −94.64 ± 4.16, −28.10 ± 3.81 kJ/mol, respectively. The formation enthalpy of Ca2AlSiO5.5 rhombohedral perovskite suggests that the enthalpy of reaction from 1/2Ca2Si2O6 (cubic perovskite) + 1/2Ca2Al2O5 (brownmillerite) is about −70 kJ/mol. This implies that disordering of Si, Al and oxygen defects may contribute to reduce energy of Ca2AlSiO5.5 rhombohedral perovskite.