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

摘要

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 14GPa. Samples synthesized below 14GPa 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 13-14GPa 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 1/2Ca(2)Si(2)O(6) (cubic perovskite) + 1/2Ca(2)Al(2)O(5) (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.