High-pressure structural configuration and phase transition in celsian, BaAl2Si2O8

Abstract In situ high pressure X-ray diffraction study was performed on celsian (Cls97Or3) from Jakobsberg, Sweden. A single-crystal of celsian was loaded in an ETH-type diamond anvil cell and unit-cell parameters were measured at 20 different pressures up to 6.0 GPa at room T. The evolution of the unit-cell parameters and volume as a function of pressure shows a discontinuity at P > 5.59 GPa indicating a displacive first-order phase transition. The P-V data were fitted by a second-order Birch-Murnaghan EoS only up to 2.55 GPa, because for P > 2.55 GPa a slight change in the compressional behavior of the unit-cell volume is observed. The resulting EoS coefficients are V0 = 1461.4(1) Å3 and KT0 = 88.1 (6) GPa. A second crystal of celsian was loaded in the DAC cell and single-crystal in situ HP X-ray diffraction was performed at P = 0.0001, 2.1, 4.2, 5.5, 5.9, 6.5 and 7.8 GPa. The data collections between 0 and 5.5 GPa show only a- and b-type reflections confirming the I2/c space group. The appearance of c and d-type reflections at 5.9, 6.5 and 7.8 GPa, the analysis of the systematic absence and the structural refinements define the HP phase transition as an I2/c - P21/c transition. The most significant changes with compression in celsian are the deformation in the Ba polyhedra and the variation in the T-O-T angles.