Phase transformation of Ca4[Al6O12]SO4 and its disordered crystal structure at 1073K

Abstract : The phase transformation of Ca4[Al6O12]SO4 and the crystal structure of its high-temperature phase were investigated by differential thermal analysis temperature-dependent Raman spectroscopy and high- temperature X-ray powder diffraction( CuKα1). We determined the starting temperature of the orthorhombic-to-cubic transformation during heating (¼711K) and that of the reverse transformation during cooling (¼742K). The thermal hysteresis was negative (¼ 31K), suggesting the thermoelas- ticity of the transformation. The space group of the high temperature phase is I43m with the unit-cell dimensions of a¼0.92426(2) nm and V¼0.78955(2) nm3 (Z¼2) at 1073K. The initial structural model was derived by the direct methods and further refined by the Rietveld method. The final structural model showed the orientational disordering of SO4 tetrahedra. The maximum-entropy method-based pattern fitting method was used to confirm the validity of the split-atom model , in which conventional structure bias caused by assuming intensity partitioning was minimized. At around the transformation temperature during heating, the vibrational spectra, corresponding to the Raman-activeSO4 internal stretching mode, showed the continuous and gradual change in the slope of fullwidth at halfmaximum versus temperature curve. This strongly suggests that the orthorhombic-to-cubic phase transformation would be principally accompanied by the statistic al disordering in orientation of the SO4 tetrahedra, without distinct dynamical reorientation.