2017 Volume 125 Issue 4 Pages 364-370
We investigated the crystal structures of high- and low-temperature phases in Sr4[Al6O12]SO4, and their thermal behavior by high-temperature X-ray powder diffraction (Cu Kα1), differential thermal analysis, and temperature-dependent Raman spectroscopy. The crystal structure at 298 K was isomorphous with that of Ca4[Al6O12]SO4 (space group Pcc2 and Z = 4). The structural model at 573 K (space group I23 and Z = 2) was characterized by the positional disordering of oxygen atoms that form [SO4] tetrahedra. The maximum-entropy method-based pattern fitting method was used to confirm the validity of these structural models, in which conventional structure bias caused by assuming intensity partitioning was minimized. The starting temperature of the cubic-to-orthorhombic transformation during cooling (= 524 K) was slightly higher than that of the reverse transformation during heating (= 519 K). The negative thermal hysteresis (= −5 K) strongly suggested the transformation being thermoelastic. At around the transformation temperature during heating, the vibrational spectra, corresponding to the Raman-active [SO4] internal stretching mode, showed the continuous and gradual change in the slope of full width at half maximum versus temperature curve. This strongly suggests that the phase transformation would be principally accompanied by the statistical disordering of oxygen-atom positions, without distinct dynamical reorientation of the [SO4] tetrahedra.