An infrared spectroscopic and single-crystal X-ray study of malayaite, CaSnSiO5

Powder infrared spectroscopy and X-ray diffraction techniques on single cry stals were used to study the thermal behaviour of malayaite, CaSnSiO5. Infr ared spectra show a discontinuity in the temperature evolution of phonon fr equencies and absorbance near 500 K. However, crystal structure data collec ted at 300, 450, 550, 670, and 750 K show no evidence of a symmetry-breakin g phase transition and no split positions. The most obvious change with hea ting is a tumbling motion of the SnO6 octahedra and an increase of the anis otropic displacement factors of Ca. The thermal evolution of the mean-squar e vibrational amplitude of the Ca atom shows a pronounced change in slope n ear 500 K. The evidence suggests that the 500 K anomaly in malayaite is mor e similar in character to the 825 K (beta-gamma) transition as opposed to t he 496 K (alpha-beta) transition in synthetic titanite.