New phase transitions in ceramic SrSnO3: Raman scattering analysis and differential thermal analysis

Abstract

We report a continuous or nearly continuous order-disorder transition from orthorhombic (pseudo-tetragonal) to orthorhombic structure on heating in SrSnO3 at 377 degrees C, and a discontinuous transition near 260 degrees C. The transition at 377 degrees C is manifested in an order of magnitude increase in Raman linewidths and in lambda-shaped anomaly in the specific heat. Previous work (Mountstevens et al 2005 Phys. Rev. B 71 220102R) has interpreted the Pnma-Imma transition at 636 degrees C as a continuous order-disorder transition and emphasized that this would be unique for an octahedron-tilting transition in oxide perovskite, as they have always been found to be displacive ('Glazer's rule'). Our work on Raman scattering and differential thermal analysis shows that the disorder sets in at much lower temperature 377 degrees C, so that the 636 degrees C transition is a disorder-disorder or a displacive transition within an already highly disordered structure. Hence it may not be as unusual as had been claimed, and Glazer's rule remains inviolate.