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

We report a continuous or nearly continuous order-disorder transition from orthorhombic (pseudo-tetragonal) to orthorhombic on heating in SrSnO$_{3}$ at 377 $^{\circ}$C. The transition at 377 $^{\circ}$C is manifested in an order of magnitude increase in Raman linewidths and in a $\lambda $-shaped anomaly in the specific heat. Previous work [Mountstevens et al., Phys. Rev. B71, 220102R (2005)] has interpreted the Pnma -- Imma transition at ca. 636 $^{\circ}$C as a continuous order-disorder transition and emphasized that this would be unique for an octahedron-tilting transition in oxide perovskite, which are always displacive. Our work on Raman scattering and Differential Thermal Analysis shows that the disorder sets in at much lower temperatures 377 $^{\circ}$C, so that the 636 $^{\circ}$C transition is disorder-disorder, or a displacive transition within an already highly-disordered lattice. Hence it may not be as unusual as had been claimed.