Structural phase transitions in ferroelastic RbAlF4. I. DSC, X-ray powder diffraction investigations and neutron powder profile refinement of the structures

TLDR: In this article, the structural phase transition of the layer compound RbAlF4 is shown to undergo structural phase transitions at 553K and 282K under investigation by DSC and X-ray powder diffraction.

Abstract: The layer compound RbAlF4 is shown to undergo structural phase transitions at 553K and 282K under investigation by DSC and X-ray powder diffraction. The structures of the three phases are determined by neutron powder profile refinement. The high-temperature phase has the ideal TlAlF4 structure P4/mmm which corresponds to the tilt system a0a0c0. The intermediate room-temperature phase is confirmed as P4/mbm, the tilt system is a0a0c+. The low-temperature phase is consistent with the space group Pmmn which corresponds to both ap+b0c+ (near the transition temperature) and ap+bp+c+ (at very low temperature). The 553K phase transition is a non-ferroic phase transition while the 282K transition is ferroelastic. According to Aizu the low-temperature phase is a 4/mmmFmmm species. The temperature behaviour of the structural parameters is discussed, particularly in the framework of ferroelasticity.