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

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 a_p⁺b⁰c⁺ (near the transition temperature) and a_p⁺b_p⁺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.