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

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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.

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