X-ray mono-Laue investigations of KAlF4 and RbAlF4 : analysis of the displacive and martensitic transformations

Abstract
The technique of « monochromatic Laue » X-ray scattering has been used to carry out a comparative study of the different phases of RbAlF4 and KA1F4. In both materials diffuse scattering has been observed. In the former case where displacive transitions occur, the diffuse scattering is surrounding the superlattice Bragg peaks rising below Tc and is well interpreted in the light of the phonon spectrum. In the latter case where the transition is « martensitic », the diffuse scattering is located along diffuse rods and several extra-spots have been observed. The domain arrangement following the martensitic transformation is studied