Lattice dynamics and structural phase transitions in the tetrafluoroaluminates AA1F4(A = Tl, Rb, K)

Abstract
The tetrafluoroaluminates AA1F4 with layer structure undergo several structural phase transitions which can be related to soft modes condensation. We propose a rigid ion model to describe the lattice dynamics properties of these materials. The results of the model applied to RbA1F4 are presented. The calculated phonon spectrum reveals the presence of low frequency modes which are consistent with the sequence of transitions observed in this compound.