Soft modes and martensitic transformations in K1—xRbxAlF4

Abstract

A phenomenological theory of phase transitions in the solid solution of tetrafluorites KAIF,-RbAlF, has been advanced. Causes have been established that give rise to a soft MX branch in the low-frequency phonon spectrum of KAIF, and K,-Jtb,AIF, (x Q: 1). It has been shown that the observed specificities in the thermodynamic behaviour and the dynamic properties of these compounds can be accounted for by the presence of a relatively "free" subsystem of potassium cations.

Crystal structure and phase transitions of a layered perovskite-like CsScF4crystal

“…A similar free energy but with expansion to the fourth power of η OP was considered in ref. 47. The given FE was analyzed and phase state diagrams were described in a standard manner similar to the one described in ref.…”

“…Below, we assume δ = 0; it was shown in ref. 47 that such an assumption should not significantly affect the results to be produced.…”