Soft modes and martensitic transformations in K1—xRbxAlF4

A phenomenological theory of phase transitions in the solid solution of tetrafluorites KAlF4-RbAlF4 has been advanced. Causes have been established that give rise to a soft MX branch in the low-frequency phonon spectrum of KA1F4 and K1-xRbxAlF4 (x << 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.

Keywords:

• Tetrafluorites
• phase transitions
• phenomenological theory
• martensitic transformation
• soft modes