Systematic Technique for the Study of Interphase Boundaries in Structural Phase Transitions

We present a group theoretic technique based on the concepts of the direction of the order parameter (OP) and the OP isotropy group to study systematically the interphase boundaries in materials undergoing displacive transitions and construct all independent OP gradient invariants, determine primary and secondary OP profiles, domain wall energy, stability, and the phonon dispersion in the vicinity of the soft mode wave vector. We illustrate the technique for an improper ferroelastic $O_h^1$- $D_{4h}^{17}$ transition driven by a soft $M_5^{-}$ mode observed in alloys with CsCl structure.