Soft Acoustic Modes and the Forming Process of the Ferroelastic Domains in KD₃(Se0₃)₂

Abstract

Many studies of an unstable lattice vibration mode (soft mode) have been reported in the structural phase transition of crystals, for instance, ferroelectric or ferroelastic crystals. The ordered phase of these crystals usually shows,below the phase transition point Tc, a domain pattern in order to lower the bulk free energy of the crystal. The domain pattern of the ferroelectric crystal is composed of the spontaneous electric polarization Ps minimizing the electrostatic energy of the crystal. The ferroelastic domains are composed of the spontaneous strains x_s. The relation between the unstable (soft) lattice vibration and the domain has not been clarified enough though a close relation might be expected. The relation would give us a microscopic understanding of the macroscopic domain pattern being consistent with a phenomenological consideration.