35Cl NQR Study of the incommensurate structural phase transitions

Abstract

Abstract In the present review the fundamental features of the successive phase transitions in ferroelectric materials from the high temperature normal phase to the incommensurate and then to the commensurate phase are described on the basis of simple model thermodynamic potential using a Landau type expansion of the free energy. It is discussed in relationship with the phase transitions of the recently studied 35Cl NQR spectra of Rb2ZnCl4 and K2ZnCl4. The NQR measurements are compatible with a transformation to an incommensurate phase due to a condensation of a soft mode with q = 1/3(1 - δ)c*, whereas the ferroelectric transition corresponds to an incommensurate to commensurate phase transformation with a discontinuous jump of the deviation δ to zero. The fact that relatively sharp 35Cl NQR lines can be seen in the incommensurate phase is compatible with the recently developed theory of the possible existence of soliton-like domains walls of the incommensurate phase, i.e. discommensurate in an otherwise ...