35 Cl NQR Study of the incommensurate structural phase transitions

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 commensurate structure. Measurements have also shown the possible existence of commensurate narrow regions which start to develop in the high temperature normal phase. In addition measurements of the ¹³C chemical shift of [N(CH₃)₄]₂ZnCl will be discussed in connection to their incommensurate to commensurate or ferroelectric phase transitions. Measurements of K₂ZnCl₄ show the possible existence of a new incommensurate transition at T ∼ 60°C.