Critical behaviour of optical birefringence near the normal–incommensurate phase transition in[N(CH3)4]2ZnCl4 crystals under the influence of hydrostatic pressure

Abstract

We have estimated the Ginzburg numberG governing crossover from the asymptotic to the classical criticalbehaviour near the normal–incommensurate phase transition in[N(CH3)4]2ZnCl4 (TMAZC) crystals andthe dependence of G on the hydrostatic pressure (0.1–330 MPa), following from the experimental datafor the optical birefringence and the quantitative analysis of temperaturederivatives of the birefringence in the framework of the approximation of weakGaussian fluctuations. The Ginzburg number found experimentally for TMAZC isG ∼ 8 × 10 − 3 at theatmospheric pressure and a considerable part of it is assumed to refer to structural defects. It is shownthat the G value for TMAZC decreases with increasing hydrostatic pressureand, based on analysis of the literature birefringence data forCs2CdBr4 andCs2HgBr4 crystals, this behaviour might be expected to be generally typical for all theA2BX4 family. The results obtained are discussed using a phenomenological theory of second-orderstructural phase transitions. In particular, they provide a basis for estimating thelimits of the asymptotic critical region in TMAZC and demonstrate that themajor part of the incommensurate phase should belong to the crossover region.