Influence of Mechanical Stress on the Birefringence Properties of (N(CH₃)₄)₂ZnCl₄Crystals

Abstract

The influence of applied mechanical stress on the birefringence of (Ν(CH3)4)2ZnCl4 crystal in the temperature region of its incommensurate phase was investigated using Senarmonth,s method (λ = 633 nm). The value of pinning force for the samples with different defectiveness was experimentally determined.PACS numbers: 77.80.BhAs it is known, the interaction of incommensurate (IC) stucture with defects causes some essential properties of IC-phases. The temperature hysteresis of IC modulation wave vector [1], dielectric permittivity [2] and linear birefringence [3] may be related to such properties. The occurrence of temperature hysteresis is an evidence of the presence of some mechanisms preventing the processes of solitons creation and annihilation [1,2]. One of such mechanisms may be soliton pinning on impurities, vacancies, dislocations and so on. Experimental studies of impure crystals confirm the above-mentioned mechanism of temperature hysteresis [4]. Irregular character of the pinning force acting on solitons breaks a regular distribution of solitons in the vicinity of Τc , and brings the solitons to metastable chaotic state. Apart from the pinning force Fpin , the solitons are influenced by the elastic coupling foree Fel acting between solitons and, in the case of a crystal sample in an external field, by the applied force Fappl. Therefore, the total force F = Fpi n + Fel + Fappl should be irregularly distributed in the sample volume.The aim of this paper is to study the influence of the applied mechanical stresses on soliton-defect interaction and an experimental determination of the pinning force Fin value. Ferroelectric (Ν(CH3)4)2 ΖnCl4 crystals were chosen as the objects of our investigation, since a smooth variation of IC-parameter δ is characteristic of the IC-phase of these crystals at the temperatures Τc = 280 K T, = 296.6 K. The samples were grown using a slow evaporation method at room temperature from aqueous solution of ZnCl2 and Ν(CH3)4Cl salts taken in (409)

Behaviour of the Optical Birefringence in Conditions of the "Viscous" Interaction

“…At the first approximation it is possible to assume that Ψι b 1 Εz , where b l is the expansion coefficient [28]. Therefore, the expression for δ(Δni) can be written in the following form:…”

“…The anomalies on temperature dependences of optical birefringence [1] and dielectric permittivity [2] are observed in conditions of the "viscous" interaction. Their shape vIew has been discussed in different works [3,4].…”