Optical-, dielectric- and elastic studies of the successive phase transitions in Rb2ZnCl4

Abstract

Abstract Optical properties such as refractive indices, birefringence and light transmission spectra of Rb2ZnCl4 at room temperature are reported. The birefringences Δni, for light propagation directions along all the three crystallographic axes (i = a,b,c) and the orientation of the indicatrix have been measured between 5 and 750 K. The spontaneous changes of the birefringence observed at the incommensurate, commensurate and low temperature phase transition have been evaluated and the relative influence of electro-optic, elasto-optic and order-parameter induced spontaneous changes of the birefringence below TI and TC are discussed. It is shown that the optical indicatrix rotates around the a- and b-axis below the low temperature transition. This indicates, that the low temperature phase must have space group symmetry P1-C1 1. The ultrasonic measurements performed with the pulse-echo technique show, that the two longitudinal modes c22 and c33 soften at TI. None of the three elastic moduli show a pronounced change at the incommensurate-commensurate phase transition temperature TC. A small step in the stiffnesses c11 and c22 however occurs at temperature TC. The velocity of transverse 10 MHz ultrasonic waves propagating along the b-direction shows a small step at TC comparable with those observed for the c11- and c22 modes. The most dramatic changes however occur for the propagation direction along c, where the ultrasonic velocity increases rapidly, approaching the phase transition temperature TC from above. In the ferroelectric phase III the ultrasonic velocity reaches a maximum at T=169.5 K (crystallographic axes: b > a > c, b ≃ □3c, a: pseudohexagonal axis). Small peaks in the dielectric constants εa and εb have been observed at the low temperature phase transition, in addition to the peak in εc at the ferroelectric lock-in transition.