Optical activity and birefringence of the incommensurate phase ofRb2ZnCl4

By using the high-accuracy universal polarimeter, all the components of optical activity tensor g and birefringence Δn of ${\mathrm{Rb}}_2$ ${\mathrm{ZnCl}}_4$, which were studied in limited part before, have been reexamined. A unique component, ${\mathit{g}}_{23}$, was definitely found to appear both in the incommensurate and ferroelectric phases, but all the other components vanished in the two phases. The previous report is revised. It manifests a similar temperature dependence to that found in [N(${\mathrm{CH}}_3$ ${)}_4$ ${]}_2$ ${\mathrm{ZnCl}}_4$; it changes the sign within the temperature region of the incommensurate phase. This phenomenon can be interpreted as due to the difference of optical gyration of soliton regions and that of the commensurate domains. It was confirmed that the critical exponent, approximately 0.42, of the temperature dependence of ${\mathit{g}}_{23}$ agrees with nearly constant values shown by other various ${\mathit{A}}_2$ ${\mathit{BX}}_4$ crystals. However, temperature, where the multisoliton region emerges, was found to differ considerably from that of [N(${\mathrm{CH}}_3$ ${)}_4$ ${]}_2$ ${\mathrm{ZnCl}}_4$.