Optical activity and birefringence of the incommensurate phase ofRb2ZnBr4

Abstract

The high-accuracy universal polarimeter (HAUP) opened a way for revealing optical activity in incommensurate phases of several ${\mathit{A}}_2$ ${\mathit{BX}}_4$-type crystals. Ortega, Etxebarria, Breczewski, and Tello found recently that a gyration tensor component ${\mathit{g}}_{23}$ of ${\mathrm{Rb}}_2$ ${\mathrm{ZnBr}}_4$, which had been reported by Meekes and Janner to appear in the incommensurate phase, vanished. They drew the conclusion that the incommensurate phase of this material lacked optical activity. In order to review these controversial results, we made a critical study of all the components ${\mathit{g}}_{\mathit{i}\mathit{j}}$ of ${\mathrm{Rb}}_2$ ${\mathrm{ZnBr}}_4$ by using the refined HAUP. It was found that the component ${\mathit{g}}_{23}$ of ${\mathrm{Rb}}_2$ ${\mathrm{ZnBr}}_4$ clearly appeared at the incommensurate transition point and increased conspicuously with decreasing temperature. Therefore the incommensurate phase of ${\mathrm{Rb}}_2$ ${\mathrm{ZnBr}}_4$ was found to be optically active.