Birefringence of mechanically stressed Rb2ZnCl4 crystals

The spectral and temperature dependences of the birefringence of mechanically free and uniaxially stressed Rb₂ZnCl₄ crystals have been investigated. It is shown that annealing a crystal leads to step changes in the birefringence at the phase transition from the incommensurate to commensurate phase, whereas the birefringence of as-prepared crystals changes continuously. It is established that the birefringence of Rb₂ZnCl₄ crystal is sensitive to uniaxial pressure; however, uniaxial stress does not change the character of its dispersion and temperature dependences. A new "pseudoisotropic" state of this crystal and a baric shift of the points of paraelectric phase–incommensurate phase–commensurate phase transitions to different temperature regions are found.