Birefringence of Mechanically Stressed Potassium Tetrachlorine Zincanate Crystals

Abstract

K2ZnCl4 crystals (TCZP) belong to the known type of ferroelectrics with incommensurate phase (ICP). They undergo successive phase transitions (PTs): a second-order one at Ti = 553 K from the paraphase (the space group Pnam) into the ICP with the wave vector q = (1−δ) ∗ 3 , a first-order one at 402 K from the ICP into the ferroelectric commensurate phase (CP) (the space group Pna12), and a first-order low-temperature one into the ferroelastic phase (the space group P2111) [1–5]. Optical studies of the TCZP dealing with temperature and spectral changes in the refractive indices and the birefringence and temperature dependences of the combined piezooptic coefficients have shown anomalies at the PTs [6–8]. Despite a great interest to the TCZP crystals, the effect of uniaxial pressures on their optical characteristics and behavior of their PT points has not been investigated. Uniaxial pressures generally do not change the crystal symmetry. Instead they affect selectively some groups of bonds and the corresponding structural units. This allows obtaining additional information for analysis of the PT mechanisms. On the other hand, studies of the refractive indices and optical birefringence are very sensitive and thus provide useful information about the structure and physical properties of crystals. The purpose of the present work is investigation of influence of the uniaxial mechanical pressure acting along the principal directions X, Y and Z and along the bisectors upon spectral and temperature changes in the birefringences ∆ni of TCZP crystals for the light propagation directions X, Y and Z.