Piezo-optic properties of LiNH 4 SO 4 crystals
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ICAL PROPERTIES OF CRYSTALS
Piezo-Optic Properties of LiNH4SO4 Crystals V. Yo. Stadnyk, R. S. Brezvin, M. Ya. Rudish, P. A. Shchepanskii, and V. Yu. Kurlyak Lviv National University, Lviv, 79000 Ukraine e-mail: [email protected] Received June 9, 2014
Abstract—The effect of uniaxial stresses applied along the principle crystallographic directions on the dispersion and temperature dependences of birefringence of LiNH4SO4 crystals is studied. The anomalies of the temperature dependence of birefringence near 220 K have been observed, and their position is shown to depend on the magnitude and direction of crystal uniaxial compression. The spectral and temperature depen0 dences of combined piezo-optic coefficients π im of these crystals are investigated. It is demonstrated that these coefficients have different signs, are characterized by significant dispersion, and weakly change with temperature; they also have equal absolute values in the vicinity of the isotropic point. DOI: 10.1134/S106377451502025X
INTRODUCTION 0 A study of combined piezo-optic coefficients π im of crystals, which are very important crystal-optics parameters, is interesting in and of itself because it allows one to analyze the temperature and spectral deformation of an optical indicatrix under mechanical strain. It is also interesting to analyze the behavior of piezo-optic coefficients in the vicinity of isotropic points (IPs) and possible phase-transition (PT) points. Previous investigations of piezo-optic coefficients 0 π im of А2ВХ4 crystals [1, 2] revealed significant anom0 alies in the vicinity of the PT point. The π im values in different phases may differ by 1–2 orders of magnitude. The aim of this study was to investigate the spectral and temperature dependences of piezo-optic coeffi0 cients π im of LiNH4SO4 crystals. Lithium ammonium sulphate (LAS) LiNH4SO4 crystals may exist in the α and β modifications. β crystals are ferroelectric at room temperature and ferroelastic below 285 K. The ferroelectric phase of LAS crystals is one possible version of ordering of tetrahedral groups relative to the rotations around the pseudohexagonal с axis. This phase is characterized by a pseudohexagonal tridymite-like structure consisting of SO4 and LiO4, vertex-sharing tetrahedra, which form six-membered rings oriented perpendicular to the Z axis. One half of the tetrahedra are vertex-down oriented; the other half are vertex-up oriented to be connected with the next
layer of tetrahedra. The thus formed voids contain an ammonium group [3, 4]. The difference between the α- and β-LAS crystals is that the SO4 and LiO4 tetrahedra may have not only common vertices, but also common edges in the Z direction. The neighboring layers interact with the nitrogen atoms of ammonium groups through hydrogen bonds to form a layered structure. The α-phase crystals are formed as a result of growth at room or lower temperatures, while the β-phase crystals are formed at temperatures above 30°С [5]. Previously we investigated the dispersion dependences of refractive indices
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