TEMPERATURE AND SPECTRAL CHANGES IN THE REFRACTIVE INDICES OF LiKSO4 CRYSTALS UNDER UNIAXIAL PRESSURES

Abstract

We have studied spectral and temperature variations of the principal refractive indices of lithium potassium sulfate crystals, LiKSO 4, subjected to uniaxial mechanical pressures. Notable baric increases in the refractive indices are found, which have been interpreted using classical single-oscillator dispersion Sellmeier equation and Lorentz–Lorenz formula. Significant pressure-induced shifts of low-temperature phase transitions in LiKSO 4 are revealed and qualitatively explained in terms of modifications of crystalline structure imposed by the pressures. It is established that high enough pressures applied along the crystallographic axis c can generate a "triple point" on the phase diagram "temperature-uniaxial pressure". It is inferred that the temperature behavior of the refractive indices below the room temperature cannot be interpreted as a pure result of spontaneous strains and should possibly involve some other mechanisms, e.g., a hypothesis of incommensurately modulated phase existing about below 255 K.