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.

Structure and refractive properties of LiNaSO4 single crystals

“…It is comparable to the corresponding parameter obtained for K 1.75 (NH 4 ) 0.25 SO 4 (~ 0.53 eV [20]) and less than the result for LiKSO 4 (1.21 eV -see Ref. [28]). Almost all of the energy levels, except for the lowest ones in the conduction band near the centre of the Brillouin zone, are characterized by a weak dispersion, which is typical for the ABSO 4 group.…”

“…4b, the uniaxial stresses directed parallel and perpendicular to the z axis of the LSS crystals give rise to the birefringence changes, which are almost the same in the magnitude but opposite in their signs. Note that increase (or decrease) in the optical anisotropy observed under the mechanical pressure parallel (or normal) to the optic axis is also characteristic for the uniaxial LiKSO 4 crystals [17,28]. The dispersion effect in the corresponding Δn(λ) curve for LSS is weakened under the influence of the stress z   and strengthened in the alternative case of  || z (see Table 3).…”

“…Its quantitative characteristics are also shown in Table 4. It is worthwhile that the birefringence of a close relative of LSS, the uniaxial LiKSO 4 crystal, increases with increasing temperature [28], whereas the behaviour of LSS is just the opposite. It is obvious that a decrease in the optical anisotropy of LSS observed with increasing temperature represents a general tendency associated with the fact that the LSS structure transforms gradually into its cubic version typical for the high-temperature α-phase.…”

“…A comparison of the refractive parameters of LSS with those peculiar to LiKSO 4 [17,28] shows that the cationic substitution K → Na leads to a slight decrease in the electronic polarizability (~ 7%) and molar refractivity (~ 13%), a displacement of the ultraviolet-oscillator position towards shorter wavelengths (~ 26 nm), and an increase of the B parameter associated with the oscillator strength (~ 10% and ~ 25% for the x and z axes). Notice also that the refractivity parameters R for LiRbSO 4 and, especially, α-LiNH 4 SO 4 are essentially (~ 50%) larger than that of LSS (see Refs.…”

“…In particular, this concerns the lattice parameters and the bandgap, as well as their anisotropies (see Refs. [17,28]). Finally, a relatively high optical anisotropy of LSS should imply the absence of optically isotropic state under the normal conditions.…”