CRYSTAL OPTICAL PARAMETERS OF UNIAXIALLY COMPRESSED LiNaSO4 CRYSTALS

Abstract

In the work, a LiNaSO 4 crystal was synthesized and spectral study of its birefringence Δ n i was carried out. It was found that the cation substitution of potassium with sodium(LiKSO 4 → LiNaSO 4 ) in the LiBSO 4 systemsignificantly increases the refractive index and birefringence (by more than an order of magnitude) compared to the substitutions K + → Rb + and K + → (NH 4 ) + . It is shown that the high optical anisotropy of the lithium sodium sulfate crystal can be related to the asymmetry of the second coordination environment of anions in its structure. First-principles calculations of the electronic and optical properties of LiNaSO 4 crystals were performed. From the calculated spectra of the real and imaginary parts of the dielectric function, the spectral dependence of the refractive indices and the extinction coefficients were obtained. The calculated dispersions of the refractive indices of the LiNaSO 4 crystalfor the visible part of the spectrum satisfy the ratio n z > n x,y . The order of the obtained curves coincides with those determined experimentally. The theoretically obtained values of refractive indices are lower (within 8%) than the experimental ones. Such a deviation can be caused by the negative contribution of infrared absorption in the crystal, which is not taken into account in the CASTEP code. It is worth noting, however, that a very good reproduction of the dispersion changes of the refractive indices was obtained from the calculations. Here, the values of d (Δ n i )/ dλ for n x,y (λ) and n z (λ) dependences are practically the same with the maximum deviation of 3.3·10 –5 nm –1 from those determined experimentally. The behavior of optical birefringence of LiNaSO 4 crystals was studied under conditions of uniaxial pressures applied along the main crystal physical directions. The uniaxial pressure applied along mutually perpendicular directions was found to lead to changes in birefringence, different in magnitude and sign. Dispersion effect of birefringence weakens under the influence of stress σ ⊥ z and strengthens in the opposite case of σ || z. Keywords : crystal, birefringence, uniaxial pressure, piezo-optical coefficients, refractive index.