Shchepanskyi P. Band structure peculiarities and optical characteristics of the ABSO4-group crystals (where A, B = Li, Na, K, NH4) with an isotropic point

The work is devoted to the first-principle calculations of the band-energy structure and the density of electronic states, as well as to the experimental and theoretical study of the dispersion dependences of refractive parameters of lithium-sodium sulfate dielectric crystals and potassium ammonium sulfate solid solution; to the changes of the refractive parameters under the influence of external fields and due to partial and complete cation substitution. Crystals of the ABSO4 group, where A and B are alkaline metal cations, or the NH4 group, are of considerable interest due to the ability to undergo various phase transitions, their ferroelectric, ferroelectric, superionic, optical and other properties. Among the optical properties, an interesting feature of these crystals is the possible existence of isotropic points, when crystal undergo transition from a biaxial to a uniaxial or from an optically uniaxial to isotropic one. Such crystals can be used as temperature and pressure sensors. Using the method of evaporation from aqueous solution, single crystals of LiNaSO4 and a K1.75(NH4)0.25SO4 of good optical quality have been grown. The structure of the obtained compounds was investigated by X-ray diffraction. The analysis of structural data in the framework of the second coordination environment approximation was carried out and the peculiarities of the influence of partial and complete cation substitution on the coordination environment of the anions was determined. The dispersion dependences of the refractive indices n(λ) of LiNaSO4 and K1.75(NH4)0.25SO4 crystals are established for the first time. On the basis of available literature data on the optical properties of isomorphic crystals, it was found that the complete cation substitution K+ → Na+ (LiKSO4 → LiNaSO4) does not change the character of dispersion changes in crystals refraction indices, but leads to a significant increase in birefringence value; partial isomorphic substitution (12.5%) K+ → (NH4) (K2SO4 → K1.75(NH4)0.25SO4) leads to the appearing of the point of the birefringence sign inversion at room temperature in K1.75(NH4)0.25SO4 for the near-infrared spectrum region (λIP ≈ 1350 nm). The results of the study of temperature and pressure influence on the birefringence of crystals are given. Two new isotropic points in K1,75(NH4)0,25SO4 are found at temperatures T01  541 K and T02  589 K, which are shifted in comparison with the corresponding ones for pure potassium sulfate towards lower temperatures. It is established that uniaxial pressures cause a different in sign birefringence changes. Herewith, the dispersion effect of birefringence increases for the LSS under the influence of the stress applied along the optical axis and weakens in a case of stress directed perpendicularly to it. Based on the birefringence changes under the action of uniaxial pressures the piezooptical coefficients dispersion is obtained. The temperature-spectral-baric diagrams of isotropic points in K1.75(NH4)0.25SO4, which determine the uniaxial state of the crystal under different temperature, pressure and spectral conditions, are constructed. Using the density functional theory, calculations of the band-energy structure of LiNaSO4 and K1.75(NH4)0.25SO4 crystals are calculated and the total and partial densities of electronic states are determined. The influence of cation substitution on electronic parameters of crystals of ABSO4 group is investigated. Theoretical spectra of refractive indices and extinction coefficients for LiNaSO4 and K1.75(NH4)0.25SO4 crystals are calculated and compared with experimentally obtained parameters of the crystals. A correlation analysis of the refractive, electronic and structural parameters of crystals of ABSO4 group was carried out. It has been found that the average value of the refractive index mainly increases with increase of crystal density, what is caused by the corresponding increase in the ionic radius of the substitutional cation.