Rudysh M. Optical-electronic parameters of lithium ammonium sulfate crystals under the action of uniaxial pressures

The work is devoted to the ascertainment of the regularities of the electronic structure, temperature and baric changes in optical properties of dielectric crystals. These are typical representatives of the ABCX4 group and may be in one of two polymorphic modifications. LiNH4SO4 single crystals of good optical quality in two (α and β) polymorphic modifications were synthesized from aqueous solution by the method of slow evaporation. The structure of the obtained crystals is investigated by X-ray diffraction method. The dispersion of refractive indices n(λ) of LiNH4SO4 crystals in two modifications is investigated using Obreimov method for the three crystal-physical directions in the optical spectral region. The refractive indices of the studied crystals are of a significant anisotropy. The presence of isotropic points is revealed. Investigated birefringence properties of the crystals in the optical region of the spectrum confirm the investigations results for the refractive indices. The presence of birefringence-sign-inversion points for α-modification is found in the long-wavelength region of the spectrum, for β – in the short-wavelength region. Given are results of study of the influence of temperature and pressure on the birefringence of the crystals. A significant temperature and baric displacement of isotropic point is established. Uniaxial pressures are established to lead to a different in sign birefringence change. The results of investigation of spectral and temperature dependences of combined piezooptical coefficients are presented. It was found that coefficients have different signs, reveal a significant dispersion and vary slightly with temperature. Minor anomalies in (T) behavior are detected. Results of the investigation of infrared reflection spectra in the range of 700-1700 cm-1 wave numbers are presented. Frequencies of reflection bands of LiNH4SO4 crystal at room temperature are obtained. Influence of uniaxial pressures on the position and intensity of the infrared reflection spectra bands was investigated. Evaluated and discussed are frequencies of longitudinal νLO and transverse νТO oscillations, damping constant  and oscillator strength f of mechanically free and clamped by the uniaxial pressure LiNH4SO4 crystal. Using the Kramers-Kronig dispersion relations for reflection spectra, baric changes in spectral dependences of optical constants were obtained and analyzed. Calculations of the band-energy structure of LiNH4SO4 crystals of two modifications were performed using density functional theory. Dispersion of electronic levels E(k) for the points of high symmetry of the Brillouin zone was constructed using various functionals for describing the exchange-correlation interaction of electrons. A low dispersion of energy levels in comparison with other isomorphic crystals of the ABCX4 group was established. Width of the bandgap is estimated and compared with literary data. Full and partial states densities are calculated. Origin of the energy levels of valence and conduction band is established. For the investigated crystals, the top of valence band is formed by 2p-states of oxygen, while the bottom of conduction band – by 1s-states of hydrogen (α-LAS) and 2s-states of lithium (β-LAS). The top of valence band and the bottom of conduction band are located in the center of the Brillouin zone (Г point). The sections of distribution of the electronic density in planes passing through atoms are presented. The character of the chemical bonding is analyzed and it is shown that bonds in tetrahedral N-H and S-O complexes are covalent. Interaction between complexes and lithium ions is ionic in nature. The spectra of the real ε1 and imaginary ε2 parts of the dielectric function were calculated, from which the spectral dependences of the refractive index and birefringence were obtained. Good agreement between calculated spectral dependences n(λ) and Δn(λ) and experimentally obtained data is revealed. Results of the study of X-ray photoelectron and X-ray emission spectra for two LiNH4SO4 crystals modifications are presented. The effect of bombarding the surface of crystals with Ar+ ions on the electronic structure of the crystals has been investigated. Bonding energies of the core electrons of constituent elements are determined. It is found that the values of binding energy of core electrons of lithium, sulfur and oxygen atoms do not change in a result of irradiation of the surface of α-LiNH4SO4 crystal with Ar+ ions for two crystal modifications. The similarity of electron bonding energies in α- and β-modification crystals testifies to the proximity of the nature of chemical bonding in two modifications. X-ray photoelectronic and X-ray emission spectra of crystals have confirmed theoretically calculated data of the band-energy structure of crystals.