The optimized crystal structures, band structures, partial and total densities of states (DOS), dielectric functions, refractive indexes and elastic constants for ZnAl 2 S 4 and ZnGa 2 O 4 were calculated using the CASTEP module of Materials Studio package. Pressure effects were modeled by performing these calculations for different values of external hydrostatic pressure up to 50 GPa. Obtained dependencies of the unit cell volume on pressure were fitted by the Murnaghan equation of state, and the relative changes of different chemical bond lengths were approximated by quadratic functions of pressure. Variations of applied pressure were shown to produce considerable re-distribution of the electron densities around ions in both crystals, which is evidenced in different trends for the effective Mulliken charges of the constituting ions and changes of contour plots of the charge densities. The longitudinal and transverse sound velocities and Debye temperatures for both compounds were also estimated using the calculated elastic constants.Key words: A. inorganic compounds; C. Ab initio calculations; D. electronic structure;
IntroductionA large group of compounds with spinel structure are generally described by the AB 2 X 4 formula (where A and B are di-and trivalent cations, respectively, and X is a divalent anion). There are two types of cation sites in the spinel structure: tetrahedral for A species and octahedral for B species.Compounds from this family usually have wide band gaps, which can be attractive for various optical applications [1]. Wide band gap also makes these compounds suitable for doping with different impurities to get luminescence in particular spectral region. Optical properties of several impurity ions in these compounds were studied previously, e.g. (X=Al, Ga, In) were calculated from first principles in Ref. [11].In spite of considerable efforts on both experimental and theoretical studies of these compounds, previously reported results for the calculated properties of the bulk materials are somewhat different (comparison between the previous and present calculations will be given below). Besides, it turned out that ab initio calculations of the optical properties and careful studies of the pressure effects on the interionic distances and electron density distribution around ions in these compounds (especially ZnAl 2 S 4 ) are lacking, to the best of the author's knowledge. So, this was one of motivations for the present study, which reports the results of first-principles calculations of a wide range of physical influence of the applied pressure on the charge density distributions, Mulliken charges, and chemical bond orders was considered.The paper is structured as follows: in the next section the crystal structure of the considered crystals and the computational details will be described briefly, then the calculated results will be presented and compared with available experimental data and results of other calculations. The paper will be concluded with a short summary.
Crystal str...
