STUDY OF LATTICE DYNAMICS AND PRESSURE DEPENDENCE OF PHONON FREQUENCIES OF ZINC BLENDE ZNS TO 13.4 GPA
ABSTRACT
The phonon dispersion of zinc sulphide (ZnS) in zinc blende structure has been investigated using ab initio calculations based on the density functional perturbation theory within the generalized gradient approximation (GGA) of exchange correlation functional as implemented in the Quantum Espresso suite of codes. The basis functions were expanded in a plane-wave basis set with kinetic energy cut-o˙ of 30Ryd. Brillouin zone integration was performed using 8 × 8 × 8 special k-point mesh. These k-point and energy cut-o˙ values were tested and provide convergence of the order of milli-electron volt (meV) in self consistent calculations. To obtain the ground state properties, the zinc blende structure of zinc sulphide was optimized. Using energy-volume variation, the equilibrium lattice constant, the bulk modulus and pressure derivative of the bulk modulus were calculated with Murnaghan equation of state. The results for the structural parameters are in good agreement with avaliable theoretical and experimental values. With the lattice constant, the dynamical matricesp were calculated on a uniform grid of 4 × 4 × 4 q-points mesh. The force constants in real space have been calculated using Fourier interpolation scheme which are further used for the calculation of the full phonon spectra in the entire Brillouin zone. The theoretically calculated phonon dispersion curves when compared with the available experimental data and previous calculations agree fairly well. The e˙ect of pressure on phonon dispersion of ZnS was also studied by comparing phonon frequencies of ZnS produced at pressures up to 13.4 GPa to ensure that the crystal structure remained zinc blende. Pressure dependence of all the phonon branches were investigated at the Γ, X and L symmetry points of the Brillounin zone as well as that of longitudinal optical (LO) and transverse optical (TO) frequency splitting (LO-TO) at the same high symmetry points. The response of the phonon branches and the LO-TO frequency splitting to pressure were found to follow a quadratic dependence in agreement with other theoretical calculations and experimental data.