By Achuka, Ebere Ifechukwudere
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 matrices were
calculated on a uniform grid of 444 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 avaliable experimental data and previous calculations agree fairly well. The
eect 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 ?, 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 presure were found to follow a quadratic dependence
in agreement with other theoretical calculations and experimental data.