Thermomechanical, electronic and thermodynamic properties of ZnS cubic polymorphs: an ab initio investigation on the zinc-blende–rock-salt phase transition

In the present work, an extensive and detailed theoretical investigation is reported on the thermomechanical, electronic and thermodynamic properties of zinc-blende (sphalerite, zb-ZnS) and rock-salt zinc sulfide (rs-ZnS) over a wide range of pressure, by means of ab initio Density Functional Theory, Gaussian type orbitals and the well known B3LYP functional. For the first time, vibrational frequencies, phonon dispersion relations, elasto-piezo-dielectric tensor, thermodynamic and thermomechanical properties of rs-ZnS were calculated with a consistent approach that allows a direct comparison with the low-pressure polymorph. Special attention was paid to the evaluation of the thermodynamic pressure–temperature stability of the mineral phases between 0–25 GPa and 0–800 K. The static (T = 0 K) bulk moduli of sphalerite and rock-salt ZnS were 72.63 (3) GPa and 84.39 (5) GPa, respectively. The phase transition in static conditions calculated from the equation of state was about 15.5 GPa, whereas the elastic constants data resulted in P_trans = 14.6 GPa. At room temperature (300 K), the zb-rs transition occurs at 14.70 GPa and a negative Clapeyron slope (dP)/(dT) = 0.0023 was observed up to 800 K. The electronic band structure showed a direct band gap for zb-ZnS (E_g = 4.830 eV at equilibrium geometry), which became an indirect one by increasing pressure above 11 GPa. The results were found to be in good agreement with the available experimental and theoretical data, further extending the knowledge of important properties of zinc sulfide, in particular the thermomechanical ones of the rock-salt polymorph here extensively explored for the first time.