A density functional theory analysis of the mechanical and dynamic stability and thermodynamic properties of pyrite-type ZnSe2 and ZnTe2

Abstract

The present work is the first quantitative theoretical investigation of elastic, dynamical and thermodynamic properties of Zinc diselenide, ZnSe2 and Zinc ditelluride, ZnTe2 based on a density functional theory plane-wave pseudopotential method. The first-principles calculations were carried out using the local density approximation, (LDA) and the generalized gradient approximation, (GGA) of Perdew Burke and Ernzerhof (PBE) in a pyrite-type structure. The structural properties obtained were consistent with existing experimental and theoretical results found with a related approach. The elastic constants were calculated by adopting a stress-strain approach and the mechanical stability of the compounds was established using elastic stability criteria. Both compounds were predicted to be ductile with good plasticity according to Pugh's and Poisson's ratios respectively. Dynamical stability was determined from the calculations of phonon dispersion spectra. ZnSe2 is predicted to be dynamically stable while ZnTe2 stability is open for verification when experimental result is available, this is because LDA predicts dynamically stable compound while PBE proves otherwise. The thermodynamic properties between the temperature range of 0–1000 K were calculated using the computed phonon density of state. There is a monotonic increase in internal energies with temperature the vibrational free energies decreases continually as temperature increases. Sharp increase was observed in specific heat capacity at constant volume between 0 and 400 K while at higher temperature CV gradually approaches the asymptotic limit of 300 J mol−1k−1. The values at ambient temperature were also reported.