First-principles insights into the structural, mechanical, electronic, and optical behavior of nickel sulfide

This study presents a spin-polarized first-principles investigation of the structural, mechanical, electronic, and optical properties of hexagonal NiS using density functional theory as implemented in the Quantum ESPRESSO package. The structure was optimized using the PBEsol functional, yielding lattice parameters of a = 3.412 Å and c = 5.076 Å in the hexagonal $$\text {P6}_{3}$$/mmc phase with favorable formation energy, confirming structural feasibility. Electronic structure calculations performed using GGA-PBEsol and HSE06 hybrid functionals reveal spin resolved band gaps of 1.73 eV majority spin and 0.79 eV minority spin, and 2.01 eV and 0.95 eV, respectively, confirming the semiconducting nature of NiS. Mechanical stability is verified through elastic constant, formation energy, and phonon dispersion analyses, all satisfying stability criteria and showing the absence of imaginary phonon modes. Optical calculations demonstrate pronounced dielectric anisotropy and strong ultraviolet visible absorption, with dominant optical transitions occurring slightly below 2 eV. The combined structural robustness, spin dependent electronic behavior, and strong optical response indicate that hexagonal NiS is a promising candidate for optoelectronic, photovoltaic, and energy conversion applications.