First-Principles Insights into the Structural, Electronic, Mechanical, and Optical Behavior of Nickel Sulfide

In this work, the structural, electronic, mechanical, and optical properties of hexagonal nickel sulfide (NiS) were investigated using spin-polarized density functional theory (DFT) as implemented in the Quantum ESPRESSO (QE) package. We have used the PBEsol functional for optimization of the crystal structure, whereas the electronic band gap was calculated within both GGA and the HSE06 hybrid functional. Structural analysis gives Ni–Ni, Ni–S, and S–S bond lengths of 3.44 Å, 44 Å, 4.69 Å, and 2.37 Å, respectively, values that align closely with previously reported experimental results. Mechanical property analysis and calculated formation energy confirm that NiS is indeed mechanically stable and ductile. Optical calculations show strong dielectric anisotropy and significant absorption in the UV-visible range, with prominent features appearing just below 2 eV. The pronounced optical response in combination with a tunable electronic structure makes NiS a promising candidate for optoelectronics, photovoltaic, and energy-conversion applications.