Electronic, structural, mechanical, and piezoelectric properties of bulk NbOX2 (X = Cl, Br, I): A DFT study

In this work, we have performed a comprehensive study of dielectric materials NbOX2 (X = Cl, Br, and I) within the framework of density functional theory, incorporating both conventional and hybrid functionals. Our study focuses on the structural, electronic, elastic, and piezoelectric properties of the compounds. It is observed that hybrid functional PBE0 gives better results for bandgap than PBE-GGA functional, which is comparable with the earlier report of HSE06. We have also performed the elastic tensor calculation, and our result of elastic constants fulfills the Born stability criteria. From the calculated elastic tensors, we have obtained mechanical properties such as Young’s modulus, Kleinmann coefficients, elastic anisotropy, and Vickers hardness. Piezoelectricity offers an innovative approach to energy extraction by manipulating material structures via mechanical stresses. The use of non-lead based materials for piezoelectricity offers a greener and carbonless approach for energy harvest. All the compounds under study show strong in-plane piezoelectricity along the y-direction and have moderate out-of-plane polarization, mainly due to shear strain (yz). Among the investigated materials, bulk NbOI2 exhibits the highest piezoelectric response of 6.32 Cm−2, which is ∼31% higher than NbOCl2, NbOBr2, and lead zirconate titanate, making it a strong piezoelectric candidate.