Ab-initio STUDIES OF STRUCTURAL, ELECTRONIC, MAGNETIC AND OPTICAL PROPERTIES OF NICKEL OXIDE

Abstract

Nickel Oxide (NiO) has many applications in solar energy utilization, due to variations in electronic, optical and magnetic properties. However, a satisfactory description of its properties is still not available. There is therefore need to understand its properties using theoretical predictions, so as to facilitate a deeper knowledge of the characteristics of this important material. In this work, the NiO ground state properties have been studied using Quantum-ESPRESSO code, while the optical properties are investigated using Yambo code. The ground state properties of NiO are determined using the Local Spin Density Approximation with Hubbard interaction energy (LSDA+U) method. The Hubbard values of 5.4 eV, 6.2 eV, 7.3 eV, 8 eV, 8.5 eV, 9 eV, 9.5 eV, 10 eV and 12 eV have been used, whereby a Hubbard term of 9.5 eV has given the results that are closer to other theoretical and experimental findings. The calculated lattice parameters are found to increase with increasing U term, and for a Hubbard term of 9.5 eV, the value of 9.652 a.u is obtained for rhombohedral (RHL) NiO with a rhombohedral angle rh = 33:5570. The bulk modulus is found to decrease with increasing U value, giving values of 224.5 GPa, for RHL NiO with rh = 33:5570, when a U term of 9.5 eV was used. LSDA and LSDA+U predicted RHL NiO with rh = 60:080 to be a metal, while it is expected to be an insulator. For RHL NiO with rh = 33:5570, the LSDA calculations have revealed very narrow indirect and direct energy band gaps of 0.61 eV and 0.85 eV, respectively, whereas they are increased up to 3.05 eV and 3.65 eV, respectively, upon the addition of a U term of 9.5 eV. The projected density of states shows that RHL NiO with rh = 33:5570 is a charge transfer insulator. The LSDA and LSDA+U calculations further have predicted both structures of RHL NiO with rh = 60:080 and rh = 33:5570 to be antiferromagnetic materials. The Green function and the dynamically screened interaction (GW), as well as the Bethe Salpeter Equation (BSE) have been used to study the absorption energy and the electron energy loss spectra. GW is found to overestimate the value of the fundamental energy band gap of NiO, while BSE gives a better prediction of the optical energy band gap. For RHL NiO with rh = 33:5570, both the optical indirect and direct band gaps are found to be 3.34 eV and 3.74 eV, respectively. The absorption spectra obtained from BSE calculations show that there is minimal absorption in the range of the infrared to the visible region and stronger absorption towards the ultraviolet region. The maximum light absorbed by NiO with rh = 33:5570 was found to be in the ultraviolet wavelength region near 330 nm (3.74 eV) and this makes NiO a good absorber in the ultraviolet region of the electromagnetic spectrum.