Symmetry considerations on band filling and first optical transition in NiO

Recent theoretical works on NiO have not agreed upon the nature of the first optical transition. By altering band filling – with highly concentrated O vacancies and Fe impurities – here, the orbital density of states is changed near the Fermi energy. The variation in optical properties, relative to the changes in orbital character, along with group theory analysis of hybridized orbitals, provides new insight when evaluating the first optical transition of NiO. Here, based on density functional theory, the first optical transition is found to have two possibilities – either superexchange site-hopping or a transition from the hybridized e g state to the hybridized a 1 u state, rather than the intra-atomic transitions which are causing disagreement in the recent literature. Graphical abstract