Electronic and optical properties of 2H-perovskite related tantalum/niobium oxides

Abstract

Quasi-one-dimensional oxides [Formula: see text] (A = Ba, Sr; [Formula: see text] = Na, Li and B = Ta, Nb) have been synthesized and found to display efficient photoluminescence. Their electronic and optical properties are calculated by using first-principles calculations. The modified Becke–Johnson exchange potential has been used to obtain accurate band gap. Our results reveal that alkali metal and alkaline-earth metal ions have very small contribution to the states around Fermi level, and for these compounds, the top valence bands and the conduction band bottom are dominated by O-2p and Nb/Ta-d states, respectively. All of these compounds have indirect band gap, with valence band maximum at K point and conduction band minimum at [Formula: see text] point. Optical absorption spectrum is characterized by two prominent peaks. The lower energy peak originates from electron transitions between Ta/Nb-[Formula: see text] and O-2p states, while the higher energy peak is determined by electron transitions between Ta/Nb-[Formula: see text] and O-2p. Despite the one-dimensional feature of the lattice structure, the electronic band structure and optical properties show three-dimensional character. We find that the band gap and optical absorption threshold are considerably larger than the energy of excitation light in the luminescence measurement. This indicates the important role of the in-gap states, which may be induced by the impurity or vacancy.