Electron structure and optical absorption properties of cubic and orthorhombic NaTaO3 by density functional theory

The electronic structures of cubic (Pm3̄m) and orthorhombic NaTaO3 (Pbnm and Pcmn) were investigated by density functional theory (DFT) method. It is shown that cubic NaTaO3 is an indirect bandgap semiconductor, whereas, orthorhombic NaTaO3 is a direct bandgap semiconductor at X point. The direct bandgap property is independent of space group for orthorhombic NaTaO3. Therefore, a steady direct optical transition can occur for this compound. The absorption spectra, calculated by DFT, fit very well with that of the experimental ones. The bandgap lowering and red shift of the absorption edge are found when the crystal structure changes from orthorhombic to cubic.