Strain-induced effects in the electronic and optical properties of Na0.5Bi0.5TiO3: An ab-initio study

The ground-state properties of sodium bismuth titanate (Na0.5Bi0.5TiO3-NBT) are studied using first-principles calculations based on density functional theory (DFT). The electronic band structure reveals that the direct and indirect band gaps are rather close (within similar to 0.07 eV of each other). This has implications for resolving the existing variance in reports regarding the nature of band gap in NBT and supports He et al.'s recent experiment-based inference. We also report hydrostatic strain-induced (epsilon = -0.02 (compression) to 0.02 (tension)) band gap variation and demonstrate electron/hole effective mass (m(e)*/ m(h)*) tunability in NBT. Importantly in compression, NBT has both direct and indirect band gaps that are rather close; but in tension, it is clearly an indirect band gap system. We identify conditions (epsilon = 0, -0.01) wherein m(e)* > m(h)*; this is known to be for a predictive measure for transparent p-type conducing oxides. For all the optical constants, we find a significant blue-shift in their spectra under compressive strains and a nominal red-shift in their values under tensile strains. Compression is pointed to as a rather facile means to tune the optical properties of NBT. Our results offer pointers that can make NBT relevant for applications involving light-harvesting, charge separation, and charge storage.