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

Abstract

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 ∼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 (ε = -0.02 (compression) to 0.02 (tension)) band gap variation and demonstrate electron/hole effective mass (me*/ mh*) 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 (ε = 0, -0.01) wherein me* > mh*; this is known to be for a predictive measure for transparent p-type conducting 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.