First-principles calculation of the impact of interstitial point defects on the Bi0.5Na0.5TiO3 (110) surface

This study explores how intrinsic interstitial defects can be used to modulate the electronic and optical properties of the Bi0.5Na0.5TiO3 (BNT) (110) surface, a promising lead-free piezoelectric. We use first-principles calculations based on the DFT+U formalism and apply a Hubbard U of 10 eV to the Ti-3d states to ensure an accurate description of the correlated electronic structure. Our results show that the pristine surface is a nonmagnetic semiconductor, but the introduction of interstitials triggers a functional transformation. Na- and Bi-interstitials are shown to act as n-type dopants that create a metallic surface and induce a magnetic moment. In contrast, Ti- and O-interstitials introduce deep, localized states within the band gap but do not produce a net magnetic moment. Our finding also points out that interstitial defects create new pathways for optical absorption at energies below the pristine band gap, causing a significant redshift in the absorption edge. This work demonstrates that native point defects can serve as functional ways, allowing the BNT (110) surface to be tuned to a potential system with metallic, magnetic, or optical characteristics. These insights also offer a viable pathway for the rational design of complex oxide surfaces for next-generation electronic and optoelectronic devices.