Influence of complex antisite defects in structural distortion, electrical and optical properties of Bi$_{0.5}$Na$_{0.5}$TiO$_{3}$(110) surface: A DFT investigation

Abstract

Bismuth sodium titanate (BNT) is a crucial lead-free multiferroic material whose functional properties are highly sensitive to surface structure and native defects. This study employs first-principles calculations based on Density Functional Theory (DFT) to systematically investigate the structural, electronic, and optical properties of the BNT(110) surface containing complex antisite defects. We first confirm that the pristine BNT(110) surface undergoes significant atomic relaxation and polarization changes compared to the bulk. Subsequent introduction of antisite defects dramatically modifies the local lattice parameters, leading to significant strain and altered coordination polyhedral. As the results, the antisite defects introduce localized deep-level states within the band gap, resulting in a substantial reduction of the band gap energy. Analysis of the projected density of states (PDOS) reveals that these states are predominantly derived from hybridized Ti-3d and O-2p orbitals from atoms adjacent to the defect site. Furthermore, the calculated optical absorption spectra indicate a strong sub-gap absorption in the visible region due to the defect-induced states, suggesting potential for enhanced light absorption. This study provides critical insights into the intrinsic effects of complex antisite defects on the functional surfaces of BNT, which is essential for optimizing defect engineering strategies in BNT-based electronic and photocatalytic devices.