First-principles calculations to investigate structural, electronic, and optical properties of Na doped TaO₃ perovskite

A comprehensive first-principles study on the structural, electronic and optical properties of perovskite NaTaO₃ using the full-potential linearised augmented plane wave plus local orbitals (FP-LAPW + lo) method, as implemented in the WIEN2k code was carried out. Five structural polymorphs (orthorhombic (Pbnm and Cmcm), tetragonal (P4/mbm and P4/mmm), and cubic (Pm-3m)) using both the generalized gradient approximation (GGA) and the Tran-Blaha modified Becke-Johnson (TB-mBJ) exchange-correlation potentials were systematically optimized. Total energy calculations show that the ground state is the orthorhombic Pbnm phase, with the Cmcm, P4/mbm, P4/mmm and Pm-3m phases at + 0.021, + 0.038, + 0.052 and + 0.074 eV per formula unit in energy, respectively. The calculated lattice parameters and bulk moduli agree well with the experimental data. The bulk modulus decreases from 197.37 GPa (Pbnm) to 192.42 GPa (Pm-3m) indicating a decreased structural rigidity with increasing symmetry. The electronic structure analysis shows that the Pbnm phase is a direct-band-gap semiconductor with a bandgap of 2.75 eV (GGA) and 3.15 eV (TB-mBJ) at the Gamma point, respectively, which is in better agreement with the experimental value of 3.98 eV. The valence band is mainly composed of hybridized O-2p and Ta-5d orbitals whereas the conduction band minimum is mainly of Ta-5d character.
A detailed analysis of optical properties, i.e., complex dielectric function, refractive index, absorption coefficient, reflectance and energy loss function, shows strong absorption in the UV region with a threshold 3.15 eV (TB-mBJ), low reflectance in the visible region and sharp plasma edge at 7.65 eV (TB-mBJ). The results presented here indicate that NaTaO₃ is a promising material for UV photocatalytic water splitting, UV optoelectronic devices, UV-transparent optical coatings, Bragg reflectors and as a dopable host matrix for visible-light photocatalysis.