Orbital-to-Optical Correlation Governing Anisotropic Light Absorption in SrTaO2N

Perovskite oxynitrides are promising visible-light photocatalysts, yet the orbital origin of their anisotropic optical response remains unclear. Here, we investigate ordered SrTaO2N using GGA-PBE, DFPT-derived GGA + U, and TB-mBJ density functional theory. The three approaches yield band gaps of 1.00, 2.33, and 1.90 eV, respectively, with TB-mBJ falling within the experimental optical range. Across all methods, N 2p and Ta 5d states dominate the valence- and conduction-band edges, with pronounced p–d hybridization within the Ta–N/O framework. Polarization-dependent calculations reveal strong anisotropy in interband transition probability: the strongest low-energy excitation occurs along the z direction and is associated with the Ta–N–Ta network. This anisotropy is preserved after gap correction and is reflected in the dielectric response, absorption, and reflectivity. These results demonstrate that the photoresponse of SrTaO2N is governed by orbital character and direction-dependent excitation rather than by the band-gap magnitude alone, highlighting z-polarized N 2p → Ta 5d interband transitions as a robust descriptor for designing visible-light-responsive oxynitride photocatalysts.