3.1. Bulk ordering and electronic structure
Anion ordering in oxynitride perovskites has long been the focus of research in the field.43-46 The experimental investigation on LaTaON2 by high resolution powder neutron and electron diffraction proposed that the cis configuration is energetically favored, in which the two O atoms in the same TaO2N4 octahedron occupying the two adjacent edge positions.47 Accordingly, we have constructed six configurations with cis -ordering for bulk LaTaON2.48 The relaxed structures are presented in Figure 1. The structure e is calculated to be the most stable, which is 0.30 eV, 0.33 eV, 0.09 eV, 0.06 eV, and 0.44 eV lower than a , b , c , d , andf , respectively. The calculated lattice parameters of structuree are a =5.727 Å, b =8.032 Å, c =5.747 Å, andα = β = γ =90˚, which are in good agreement with experimental values. Hence, we only use e configuration to model the bulk structure of LaTaON2 and carry out the following calculations. Band structure along high-symmetry points in the Brillouin zone and density of states (DOS) are calculated by PBE+U method and summarized in Figure 2. The results demonstrate that LaTaON2 is an indirect semiconductor with the VBM atD point and the conduction band minimum (CBM) at Γ point, and the band gap is 2.15 eV, which are consistent with the experiments.22, 26, 42 DOS and electron cloud distributions of frontier orbitals demonstrate that the VBM is mainly composed of N 2p states, and the CBM dominantly consists of Ta 5d states. La 6s states have little contribution to the VBM and CBM.