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.