  As a compound in between the tantalum oxide and nitride, the tantalum
oxynitride TaON is expected to combine their advantages and act as an efficient
visible-light-driven photocatalyst. In this letter, using hybrid functional
calculations we show that TaON has different defect properties from the binary
tantalum oxide and nitride: (i) instead of O or N vacancies or Ta
interstitials, the $O_N$ antisite is the dominant defect, which determines its
intrinsic n-type conductivity and the p-type doping difficulty; (ii) the $O_N$
antisite has a shallower donor level than O or N vacancies, with a delocalized
distribution composed mainly of the Ta $5d$ orbitals, which gives rise to
better electronic conductivity in the oxynitride than in the oxide and nitride.
The phase stability analysis reveals that the easy oxidation of TaON is
inevitable under O rich conditions, and a relatively O poor condition is
required to synthesize stoichiometric TaON samples.
