The energy gap of the valence band maximum of NaNbO₃ is determined from the Schottky barrier heights at the contacts with low work function Sn-doped In₂O₃ and high work function RuO₂ by means of X-ray photoelectron spectroscopy with in-situ interface preparation. The measurements reveal that the valence band is at a similar energy than that of SrTiO₃ and BaTiO₃. The energy gap of SrTiO₃ and BaTiO₃ of 3.2 eV is comparable to the values of 3.2-3.5 eV, which are determined by means of optical and electron energy loss spectroscopies for NaNbO₃. It is therefore expected that the conduction band minimum of NaNbO₃ is also located at a similar energy than the conduction band minimum of SrTiO₃ and BaTiO₃. If this is the case, it is expected that donor doping of NaNbO₃ leads to electrical conductivities, which are comparable to those of donor-doped SrTiO₃ and BaTiO₃ (up to ~ 1 S/cm). In contrast, Sr- and Ca-doped NaNbO₃ bulk ceramics exhibit room temperature conductivities <10⁻¹⁰ S/cm, only slightly higher than those of undoped NaNbO₃. High-field conductivity measurements and impedance spectroscopy give no indication that the low conductivity is caused by insulating grain boundaries separating electrically conductive grains. It is therefore suggested that the energy gap of NaNbO₃ is substantially higher than the gap of 3.2-3.5 eV determined from optical absorption and electron energy loss measurements. A comparable conclusion has been derived from electronic structure calculations of LiNbO₃ [1].
[1]    W.G. Schmidt et al., Phys. Rev. B 77 (2008), 035106.