Fermi energy, electrical conductivity, and the energy gap of NaNbO₃

The energy of the valence band maximum of NaNbO₃ is determined from the Schottky barrier heights at the contacts with low work function Sn-doped In⁢₂O₃ and high work function RuO₂ by means of x-ray photoelectron spectroscopy with in situ interface preparation. The measurements reveal a valence-band edge energy, which is comparable to that of SrTiO₃ and BaTiO₃. The energy gap of SrTiO₃ and BaTiO₃ is 3.2 eV and comparable to the values of 3.4 eV to 3.5 eV, which are determined by means of optical and electron energy loss spectroscopy for NaNbO₃. It is therefore expected that the conduction band minimum of NaNbO₃ is also located at a similar energy as the conduction band minimum of SrTiO3 and BaTiO₃. If this is the case, it can be expected that donor doping of NaNbO₃ leads to an electrical conductivity, which is comparable to those of donor-doped SrTiO₃ and BaTiO₃ (up to ∼ 1 S/c⁢m⁻¹). In contrast, Sr- and Ca-doped NaNbO₃ bulk ceramics exhibit a room temperature conductivity up to 10 × 10⁻¹⁰ S/c⁢m⁻¹, only slightly higher than that of 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 3.4 eV to 3.5 eV determined from optical spectroscopy reported in literature and from electron energy loss spectroscopy within this paper, as also suggested from electronic structure calculations of LiNbO₃ [Phys. Rev. B 77, 035106 (2008)].