The electronic structure and charge transport of NaNbO3 have been studied by X-ray photoelectron spectroscopy, transmission electron microscopy, impedance spectroscopy, and with direct current conductivity measurements in dependence on electric field, temperature, and oxygen partial pressure. The measurements reveal that the fundamental gap is considerably larger than the optically determined energy gap of 3.5 eV [1]. The optically derived band gap is assigned to excitonic transitions involving trapped charge carriers in agreement with theoretical calculations for LiNbO3 and KNbO3 [2,3]. AC and DC conductivity indicate that i) DC conductivity is probing only electronic conduction, ii) ionic conductivity is higher than electronic conductivity, and iii) two types of ionic and two types of electronic carriers are involved in electric conduction. By assuming that electronic conductivity is dominated by hopping of electrons, suggested to be localized on Nb, and of holes, suggested to be localized on oxygen, the temperature dependence of electronic transport in air and in nitrogen can be consistently described for undoped and for donor-doped NaNbO3. The model reveals a fundamental energy gap of 4.5 eV and trapping energies of electrons and holes of 1 and of 0.2 eV, respectively. The model is particular capable of describing the change of activation energy and of conduction type (n- vs. p-type) at ~300°C. 
[1] N Bein et al, Phys. Rev. Mater. 6, 084404 (2022)
[2] W G Schmidt at al., Phys. Rev. B 77, 035106 (2008).
[3] F Schmidt et al., Phys. Rev. Mater. 3, 054401 (2019)