PHYSICAL REVIEW MATERIALS 6, 084404 (2022) Fermi energy, electrical conductivity, and the energy gap of NaNbO 3 Nicole Bein , 1 Brigita Kmet , 2 Tadej Rojac , 2 Andreja Benˇ can Golob , 2 Barbara Maliˇ c , 2 Julian Moxter , 3 Thorsten Schneider , 4 Lovro Fulanovic , 4 Maryam Azadeh , 4 Till Frömling , 4 Sonja Egert , 5 Hongguang Wang, 6 Peter van Aken , 6 Jutta Schwarzkopf, 7 and Andreas Klein 1 , * 1 Technische Universität Darmstadt, Institute of Materials Science, Otto-Berndt-Str. 3, 64287 Darmstadt, Germany 2 Jožef Stefan Institute, Electronic Ceramics Department, Jamova cesta 39, 1000 Ljubljana, Slovenia 3 Technische Universität Darmstadt, Department of Electrical Engineering and Information Technology, High-voltage engineering, Fraunhoferstr. 4, 64283 Darmstadt, Germany 4 Technische Universität Darmstadt, Institute of Materials Science, Alarich-Weiss-Str. 2, 64287 Darmstadt, Germany 5 Technische Universität Darmstadt, Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Alarich-Weiss-Str. 8, 64287 Darmstadt, Germany 6 Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany 7 Leibniz-Institut für Kristallzüchtung, Max-Born-Str. 2, 12489 Berlin, Germany (Received 22 March 2022; accepted 8 July 2022; published 3 August 2022) The energy of the valence band maximum of NaNbO 3 is determined from the Schottky barrier heights at the contacts with low work function Sn-doped In 2 O 3 and high work function RuO 2 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 SrTiO 3 and BaTiO 3 . The energy gap of SrTiO 3 and BaTiO 3 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 NaNbO 3 . It is therefore expected that the conduction band minimum of NaNbO 3 is also located at a similar energy as the conduction band minimum of SrTiO 3 and BaTiO 3 . If this is the case, it can be expected that donor doping of NaNbO 3 leads to an electrical conductivity, which is comparable to those of donor-doped SrTiO 3 and BaTiO 3 (up to ∼ 1S/cm −1 ). In contrast, Sr- and Ca-doped NaNbO 3 bulk ceramics exhibit a room temperature conductivity up to 10 × 10 −10 S/cm −1 , only slightly higher than that of NaNbO 3 . 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 NaNbO 3 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 LiNbO 3 [Phys. Rev. B 77, 035106 (2008)]. DOI: 10.1103/PhysRevMaterials.6.084404 I. INTRODUCTION NaNbO 3 is a prototype antiferroelectric perovskite and one of the two end members of the important lead-free piezoelec- tric (K, Na)NbO 3 (KNN) [1–3]. Understanding the electrical conductivity of these materials, which are operated at high electric fields, is of particular relevance. The (relatively) high leakage current is a major obstacle for the application of KNN-based ceramics. It has been reported that Mn-doping can reduce the leakage current of NaNbO 3 [4,5]. This behav- ior is comparable to that of BaTiO 3 , in which Mn is a typical acceptor dopant used in BaTiO 3 -based multilayer ceramic capacitors [6–9]. In BaTiO 3 oxygen vacancies are donors with energy levels close to the conduction band, which will make the material n-type. Hence, acceptor doping is necessary to stabilize a low electrical conductivity [10–13]. Donor doping by partially substituting Ti 4+ by Nb 5+ or Ba 2+ by La 3+ results in n-type conduction of BaTiO 3 , indicating that the energy * andreas.klein@tu-darmstadt.de levels of the donors are close to or even above the conduction band minimum [14]. The relation between band-edge energies and electrical properties, which is related to the alignment of defect energy levels, is well known for semiconducting materials [15–22], but much less studied for dielectric oxides. BaTiO 3 and SrTiO 3 are two intensively studied prototype perovskite-type oxides with comparable valence and conduction band en- ergies [23,24]. The n-type conductivity of BaTiO 3 , which is, hence, comparable to that of SrTiO 3 [25,26], has been related to the energetic position of the band edges, which is determined by the orbital contributions to the energy bands [23,27,28]. In BaTiO 3 , the electronic states near the valence band maximum E VB , and those near the conduction band minimum E CB , are formed mostly by the O 2 p and Ti 3d states shown in Fig. 1. In Pb-containing compounds, the hybridization of the (occupied) Pb 6s orbitals with the O 2 p states results in an upward shift of the valence band maximum by more than 1 eV compared to BaTiO 3 illustrated in Fig. 1 in comparison to PbTiO 3 . The higher valence band maximum of these materials is related to their preference for p-type 2475-9953/2022/6(8)/084404(13) 084404-1 ©2022 American Physical Society