Electronic Structure and Electrical Conduction Analysis of Na₀.₅Bi₀.₅TiO₃-6BaTiO₃ based Piezoceramics

Na₀.₅Bi₀.₅TiO₃ (NBT-based) ceramics are considered promising candidates to replace lead-based materials for piezoelectric applications; however, the functional properties are not the focus of this study. Instead, a comprehensive investigation was conducted on the electronic structure and electrical conduction behavior of the morphotropic phase boundary composition (6%) in the solid solution of (1−x)(Na₀.₅Bi₀.₅)TiO₃–xBaTiO₃ system (NBT-BT) with different A-site stoichiometry, A-site to B-site ratios, and 1% Zn doping. The electrical conduction behavior was analyzed using a combination of direct current (dc) and alternating current (ac) conductivity measurements, conducted over a wide range of oxygen partial pressures and temperatures under various atmospheric conditions. Additionally, multiple X-ray photoelectron spectroscopy (XPS) techniques were employed, including high-temperature XPS, near-ambient pressure XPS (NAP-XPS) on bare surfaces of oxidized and reduced samples, as well as in situ XPS measurements at interfaces between NBT-6BT and 10% Sn-doped In₂O₃ under vacuum annealing and cathodic polarization, using an electrochemical cell setup. In oxygen partial pressure-dependent conductivity measurements, assuming that the dc method reflects electronic conductivity and the ac method measures total conductivity, the ionic contribution was determined by subtraction. This approach is valid under conditions ranging 10⁵ – 10⁰ ppm. Then, two distinct electrical conduction behaviors were identified among the samples by comparing their dc and ac conductivity: Type I (p-type) behavior is observed in Na-rich and acceptor-doped compositions, which are dominated by high ionic conductivity, with total conductivity reaching up to 4 ×10⁻⁴ S/cm at 450°C, accompanied by p-type electronic conduction. In contrast, Type II (n-type) behavior is found in slightly Na-rich and Bi-rich samples, which primarily exhibit n-type electronic conductivity with significantly lower conductivity, around 10⁻⁸ S/cm at 450°C. Then a new defect model for multivalent A-site perovskite NBT-based materials is proposed to interpret these observations, accounting for the effects of A-site nonstoichiometry through the formation of NaBi and BiNa antisite defects, and the A:B ratio through the formation of A-site or B-site vacancies. This model calculates antisite defect and vacancy concentrations to predict effective doping effects by defining the Na-to-Bi ratio as X and the A-to-B ratio as Y. Represented as a two-dimensional map of effective doping concentration versus X and Y, the model predicts that: only half of the third quadrant (X<1, Y<1) and a quarter of the second quadrant (X<1, Y>1) show an effective ''donor'' effect, while the remaining regions exhibit an effective ''acceptor'' effect. When accounting for the intrinsic donor background in our samples and the acceptor background observed in the University of Sheffield samples, described in literature, the experimental results confirm the model’s predictions. In temperature-dependent conductivity measurements at 10⁵ and 10⁰ ppm, the observed behavior cannot be explained by free carriers alone—polaron conduction must also be considered. The hole polaron binding energy, attributed to the Bi³⁺/Bi⁴⁺ charge transition levels, is found to be 1.08 eV above the valence band maximum by Na₀.₅₁Bi₀.₄₉TiO₃–6BT sample. In contrast, the electron polaron binding energy, associated with the Bi³⁺/Bi⁰ charge transition level, is found to be 2.04 eV below the conduction band minimum. These hole and electron polaron levels also define the lower and upper limits of the Fermi level, respectively. The investigation of oxygen partial pressure and temperature-dependent conductivity was further extended into the strongly reducing regime (10⁻¹² to 10⁻¹⁸ ppm). The results show that under strongly reducing conditions, dc conductivity includes significant ionic contributions, invalidating the assumption that it represents purely electronic conduction. The activation energies extracted from the Arrhenius plots are 0.4–0.68 eV, corresponding to the migration energy of oxygen vacancies. After temperature-dependent measurements at 10⁻¹⁸ ppm, all samples darkened from light yellow, indicating reduction. XPS on bare surfaces confirms significant Bi³⁺reduction to metallic Bi upon heating to 350°C. Despite their high ionic conductivity, the chemical instability of Type I samples under strongly reducing conditions limits their suitability for solid oxide fuel cell applications. A clear trend in surface composition emerges from high-temperature XPS measurements on oxidized samples: a higher A:B ratio corresponds to an increased Na:Bi ratio, as Bi content rises and Na content decreases with increasing A:B ratio. Lastly, the Fermi level positions show a strong correlation with electrical conductivity: p-type samples exhibit Fermi levels that are 0.25 ± 0.05 eV lower than those of n-type samples, which may account for the higher electronic conductivity observed in p-type conduction. Finally, electron traps in NBT-6BT were identified using in situ X-ray photoelectron spectroscopy (XPS) under vacuum annealing and cathodic polarization, employing an electrochemical cell setup. This approach comfirmed Bi³⁺/Bi⁰ as a key electron trap, with its charge transition level located at 2.47 ± 0.10 eV above the valence band maximum. Including the electron polaron binding energy, the electrical band gap for NBT(-6BT)-based materials is 4.51 eV.