First-principles investigation of small polarons in rhombohedral NaNbO 3

Sodium niobate ( NaNbO 3 ) is a perovskite oxide and a key component of emerging lead-free antiferroelectric capacitors for high-energy-density applications. However, its performance can be hindered by irreversible phase transitions and leakage currents associated with low electrical resistivity. Defect and doping engineering offers a potential way to overcome these problems, but its use requires a detailed understanding of electronic, ionic, and polaron charge-compensation mechanisms, where the role of polarons remains largely unexplored. Here, we investigate the stability of small hole and electron polarons in rhombohedral NaNbO 3 , which is a structurally well-defined model system that avoids lattice-dynamical instabilities. Trapping energies are calculated using density functional theory corrected by a Hubbard U , using the enforced-piecewise-linearity approach including finite-size scaling. For the small hole-polaron centered on O- 2 p orbital, we find a trapping energy of − 0.65 eV and an adiabatic migration barrier of 0.32 eV determined by nudged-elastic-band calculations. In contrast, we show that excess electrons do not self-trap on Nb- 4 d orbitals, reflecting weak electron-phonon coupling in the conduction band manifold. These results identify oxygen as an intrinsic hole trap in NaNbO 3 and highlight the importance of including hole polarons in defect models of NaNbO 3 -based electroceramics.