Defect-mediated remarkable ionic conductivity in undoped nickel oxide at low temperature

Undoped nickel oxide (NiO) is conventionally known as a p-type semiconductor with dominant hole transport. However, for the first time, this study reveals an exceptionally high ionic conductivity in dense, undoped NiO ceramic at remarkably low temperature of 375 °C. High density NiO ceramics (relative density > 97%) sintered at 1300 °C demonstrate ionic conductivity σ ∼ 5 × 10−3 S/cm at 375 °C, surpassing many conventional solid oxide fuel cells applications and leveling up to the performance of lithium (Li)-based solid electrolytes at comparable temperatures. The temperature-dependent DC conductivity relation follows Arrhenius behavior with an activation energy of ∼0.78 eV. Impedance spectroscopy analysis reveals a systematic evolution from semicircular to characteristic teardrop-shaped Nyquist plot (Z″ vs Z′) with increasing temperature, indicating the emergence of ambipolar transport. In situ x-ray diffraction at high temperatures demonstrates lattice expansion and increasing non-stoichiometry with temperature due to the formation of cation vacancies that facilitate ion migration. Chronoamperometric transference number measurements yield an ionic transference number of approximately 0.98, indicating that ionic transport constitutes the dominant contribution to the measured conductivity. UV-Visible absorption spectroscopy and photoluminescence spectroscopy further identify sub-bandgap defect states and defect-related transitions arising from strong hole-phonon coupling. The combined structural, electrical, and optical evidence establishes defect-mediated ionic transport as the origin of the remarkable conductivity. These findings position undoped NiO as a promising low-temperature ionic conductor for next-generation material for electrochemical energy conversion.