High-Temperature Defect Structure and Electrical Properties of NiO

The electrical conductivity and the Seebeck coefficient of single‐crystal and polycrystalline NiO were measured as a function of temperature and of oxygen partial pressure over a large part of the phase field of stable NiO. In the temperature range 1000°–1600°C the electrical conductivity was found to be proportional to the fourth root of the oxygen partial pressure. This pressure dependence is that which one would expect for pure, nonstoichiometric NiO containing singly ionized metal vacancies as the predominant point defects. The activation energy for conduction, ${\mathrm{\Delta H}}_c$ , computed from ${\mathrm{\sigma \hspace{0.17em}=\hspace{0.17em}\sigma }}_0\exp {\mathrm{(-\Delta H}}_c\mathrm{\hspace{0.17em}/\hspace{0.17em}kT)}$ , was found to be 0.92 ± 0.02 eV. The Seebeck coefficient of NiO was found to be $p$ type over the entire range of temperatures and oxygen partial pressures measured. From the temperature dependence of the Seebeck coefficient, the enthalpy of formation of defects in NiO was calculated to be 0.66 ± 0.03 eV. From the measured temperature dependence of the electrical conductivity and the Seebeck coefficient it was found that the charge‐carrier mobility increases with the temperature. Considering the drift mobility to be of the form ${\mu }_p{\mathrm{\propto T}}^{\text{−1}}\exp {\mathrm{(-\Delta H}}_m\mathrm{\hspace{0.17em}/\hspace{0.17em}kT)}$ a value of 0.37 eV could be attributed to the activation energy for motion of the charge carrier, ${\mathrm{\Delta H}}_m$ . The existence of such an activation energy supports the suggestion of a hopping process for the charge‐carrier transport mechanism in NiO above about 1000°C.