Electrical Conductivity and Thermodynamic Equilibrium in Nickel Oxide

Abstract

The electrical conductivity of nickel oxide has been studied at temperatures between 600° and 1350°C and partial pressures of oxygen between 1 atm and 10⁻⁴ atm. The slope of log conductivity vs reciprocal temperature plots increases when the temperature is high enough for the sample to come into thermodynamic equilibrium with the atmosphere, and the temperature at which the change in slope occurs is observed to depend on the rate of heating or cooling of the crystal. The heat of formation of nickel vacancies may be determined from the magnitude of the change in slope by a simple calculation. Nickel oxide is a p‐type semiconductor [M. Verwey, M. Haaijman, H. Romeijn, and M. van Oosterhout, Philips Research Repts. 5, 173 (1950)] in which the conductivity is proportional to the concentration of Ni³⁺ ions in the lattice. When thermodynamic equilibrium is established with the atmosphere the conductivity is proportional to the ⅙th power of the oxygen partial pressure. The analysis of the conduction mechanism and the conduction data may be self‐consistently correlated with nickel diffusion, nickel vacancy diffusion, and weight changes resulting from equilibration with different oxygen partial pressures. The following constants may be determined from these correlations: heat of formation of nickel vacancies, activation energy for nickel vacancy migration, activation energy for electron hole migration, and concentrations of nickel vacancies and Ni³⁺ ions as a function of temperature and oxygen pressure. Combined results give the following for the concentration of nickel vacancies: 0.11 (P_O2)^⅙ exp(−17 800/RT) vacancies per ion pair.