Electrical Properties of Cobalt Monoxide

Abstract

The electrical conductivity of polycrystalline CoO_1+x was measured as a function of temperature and partial pressure of oxygen, P(O₂). The range of temperature was 900°—1450°C and the range of partial pressure was 1 − 10⁻¹² atm. The plot of the conductivity against partial pressure of oxygen gave two distinct regions: Region A, where the conductivity is proportional to P(O₂)^¼, and Region B, where the conductivity is proportional to P(O₂)^⅙. From these results, together with considerations of ionic radii, we concluded that the dominant defects in CoO_1+x at high temperatures are cation vacancies, with the majority being singly ionized in Region A and doubly ionized in Region B. A combination of gravimetric and electrical measurements showed that the mobility of the holes contributed by these vacancies rises exponentially with temperature. The activation energy of mobility is E_u=0.3 eV. At 1350°C, the mobility is about 0.4 cm² V⁻¹·sec⁻¹. These values are discussed in terms of polaron theory. The sum of the enthalpy of formation of a neutral vacancy and the first ionization energy is Δh_v+E₁=0.56 eV. The second ionization energy was measured separately and found to be E₂=0.65 eV. A rough estimate of the first ionization energy gives E₁≅0.45 eV. Analysis of Seebeck‐coefficient measurements performed in the same range of temperature and oxygen pressure is consistent with the results of the conductivity measurements regarding the sign and number of charge carriers. The term A_p, the so‐called heat of transport, was calculated on the assumption that p₀, the molar concentration of available states for charge carriers, is about 1. This assumption gave negative values for A_p. The term dA_p/d(kT)⁻¹, which is independent of the value of p₀, is about 0.24 eV in Range A, and 0.54 eV in Range B. The value of dA_p/d(kT)⁻¹ in range A is close to E_μ, as the theory suggests. The resistivity of quenched samples of CoO_1+x was measured at temperatures below 70°C for different x. Above the Néel temperature (t_N≅19°C) the activation energy of the conductivity is ½E₁+E_μ≅0.5 eV in a wide range of concentrations. This value is consistent with E₁ and E_μ evaluated at high temperatures. At very low deviations from stoichiometry the activation energy increases, due perhaps to the appearance of electrons donated by interstitial cobalt atoms. Below the Néel temperature the activation energy ε decreases by about 0.1 eV, the difference between ε(t>t_N) and ε(t<t_N) depending on the departure from stoichiometry. This decrease is not understood. The appendix contains a slight correction to the phase diagram that was previously published. This correction is due to a more refined analysis of the data.