High-Temperature Electrical Properties of Manganese Monoxide

The deviation from stoichiometry, the electrical conductance G, and the Seebeck coefficient Q of MnO were measured as function of temperature T and oxygen partial pressure, P(O2), where T varied between 1100° and 1557°C and P(O2) varied from 10−18 to 10−1 atm. We found that MnO is an oxygen-excess compound, the dominant defects being manganese vacancies. In the range of P(O2) which corresponds to P(CO2)/P(CO)<50, these vacancies are mainly doubly ionized, the concentration of holes is proportional to the one-sixth power of P(O2) and exceeds the concentration of free electrons in all or most of this range. The mobility of the free electrons was shown to be greater by a factor of 10 or more than that of the holes. This feature is responsible for the fact that for P(CO2)/P(CO)<1 the oxide behaves like an n-type semiconductor, i.e., Q is negative and G is proportional to a negative power of P(O2). The intrinsic point, where n=p, occurs at much lower oxygen pressures and may indeed fall outside the range of our measurements. The conductance and the deviation from stoichiometry plotted against 1/T, show a break near 1375°C, indicating a possible phase transition in MnO. The mobility of the holes was found to rise exponentially with temperature, the activation energy being 0.37±0.07 eV over the whole range of our measurements. This value is consistent with small-polaron theory. The analysis of Q supports the hopping model. The measurements of G and Q on single crystal gave the same results as on polycrystalline samples, except in the region of high temperature and high P(O2), where the G's differ. This difference is explained by the presence of grain boundaries in the polycrystalline samples.