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We report on a comparative study of the transport properties of lightly La-doped layered CaO (CaMnO 3 ) m (mϭ1, 2, and ϱ͒ and similar Mn 3ϩ /Mn 4ϩ ratios. In contrast to the large and rather nonsystematic variations of the resistivity (T) for samples with the same doping and various m, the absolute thermopower ͑S͒ exhibits remarkable systematics. In the paramagnetic ͑PM͒ state, S varies linearly with temperature and its extrapolation to Tϭ0 has a finite intercept. The intercept and the slope of these lines vary systematically with m. In particular, within the experimental error, the inverse slopes of the straight lines increase linearly with 1/m, which in its turn varies linearly with the average number of Mn-Mn near neighbors. The implications of this simple systematics are discussed using a model developed by Cutler and Mott ͓Phys. Rev. 181, 1336 ͑1968͔͒ for transport by activated hopping of carriers obeying nondegenerate statistics. This model ͑or a possible extension into the semidegenerate regime͒ leads also to a simple, qualitative interpretation of S(T) of these materials on crossing the Néel temperature (T N ) from the PM state and below T N .

The electrical conductivity of polycrystalline CoO1+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 CoO1+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 Eu=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 Δhv+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 Ap, 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 Ap. The term dAp/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 dAp/d(kT)−1 in range A is close to Eμ, as the theory suggests. The resistivity of quenched samples of CoO1+x was measured at temperatures below 70°C for different x. Above the Néel temperature (tN≅19°C) the activation energy of the conductivity is ½E1+Eμ≅0.5 eV in a wide range of concentrations. This value is consistent with E1 and Eμ 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>tN) and ε(t<tN) 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.

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