A defect equilibrium model is suggested for CaMnO 3−δ based on oxygen non-stoichiometry and conductivity data. The model includes reactions of oxygen exchange and thermal excitation of electrons. The respective equilibrium constants, enthalpies and entropies for the reactions entering the model are obtained from the fitting of the calculated and experimental data for oxygen nonstoichiometry. The energy parameters obtained from the model and variations in the concentration of manganese species enable explanation of temperature and oxygen pressure dependencies of thermopower and conductivity within frameworks of a small polaron type model.
The drift mobility of electron charge carriers in oxygen non-stoichiometric manganite CaMnO 3− δ was calculated by combining the total electrical conductivity and oxygen non-stoichiometry data at 700-950°С and oxygen partial pressure varying between 10 −6 and 1 atm. The carrier concentration changes with pressure and temperature were obtained with the help of the earlier-developed defect model involving reactions of oxygen exchange and thermal excitation of manganese sites. The activation energy for mobility is found to increase with oxygen non-stoichiometry. High-temperature electron transport properties of the manganite CaMnO 3−δ can be explained in terms of activated jumps of n-type small polarons in adiabatic regime. The relatively small mobility of charge carriers is explained by strong localization of polarons on manganese sites.
The experimental data for equilibrium oxygen content were used in order to extract increments of partial molar thermodynamic functions of oxygen with changes of oxygen stoichiometry in calcium manganite CaMnO 3−δ . It is shown that along with the oxygen exchange reaction, thermal excitation of Mn 4+ cations plays an important role in equilibration of charged manganese species that appear in response to the loss of oxygen at heating. The interrelation of partial molar enthalpy and entropy of oxygen with electron and ion defect formation parameters is obtained in approximation of the point defect model. The nearly linear changes of oxygen partial molar enthalpy are shown to directly reflect thermally driven changes in concentration of Mn 3+ cations.
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