Thermodynamics of the movable oxygen and conducting properties of the solid solution YBa₂Cu_3.xCo_xO_6+δ at high temperatures

High precision coulometric measurements of the equilibrium oxygen content in the solid solution YBa₂Cu_3-xCo_xO _6+δ, where x = 0, 0.2, 0.4, 0.6 and 0.8, were carried out using a double-cell technique in the temperature range 600 - 850 °C and at oxygen pressure varying between 10^-5 and 1 atm. The data were employed to determine the partial molar enthalpy and entropy of the movable oxygen depending on δ and x. The electrical conductivity and thermopower were also measured in the same range of the external parameters, and their dependence on the oxygen concentration was determined at different cobalt content. The data reveal several types of oxygen sites participating in the gas-solid equilibrium. The behavior of thermodynamic functions is indicative of the partial ordering of the complex species which form the structural layer CU_1-xCo _xO_δ with variable content of oxygen and cobalt. It was shown that replacement of copper by cobalt does not result in appearance of the electronic charge carriers. The behavior of the thermopower and electric conductivity was explained with a narrow band model. The energy change with δ and x of the p-band, which dominates the conductivity, was found to follow the respective change in the oxygen partial enthalpy. Thus, electronic carriers in the layered structure of the cuprate are strongly influenced by the labile oxygen ions.