The relation between high-temperature thermopower, conductivity and oxygen content in YBa₂Cu₃O_6+x

Experimental data, more detailed and complete than previously available, for thermopower and conductivity in YBa₂Cu₃O_6+x was obtained in the temperature range 600 to 850°C and oxygen content varying in the limits 0.1≤x≤0.8 under equilibrium pressure of oxygen. The concentration dependencies of conductivity and thermopower were derived using an equilibrium P(O₂)-T-x diagram. Isothermal plots of thermopower versus the logarithm of conductivity show unusual features in the extrinsic hole regime, i.e. a small inward curvature, an anomalously small slope and a weak temperature dependence were observed. The decrease in the oxygen content at high temperature results in the expected transition to intrinsic behavior and the contribution of electrons to the conductivity. The results obtained have been interpreted in the framework of the band structure comprised of two narrow bands on the background of two broad bands. The main influence of the broad bands is on the position of the chemical potential, whereas the location of the narrow bands relative to the chemical potential of the electrons impacts the electron transport.