Defects and Transport in YBa2 Cu3 O7 − x
In an attempt to understand the role of stoichiometry in controlling the transport properties of the system YBa2Cu3O7−x (0 < x < 1), we have performed electrical conductivity (σ) and thermoelectric power (Q) measurements, in situ, as a careful function of temperature, oxygen partial pressure, and oxygen stoichiometry.
In the low temperature orthorhombic phase, a continuous transition from semiconducting to metallic conduction was observed as temperature decreases and Po2 increases as a result of exothermic generation of holes. Both the magnitude of Q and the Po2 dependence of σ and Q confirm the p-type nature of the carriers. In the high temperature tetragonal phase a p-n semiconducting transition is detected which shifts to higher temperature as the Po2 is increased. Based on these results and results of measurements performed as a function of temperature for fixed values of x, we conclude that the n-type contribution is a result of the disproportionation of Cu1+ to Cu0 and Cu2+ which is characterized by an energy of 1.24 eV. A preliminary defect model is proposed which provides a framework for understanding the Po2 dependence of σ and Q.
KeywordsOxygen Partial Pressure Thermoelectric Power Oxygen Stoichiometry Mass Balance Relation Absolute Thermoelectric Power
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