Defects and Transport in YBa₂ Cu₃ O_{7 − x}

Abstract

In an attempt to understand the role of stoichiometry in controlling the transport properties of the system YBa₂Cu₃O_7−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.