Defect Equilibria and Transport in YBa2Cu3O7‐x at Elevated Temperatures: I, Thermopower, Electrical Conductivity, and Galvanic Cell Studies

Abstract

Studies of the Y‐Ba‐Cu‐O system at elevated temperatures (300 to 1173 K) and under controlled oxygen activity (102 to 105 Pa) were performed by using electrical methods such as thermopower, electrical methods such as thermopower, electrical conductivity, and emf of solid‐state galvanic cell measurements. The discontinuities of measured electrical parameters demarcate regions of different properties and phenomena: (a) the transition between the range when the oxide sample is quenched on one side and the equilibrium range on the other side and (b) the transition between the quasi‐metallic and semiconducting (m/s) regimes. The temperature of the m/s transition increases from 950 K at po2= 102 Pa to 1150 K at 104 Pa. These data have served to derive a T‐po2 diagram for YBa2Cu3O7‐x. NO discontinuity of the measured electrical parameters has been observed under conditions accompanying the phase transition between the tetragonal and orthorhombic structures. A p‐n type transition has been observed within the semiconducting regime. Electrochemical oxygen titration has led to the determination of the relationship between the lattice oxygen content and the oxygen activity in the ambient gas atmosphere.