Pressure-composition isotherms for the oxygen solution in YBa 2Cu 3O 6+x

Abstract

The oxygen content x in YBa 2Cu 3O 6+x was measured at oxygen pressures between about 1 and 1500 mbar and temperatures between 400 and 750°C. Additionally, the locus of the orthorhombic (O)-to-tetragonal (T) phase boundary was determined in the range 550–700°C. From the results, the chemical potential, Δμ O( x), and the partial molar enthalpy, Δ H O( x), and entropy, Δ S O( x), of oxygen in YBa 2Cu 3O 6+x were derived for both phases. Between 525 and 675°C the Δμ O( x) curves clearly change in curvature around concentrations coinciding with those obtained for the O-T transition. Δ H O decreases significantly with changing x going from the T to the O phase region. In the O phase, Δ S O exhibits large negative deviations from the ideal configurational entropy. These features are typical of the thermodynamic behaviour of a lattice gas with an order-disorder transition.