Equilibrium pressures of oxygen above YBa₂Cu₃O_7±x up to 2,000 bar

Abstract

The interaction of oxygen with the high-temperature superconducting oxides is very important because several phenomena that influence the superconducting properties, such as the orthorhombic to tetragonal phase transformation, are dependent on the oxygen stoichiometry. In spite of this, many of the basic thermodynamic pressure data remain unpublished. The existing information indicates that the decomposition of YBa₂Cu₃O_7–x starts at T = 400 °C at Po₂ = 1 bar (ref. 1), and that the compound melts incongruently². Pressure data and corresponding melting points are therefore needed, to ascertain if it is possible to change the phase diagram at higher oxygen pressures so that the compound melts congruently. If this were feasible, then the growth of large single crystals from a stoichiometric melt could be possible. Here we present the equilibrium oxygen pressure curve for the YBa₂Cu₃O_7±x decomposition (P–T phase diagram) up to 2,200 bar and 1,200 °C. To construct the three-dimensional P–T–x phase diagram of the pseudo-binary system O₂–YBa₂Cu₃O_7±x, we have also developed methods for measuring the T–x diagram at high pressure, and we present the first data at 10 bar oxygen pressure.