Effect of oxygen pressure on the orthorhombic-tetragonal transition in the high-temperature superconductor YBa₂Cu₃O_x

High-resolution in situ x-ray-diffraction measurements were made of the lattice constants of YBa₂Cu₃O_x as it was heated and cooled through the orthorhombic-tetragonal transition at a series of O₂ pressures. The transition is sharp, continuous, and reversible in temperature and pressure. As O₂ pressure is reduced from 1.0 to 0.005 atm, the transition temperature falls from 676±5 °C to 521±10 °C while oxygen content at the transition falls from 6.66±0.01 to 6.59±0.02. Dilation along the c axis is observed as oxygen is removed. Under helium, an irreversible but time-dependent orthorhombic-to-tetragonal transition occurs during heating.