Effect of oxygen pressure on the orthorhombic-tetragonal transition in the high-temperature superconductor YBa2Cu3O/sub x/

High-resolution in situ x-ray-diffraction measurements were made of the lattice constants of YBa2Cu3O/sub 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 0C to 521 +- 10 0C 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