Orthorhombic — tetragonal phase transition and oxygen index of YBa2Cu3O6+δ

The orthorhombic — tetragonal phase transition of YBa₂Cu₃O_6+δ has been investigated by X-ray powder diffraction methods. The transition is of the second order or very close to second order and no coexistence of tetragonal and orthorhombic phases has been observed. The transition strongly depends on the oxygen index δ which changes with temperature. Typical patterns of the triplet 006, 020, and 002 obtained by precise X-ray powder diffraction measurements have been presented and shown to be useful for identification of YBa₂Cu₃O_6+δ and estimation of oxygen index δ.