Local ordering and charge transfer during room-temperature annealing of quenched tetragonal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.25}$

Neutron powder diffraction was used to study the crystallographic structure of a quenched tetragonal sample of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.25}$ during room-temperature annealing. It is found that the lattice parameters decrease with anneal time. The time dependence of the two lattice parameters is characterized by a single relaxation time ∼760 min. Similar relaxation times were recently found for the electrical resistances ${\mathit{R}}_{\mathit{a}\mathit{b}}$, ${\mathit{R}}_{\mathit{c}}$, in tetragonal single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.25}$. These results are interpreted in terms of local ordering of the oxygen ions in the basal ${\mathrm{CuO}}_{\mathit{x}}$ plane, leading to a charge transfer between the ${\mathrm{CuO}}_{\mathit{x}}$ and ${\mathrm{CuO}}_2$ planes. It is the same effect that leads to the rise in ${\mathit{T}}_{\mathit{c}}$ observed earlier in orthorhombic ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ (x>0.35). The mechanism of the local ordering and the concept of oxygen ions ordering in a structure with tetragonal symmetry are discussed.