Orthorhombic low-temperature superstructures in YBa2Cu3O6+x

The emergence of the cell-doubled (OII) and both cell-tripled (OIII and ${\mathrm{OIII}}^{*})$ phases of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ are observed on oxygen-chain-equalized and order-stabilized pair samples at an extended range of oxygen concentrations $(0.28lxl0.83),$ including the interval of x, which is characteristic of the onset of superconductivity. The phase diagram for the asymmetric next-nearest-neighbor interaction model, extended to include an additional longer-range interaction potential between oxygen atoms, is calculated and compared to two experimental phase diagrams, obtained for intercalated as well as for deintercalated members of the pair samples. A fairly good agreement with experimental data on locations of the OII, OIII, and ${\mathrm{OIII}}^{*}$ phases in the phase diagram and on the average length of the O-Cu-O chains is obtained.