Order-disorder transitions and metastable phases produced by mechanical deformations in the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ superconducting system

High-energy mechanical deformations have been used to induce order-disorder transitions in the ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ orthorhombic structure and to synthesize metastable phases. During the milling process, a disorder is produced on the oxygen sublattice and on the Y- and Ba-cationic sites. A metastable simple cubic (${\mathrm{Y}}_{0.33}$ ${\mathrm{Ba}}_{0.67}$)${\mathrm{CuO}}_{3\mathrm{-}\mathit{y}}$ structure is formed. This cubic phase can also be synthesized by mechanical alloying a mixture of the elemental oxides. Discussions on the mechanism of formation, the thermal stability of the metastable structure, and the superconducting properties are presented.