Ordering and Decomposition in the High-Temperature Superconducting CompoundYBa2Cu3Ox

The method of concentration waves is applied to structural transformations in superconducting Y-Ba-Cu oxide. The method predicts an ordering reaction that results in a tetragonal-to-orthorhombic phase transformation, and also yields the correct structure of the Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ orthorhombic ordered phase. The results suggest that at low temperature the off-stoichiometric orthorhombic phase is thermodynamically unstable with respect to secondary decomposition into a mixture of ordered orthorhombic and disordered tetragonal phases, whose microstructure can be anticipated.