Transient homologous structures in nonstoichiometric YBa2Cu

Abstract

We propose that a homologous series of transient ordered structures may form during the spinodal decomposition of the nonstoichiometric superconducting oxide $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$. These transient phases provide the best kinetic accommodation of the local vacancy concentration. Their structures should be similar to those of the ${\mathrm{Ti}}_{m}{\mathrm{O}}_{2m\ensuremath{-}1}$ and ${\mathrm{Mo}}_{m}{\mathrm{O}}_{2m\ensuremath{-}1}$ series of Magneli phases. The transient phases in the oxide have the stoichiometry $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{\frac{7\ensuremath{-}n}{(2n+1)}}$, where $n$ is an integer. The structure of the $n$ phase has a diffraction maximum at the point ($\frac{n}{(2n+1) 00}$). Examples have been experimentally observed.