Monoclinic phase transition and twinning in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{y}}$ single crystals

An orthorhombic-to-monoclinic phase transition with associated (100) twinning has been observed by transmission electron microscopy in superconducting ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{\mathit{y}}$ single crystals after heating to between 350 and 400 °C. A model describing the monoclinic phase structure and the twin boundary geometrical configuration is presented. We propose that the monoclinic phase transition results from oxygen ordering in the ${\mathrm{CuO}}_2$ planes associated with oxygen diffusion out of the crystal at high temperature and high vacuum. This transition is accompanied by (100) twinning to reduce the strain energy.