Single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ were studied by transmission Laue photography and monochromatic diffraction techniques, using the Cornell High Energy Synchrotron Source and a conventional rotating anode x-ray source. Two types of twinning were observed: the well-known (110)-type twinning in one sample and a 90° twinning about the c axis in another crystal whose superconducting properties were excellent, thereby precluding the necessity of (110) twins for high $T_c$. Diffraction experiments on the sample with the (110) twins reveal subgrains, each of which has a value of the orthorhombic parameter (a-b)/a differing by as little as 0.003 from the others. Such a microstructure distribution suggests a multiphase character and is discussed in relation to recent evidence of granular superconductivity.

• Received 30 September 1987

DOI:https://doi.org/10.1103/PhysRevB.37.2301