High-resolution synchrotron x-ray study of the structure of ${\mathrm{La}}_{1.8}$${\mathrm{Ba}}_{0.2}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$

X-ray diffraction of ${\mathrm{La}}_{1.8}$ ${\mathrm{Ba}}_{0.2}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$ reveals two macroscopically segregated tetragonal (${\mathrm{K}}_2$ ${\mathrm{NiF}}_4$-type) phases of nearly identical lattice parameter. Many peaks show additional broadening upon cooling. This broadening is consistent with a spontaneous monoclinic distortion, with an onset temperature of ≊150 K, and is possibly relevant to the superconducting properties. Small single crystals of ≊70 μm diameter within the powder aggregate are also studied and show a similar two-phase constituency and a resolvable peak splitting at low temperature.