X-Ray Diffraction Study on Single Crystal of La_1.91Ba_0.09CuO₄

Abstract

X-ray diffraction study has been performed on a single crystal of La_1.91Ba_0.09CuO₄ using a four-circle diffractometer. It is found that the crystal structure is orthorhombic (low-temperature-orthorhombic; LTO), but an extra reflection is observed at (½, ½, 0) in the high-temperature-tetragonal (space group I4/mmm) coordinates, which is forbidden by the extinction rule for the well-known LTO structure (space group Bmab), at 300, K. The intensity of the extra reflection is lower than that of the most intense fundamental reflection (1, 1, 0) by three orders of magnitude, but the sharp line shape suggests a long-range lattice modulation of the unit cell. The intensity exhibits no significant change in the temperature range down to 14, K. We also report the same kind of reflections in high-quality single crystals of La_2-xSr_xCuO₄.