DISAPPEARANCE OF COPPER DISPLACEMENTS IN THE 20-K ORTHO-II STRUCTURE OF YBA2CU3O6+X OBSERVED BY X-RAY-DIFFRACTION

The ortho-II structure of YBa2Cu3O6+x (x = 0.5 and x = 0.6), characterized by alternating Cu-O-Cu and Cu-Cu chains, is determined at 295 K and at 20 K by single-crystal X-ray diffraction on a four-circle diffractometer for three samples. About 260 main and superstructure reflections have been used for one structure refinement. The buckling of the copper plane in the superconducting layer is at room temperature +/-0.0015(3) (0.018 Angstrom) for O-6.50 +/-0.0031(4) (0.036 Angstrom) for O-6.60 and +/- 0.0014(4) (0.016 Angstrom) for tempered O-6.60, relative to the averaged positions. The buckling has disappeared at 20 K in all three samples.