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

The ortho-II structure of YBa2Cu3O6+x (x = 0.5 and x = 0.6), character ized by alternating Cu-O-Cu and Cu-Cu chains, is determined at 295 K a nd at 20 K by single-crystal X-ray diffraction on a four-circle diffra ctometer for three samples. About 260 main and superstructure reflecti ons have been used for one structure refinement. The buckling of the c opper plane in the superconducting layer is at room temperature +/-0.0 015(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 al l three samples.