Observation of real space electronic state by laboratory X-ray diffractionin YBa2Cu3O6.9 and YBa2Cu3O6.0

The electron-density distribution in orthorhombic superconductor YBa2Cu3O6. 9 (T-c = 91K) and tetragonal insulator YBa2Cu3O6.0 was studied with laborat ory powder X-ray diffraction. At 300K, there is an observable electron dens ity distribution bridging between Cu and O along the a- and h-axes in the t wo-dimensional (2D) CuO2 planes of YBa2Cu3O6.9, which originates from the o verlapping electronic orbitals from Cu 3d and O 2p(sigma) states. The obser ved electron density forms the network of the -Cu-O-Cu- linkage in the 2D C uO2 plane. The electron density profile at 11 K remains similar to the dens ity profile at 300K, which show?, that the network of the electron-density distribution along the -Cu-O-Cu- interatomic linkage in the 2D CuO2 plane i s not due to the thermal vibration, but is intrinsic. On the contrary, no o bservable trace of the interatomic electron density is found in the ZD CuO2 planes of YBa2Cu3O6.0, both Cu and O are rather ionic. The results provide a clue to the role of valence and/or bond state with Cu and O in 2D CuO2 p lanes on the appearance of the metallic state leading to the onset of super conductivity. (C) 1998 Elsevier Science Ltd. All rights reserved.