A SYSTEMATIC NEUTRON-DIFFRACTION STUDY OF RBA2CU3O7 (R=YTTRIUM AND RARE-EARTHS) HIGH-T(C) SUPERCONDUCTORS

The 10 K crystal structures of high-T(c) RBa2Cu3O7 (R = Y and rare earths) ceramic materials investigated by neutron diffraction were consistently analysed concerning systematic trends. Several interionic distances exhibit the well known lanthanide contraction behaviour, i.e., a linear relationship with the ionic radii of trivalent rare-earth ions. The only exceptions are associated with the apex oxygen O(1) ions: the chain copper Cu(1)-O(1) distances are constant, and the plane copper Cu(2)-O(1) distances are increasing across the rare-earth series. For R = Pr the 4f-CuO2 valence band hybridization effects are found to be highly anisotropic, since exclusively the interionic distances involving the b-direction (i.e, the chain direction) indicate that the Pr ions have an intermediate valence of about +3.4.