A SYSTEMATIC LOW-TEMPERATURE NEUTRON-DIFFRACTION STUDY OF THE RBA2CU3OX (R = YTTRIUM AND RARE-EARTHS, X = 6 AND 7) COMPOUNDS

The crystal structures of RB22Cu3Ox (R = Y and rare earths; x = 6 and 7) ceramic materials were investigated at 10 K by neutron diffraction and consistently analysed concerning systematic trends. Other than for non-superconducting PrBa2Cu3O7 the lattice parameters and most interi onic distances exhibit the well known lanthanide contraction behaviour , i.e., a linear relationship with the ionic radii of trivalent rare-e arth ions. The only exceptions are associated with the apex oxygen O(1 ) ions: the chain copper Cu(1)-O(1) distances are constant within erro r limits, and the plane copper Cu(2)-O(1) distances are increasing acr oss the rare-earth series. The much stronger increase of the distance Cu(2)-O(1) in the RBa2Cu3O6 series compared to the RBa2Cu3O7 series ca n be explained by the increase of T(c) from 90 K for YbBa2Cu3O7 to 96 K for NbBa2Cu3O7. The smaller distance Cu(1)-O(1) for the RBa2Cu3O6 se ries compared to the RBa2Cu3O7 series may be related to the suggested double-well potential of the apical oxygen ion For some interionic dis tances of PrBa2Cu3O7 approximately parallel to the b direction (i.e., the chain direction) we determine by extrapolation a valence of +3.4 f or the Pr ions. This indicates for PrBa2Cu3O7 a highly anisotropic 4f- CuO2 valence band hybridization. An important structural property with respect to the superconductivity is the puckering of the CuO2 planes: the superconductivity is lost when the puckering angle exceeds a crit ical value of about 167.3-degrees.