A systematic low-temperature neutron diffraction study of the RBa₂Cu₃O_x(R=yttrium and rare earths; x=6 and 7) compounds

Abstract

The crystal structures of RBa2Cu3Ox (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 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 within error limits, and the plane copper Cu(2)-O(1) distances are increasing across the rare-earth series. The much stronger increase of the distance Cu(2)-O(1) in the RBa2Cu3O6 series compared to the RBa2Cu3O7 series can be explained by the increase of Tc from 90 K for YbBa2Cu3O7 to 96 K for NbBa2Cu3O7. The smaller distance Cu(1)-O(1) for the RBa2Cu3O6 series compared to the RBa2Cu3O7 series may be related to the suggested double-well potential of the apical oxygen ion. For some interionic distances of PrBa2Cu3O7 approximately parallel to the h direction (i.e., the chain direction) we determine by extrapolation a valence of +3.4 for 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 critical value of about 167.3.

The Dependence of the Lattice Parameters on Oxygen Content in Orthorhombic YBa2Cu3O6+x: A High Precision Reinvestigation of Near Equilibrium Samples

“…In samples synthesized by a similar method as in this work, structure refinement by neutron powder diffraction showed a 4% occupancy (with a standard deviation of 1%) of the O5 site for x"0.996 (25). Also in R-123 samples differently synthesized but still not quenched a similar O5 occupation has been found (26). We note that O(4) sites in twin domains smaller than approx.…”

Structure of Al Defect in High-Temperature Superconductor, Al-Doped Sm-123: An Electron Density Study

“…In fact, the simultaneous occupation by O3 ions of the ( , 0, 0) and (0, , 0) sites among the aluminium ions statistically saves the symmetry of the tetragonal P4/mmm space group also in the oxidized sample. In contrast, in undoped samples, when oxygen enters in the structure by annealing under oxidizing conditions, O3 ions order along the b direction, determining a tetragonalPorthorhombic phase transition (21).…”

“…Pure SmBa Cu O >B ( 0) crystallizes in the P4/mmm tetragonal space group (21). With the addition of oxygen it undergoes a tetragonalPorthorhombic phase transition and, when 1, the space group is Pmmm (21).…”

“…With the addition of oxygen it undergoes a tetragonalPorthorhombic phase transition and, when 1, the space group is Pmmm (21).…”

THE DIMPLING IN THE CuO2 PLANES OF YBa2Cu3Ox (x=6.806–6.984, T=20–300K) MEASURED BY YTTRIUM EXAFS

“…The dimpling in metallic Y-123 is ≥ 0.25Å [3,4], but ≤ 0.05Å in metallic Hg-1212 [2]. We conclude that also other physical properties than metallicity and superconductivity may be at the origin of the dimpling, and the net effect possibly attributable to the superconductivity in the high-T c cuprates may be as small as 0.05Å.…”