Reciprocal and real space analysis of local disorder in Al doped SmBa2Cu3O6+d superconductor

Relations between structure and physical properties in REBa2Cu3O6+d (RE-123) High Temperature Superconductors (RE = Y, Ln; 0 < d < 1) are not fully understood. As an example, the effect of long range or local Cu-O chain ordering on the superconducting properties has still to be unambiguously probed by experiment [1]. A suitable way to handle this problem is to perform cation substitutions for copper in RE-123 and study the physical and structural properties of the doped materials. Aluminium seems to be an optimal substituent as it is a non-magnetic ion and it substitutes for copper only on one of the two crystallographic non-equivalent positions, Cu1 (0,0,0) [2]. As a part of a wider study on the relation between the short/long range structure and the physical properties of Al doped REBa2Cu3O6+d, the attention in the present work is focused on the structural disorder induced by Al-doping in SmBa2Cu3-xAlxO6+d For this purpose samples of SmBa2Cu3-xAlxO6+d0< d < 1, x = 0, 0.15, 0.33) have been investigated by means of X-ray Powder Diffraction (XRPD) at the ID31 beamline of the European Synchrotron Radiation Facility, as a function of aluminium concentration, oxygen non-stoichiometry d and temperature. The XRPD data have been refined also in real space through the analysis of the pair distribution function (PDF) thanks to the high counting statistics and to the wide Q range (Qmax ~ 18 Å-1). In highly doped samples the tetragonal-to-orthorhombic phase transition is inhibited because the local tetrahedral environment for Aluminium inhibits the formation of long Cu-O chains while oxidising. The actual length of Cu-O chains (at d ~ 1) is a function of both aluminium concentration and clustering. The aluminium insertion induces local disorder due to heavy ion displacements as it can be evaluated comparing the atomic thermal factors (Uiso) of doped and undoped samples and by the PDF analysis.