Neutron diffraction and EM structure determination of the oxide PbBa 0.8Sr 1.2PrCeCu 3O 9, a perfectly ordered intergrowth of rock salt, oxygen-deficient perovskite, and fluorite-type structures

Abstract

The structure of PbBa 0.8Sr 1.2PrCeCu 3O 9 has been determined with powder neutron diffraction techniques and profile analysis. The symmetry of this oxide is pseudotetragonal P4 mm, but ED study shows that it is orthorhombic, space group Cmm2, with cell parameters a = 5.4816(1) Å, b = 5.4703(1) Å, and c = 16.4017(4) Å. The structure of this oxide corresponds to a triple intergrowth of single and double oxygen-deficient perovskite layers, with single rock salt and double fluorite-type layers. The analysis of the structure does not show any statistical distribution of copper and lead in the same sites, contrary to what was found for similar compositions. The main original structural features are: (a) the disordered distribution of oxygen in the PbO layers and the distorted tetrahedral coordination of lead PbO 3L due to the 6 s 2 lone pair of Pb(II)(L); (b) the angle of the OCuO bond of univalent copper (164°); (c) the off-centering of rare earth elements in the “O 8” cages, which move away from the copper layers; and (d) the displacement of Cu(II) inside the CuO 5 pyramids toward the apical oxygen. A comparison with Pb 2Sr 2YCu 3O 8-type structures is made.