Powder neutron diffraction and TEM investigations of Bi(0.775)Ln(0.225)O(15) oxide conductors (Ln=La, Pr, Nd, Sm,Tb, Dy) with rhombohedral Bi-Sr-O type: Structural relationships with monoclinic epsilon-Bi4.86La1.14O9 form

摘要

Structural investigations of Bi(0.775)Ln(0.225)O(1.5) phases with rhombohedral Bi-Sr-O type, have been carried out using complementary techniques. Average crystal structures were refined from neutron diffraction powder data using the Rietveld method for Ln = La, Pr, Nd and Th. The comparison with the monoclinic Bi4.86La1.14O9 structure previously determined using the same technique has been done. For both types of structure, a piling of slabs constituted of a (Bi-(1), Ln) oxide layer, inserted between two Bi-(2) oxide layers, is observed. In the rhombobedral phases there are moreover oxide ions in the inter-slab spaces responsible for the well-known conductivity of these phases, while in the monoclinic Bi4.86La1.14O9 structure, the absence of oxygen in the inter-slab spaces, is compensated with the presence of the B-(2) lone pair electrons localized at a significant distance (congruent to1.20 Angstrom), which participate in the cohesion of the crystal structure. TEM study of Bi(0.775)Ln(0.225)O(1.5) samples, has confirmed the existence of superstructures for these phases and has allowed to propose two types (respectively for La and for Nd, Sm, Tb or Dy) of q(1)* and q(2)* modulation wave vectors. (C) 2002 Elsevier Science (USA). [References: 30]