Bi-Sr-O type: Structural Relationships with Monoclinic epsilonBi4.86La1.14O9 Form

Structural investigations of Bi0.775Ln0.225O1.5 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 Tb. 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 rhombohedral 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 interslab spaces, is compensated with the presence of the Bi(2) lone pair electrons localized at a significant distance (~=to1.20 ?), which participate in the cohesion of the crystal structure. TEM study of Bi0.775Ln0.225O1.5 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 q1* and q2* modulation wave vectors.