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 Tb. The comparison with the monoclinic Bi 4.86 La 1.14 O 9 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 Bi 4.86 La 1.14 O 9 structure, the absence of oxygen in the inter-slab spaces, is compensated with the presence of the Bi (2) lone pair electrons localized at a significant distance (D1.20 ( A), 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. # 2002 Elsevier Science (USA)