SYNTHESIS AND STRUCTURE OF BI2LAO4CL - A NOVEL VARIANT OF THE SILLEN PHASES

A new series of layered oxyhalides, Bi(2)MO(4)Cl (M = La, Nd, Y), has been synthesized. Bi2LaO4Cl has been characterised by a combination of X-ray and neutron powder diffraction: space group P4/mmm, a = 3.9547( 1), c = 9.1275(3) Angstrom. The structure consists of alternating laye rs of 'triple fluorite', [M(3)O(4)](+) and halide, Cl-. Within the flu orite layers there is perfect ordering of the cations, -Bi-O-2-La-O-2- Bi, such that La is coordinated only by oxide layers and Bi by both ox ide and chloride layers. This phase represents the simplest example of an intergrowth structure containing the [M(3)O(4)] block layer, which is found in several of the copper oxide superconductors. The structur e also suggests the existence of a new family of layered oxyhalides pa rallel with the well known Sillen phases, [M(2)O(2)]X(n), with triple [M(3)O(4)] fluorite layers replacing the normal double [M(2)O(2)] laye rs.