Synthesis and structure of Bi₂LaO₄Cl: A novel variant of the sillen phases

A new series of layered oxyhalides, Bi₂MO₄Cl (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) Å. The structure consists of alternating layers of 'triple fluorite', [M₃O₄] ⁺ and halide, Cl^-. Within the fluorite layers there is perfect ordering of the cations, -Bi-O₂-La-O₂-Bi, such that La is coordinated only by oxide layers and Bi by both oxide and chloride layers. This phase represents the simplest example of an intergrowth structure containing the [M₃O₄] block layer, which is found in several of the copper oxide superconductors. The structure also suggests the existence of a new family of layered oxyhalides parallel with the well known Sillen phases, [M₂O₂]X_n, with triple [M ₃O₄] fluorite layers replacing the normal double [M ₂O₂] layers.