A neutron, X-ray and electron diffraction study of the structures of Pb₃O₂X₂ (X = Cl, Br)

The crystal structures of the compounds Pb3O2Br2 and Pb₃O₂Cl₂ have been re-refined from neutron powder diffraction data. They are found to be more closely isostructural than previous reports suggested. The major difference between the present refinements and previous refinements is in the oxygen position of the bromide compound which has moved significantly along b from y =0.024(5) to 0.9933(4), rather more in keeping with the previously reported structure of the chloride compound. Electron diffraction was used, prior to refinement, to determine the appropriate space group symmetry, given some previous ambiguity in the literature. The space group symmetry was thereby shown to be at least Pn21a, but most probably Pnma. Evidence for some displacive disorder along b was also found in the form of weak diffuse streaking perpendicular to b^*.