Acta Crystallographica Section B

Structural Science

Volume 57, Part 3 (June 2001)

research papers

Acta Cryst. (2001). B57, 237-243 [ doi:10.1107/S0108768101001628 ]

Neutron diffraction study of -Bi₈Pb₅O₁₇: structure refinement and analysis of cationic ordering

L. Righi, G. Calestani, M. Gemmi, A. Migliori and M. Bettinelli

Abstract: The triclinic crystal structure of the [varphi] phase in the structure of the Bi₂O₃-PbO phase diagram has been recently solved by the synergetic use of electron and X-ray diffraction on a polycrystalline Bi₈Pb₅O₁₇ sample. In the present work the problem is re-examined on the basis of powder neutron diffraction data: the structure has been confirmed and refined by the Rietveld method. The increased accuracy of the O-atom positions allowed the study of the cationic ordering in this structure by means of bond-valence calculations and charge distribution analysis. The results, confirmed by the refinement of the site occupancies, indicate that the structure is ordered to a large extent, with Bi and Pb atoms occupying preferentially different lattice sites. In this frame, the [varphi] phase being the most ordered one in the considered region of the Bi₂O₃-PbO phase diagram, it should represent the thermodynamically stable low-temperature polymorph.

Keywords: order; neutron powder diffraction; bond valence.