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

The triclinic crystal structure of the phi 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 phi 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.