A new fluorite type compound Pb 5Bi 17X 5O 43: synchrotron and neutron structure determination (X=P) and conduction properties (X=P, V and As)

Abstract

Recently, many new compounds have been discovered in the ternary system PbO–Bi 2O 3–P 2O 5: PbBi 6P 4O 20, nPbO–BiPO 4 (n=1, 2, 4), and Pb 5Bi 18P 4O 42. The structure of Pb 5Bi 18P 4O 42 was solved in a monoclinic I-centered cell, space group I2/ m. The structure of a new compound Pb 5Bi 17P 5O 43, monoclinic I-centered cell, is closely related and has been solved in the non centro symmetric Im space group from a joint refinement of X ray synchrotron single-crystal data and a powder neutron diffraction data; the cell parameters are a=11.341(2) Å, b=16.604(3) Å, c=11.432(2) Å, β=93.73(3)°. The atoms of lead, bismuth and phosphorus take the same positions as the bismuth atoms in the structure of δ-Bi 2O 3 with a distorted 3×3×3 superstructure of a tetragonal variety, as in Pb 5Bi 18P 4O 42. The homologous vanadate and arsenate phases are isostructural. Conductivity measurements were performed on the compounds of the series from 300°C to 800°C. The vanadate phase has the highest conductivity. An increase of the volume of the unit cell from the phosphate to the vanadate eases the ion migration through the structure.