On the Existence of Bi6.67O4(XO4)4 and PbBi6O4(XO4)4 (X=P,V, and As)

PbBi6O4(PO4)4 obtained at room temperature is isomorphous with the high-temperature phase Bi6.67O4(PO4)4. The Pb atom replaces 0.67 Bi in the same crystallographic site. The vanadate and arsenate with the composition PbBi6O4(XO4)4 were synthesized and yielded isomorphic phases at room temperature. All three compounds adopt a triclinic cell, space group P Z=1. The structure refinement of PbBi6O4(PO4)4 was performed using the Rietveld method on X-ray powder diffraction data. The starting parameters were the atomic positions of Bi6.67O4(PO4)4. Conductivity measurements were made from 300°C to 800°C on samples of the three homologues. The highest conductivity was observed for the vanadate. Attempts to synthesize the binary vanadate and arsenate compounds isostructural with the hightemperature phase Bi6.67O4(PO4)4 were unsuccessful.