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-t emperature phase Bi6.67O4(PO4)(4). The Pb atom replaces 0.67 Bi in the same crystallographic site. The vanadate and arsenate with the composition PbBi 6O4(XO4)(4) were synthesized and yielded isomorphic phases at room temperat ure. All three compounds adopt a triclinic cell, space group P (1) over bar , Z=1. The structure refinement of PbBi6O4(PO4)(4) was performed using the Rietveld method on X-ray powder diffraction data. The starting parameters w ere the atomic positions of Bi6.67O4(PO4)(4). Conductivity measurements mer e made from 300 degreesC to 800 degreesC on samples of the three homologues . The highest conductivity was observed for the vanadate. Attempts to synth esize the binary vanadate and arsenate compounds isostructural with the hig h-temperature phase Bi6.67O4(PO4)(4) were unsuccessful. (C) 2000 Academic P ress.