Structures and ionic conductivities in two fluorite type families: Pb 5Bi 17X 5O 43 and Pb 5Bi 18X 4O 42 (X=P, V and As)

Abstract

The phases Pb 5Bi 17P 5O 43 and Pb 5Bi 18P 4O 42 are among many recent new oxyphosphates discovered in the ternary system PbO–Bi 2O 3–P 2O 5. The syntheses of the vanadates and arsenates led to isostructural compounds. Both series display a distorted 3×3×3 superstructure of the tetragonal δ-Bi 2O 3 polymorph. These types of phases display interesting anion conductivities and measurements were performed from 300 °C to 800 °C on the phosphates, arsenates and vanadates of the two families. The Pb 5Bi 18X 4O 42 phases have higher conductivity values than those of Pb 5Bi 17X 5O 43. Of the three homologues the vanadates always have the highest conductivities, i.e., at 800 °C: 1.6×10 −2 Siemens·cm −1 for Pb 5Bi 18V 4O 42 and 1.6×10 −3 Siemens·cm −1 for Pb 5Bi 17V 5O 43. An increase of the volume of the unit cell due to the increasing radius of the pentavalent cations from P, As to V, 0.34 Å, 0.47 Å, 0.59 Å respectively, helps the anion migration through the structure.