Structures and ionic conductivities in two fluorite type families: Pb5Bi17X5O43 and Pb5Bi18X4O42 (X=P, V and As)
Résumé
The phases Pb5Bi17P5O43 and Pb5Bi18P4O42 are among many recent new oxyphosphates discovered in the ternary system PbO-Bi2O3-P2O5. The syntheses of the vanadates and arsenates led to isostructural compounds. Both series display a distorted 3×3×3 superstructure of the tetragonal δ-Bi2O3 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 Pb5Bi18X4O42 phases have higher conductivity values than those of Pb5Bi17X5O43. Of the three homologues the vanadates always have the highest conductivities, i.e., at 800 °C: 1.6×10−2 Siemens*cm−1 for Pb5Bi18V4O42 and 1.6×10−3 Siemens*cm−1 for Pb5Bi17V5O43. 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.