Structures and ionic conductivities in two fluorite type families: Pb_5Bi_(17)X_5O_(43) and Pb_5Bi_(18)X_4O_(42) (X = P, V and As)

摘要

The phases Pb_5Bi_(17)P_5O_(43) and Pb_5Bi_(18)P_4O_(42) are among many recent new oxyphosphates discovered in the ternary system PbO-Bi_2O_3P_2O_5. The syntheses of the vanadates and arsenates led to isostructural compounds. Both series display a distorted 3 X 3 X 3 superstructure of the tetragonal delta-Bi_2O_3 polymorph. These types of phases display interesting anion conductivities and measurement were performed from 300 deg C to 800 deg C on the phosphates, arsenates and vanadates of the two families. The Pb_5Bi_(18)X_4O_(42) phases have higher conductivity values than those of Pb_5Bi_(17)X_5O_(43). Of the three homologues the vanadates always have the highest conductivities, i.e., at 800 deg C: 1.6 X 10~(-2) Siemens centr dot cm~(-1) for Pb_5Bi_(18)V_4O_(42) and 1.6 X 10~(-3) Siemens centre dot cm~(-1) for Pb_5Bi_(17)V_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 A, 0.47 A, 0.59 A respectively, helps the anion migration through the structure.