Structural and electrical behavior of Bi2VO5 with temperature. Stabilization at room temperature of the high temperature polymorphs

Mixed bismuth(III)-vanadium(IV) oxide BI2VO5 was prepared as a very crystal line powder by solid-stare reaction under vacuum and N-2 flow. Thermal anal ysis and powder X-ray diffraction at high temperatures confirmed the existe nce of two phase transitions, alpha --> beta at 397 degrees C and beta --> gamma at 778 degrees C due to the progressive loss of oxygen vacancy orderi ng, Impedance spectroscopy measurements showed that this material is a good ionic conductor. This is particularly true of the beta-phase, which exhibi ts a conductivity as high as 5 x 10(-2)Omega(-1).cm(-1) at 700 degrees C. T hree solid solutions Bi2V1-xGexO5 (0 less than or equal to x < 0.5), Bi2V1- yTiyO5 (0 less than or equal to y less than or equal to 0.1) and Bi2V1-zFez O5-z/2 (0 less than or equal to z less than or equal to 0.4) were studied. The X-ray powder diffraction patterns show that partial substitution of iso valent Ge(TV) and Ti(IV) or aliovalent Fe(III) on the V(IV) site stabilizes the high-temperature beta-phase (x < 0.5, z less than or equal to 0.4) and gamma-polymorph (y less than or equal to 0.1). (C) 2000 Elsevier Science L td. All rights reserved.