New oxide ion conducting solid electrolytes, Bi4V2O11:M; M=B, Al, Cr, Y, La

The compositional ranges of Bi4V2O11 solid solutions containing trivalent cations: B, Al, Cr, Y and La have been determined by means of a phase diagram study at solidus temperatures, ca. 850 degrees C. At least three mechanisms for accommodating variable cation contents are required. These are nominally V-->Bi V-->M, Bi-->M;the first two also involve the formation of anion vacancies. The detailed mechanisms by which ions as different in size as B and La can enter the Bi4V2O11 structure cannot, however, be inferred from the phase diagrams. Phase diagrams at 500 degrees C have much smaller solid-solution areas, indicating the stability and extent of the solid solutions to be very temperature-dependent. Conductivity studies show that La-, Y- and Al-doped materials, with a composition around Bi(4)V(1.8)M(0.2)O(10.8), are the best conductors at 300 degrees C with a conductivity value of up to 1.4 x 10(-4) Omega(-1) cm(-1) At 600 degrees C, Al-doped materials have the highest conductivity, ca. 1 x 10(-1) Omega(-1) cm(-1). Conductivity Arrhenius plots show changes of slope associated with the phase transitions: alpha-->beta-->gamma for B, Al, Cr and beta-->gamma for Y; data for La (gamma-polymorph only) are almost linear.