Oxide Ion Conductivity in Ln5Mo3O16+x) (ln = La, Pr, Nd, Sm, Gd; X .apprx. 0.5) with a Fluorite-Related Structure

High oxide ion conductivity has been observed in the Ln(5)Mo(3)O(16+x) system with Ln = La, Pr, Nd, Sm, and Gd, x similar to 0.5, by ac complex impedance methods in the temperature range 250-700 degrees C. The Ln(5)Mo(3)O(16+x) phases form in a fluorite-related cubic structure. The oxygen content varies in the range 0 <= x <= 0.5. In the reduced phases (Ln(5)Mo(3)O(16+x), x similar or equal to 0) the conductivity is dominated by electron hopping due to mixed valent Mo5+/Mo6+ and is several orders of magnitude higher than the conductivity of the fully oxidized phases (Ln(5)Mo(3)O(16.5-y), y similar or equal to 0.0). The conductivity in Ln(5)Mo(3)O(16.5-y) (y similar to 0) is ascribed to the motion of O2- ions in the fluorite-related lattice, where some of the empty cavities are partially occupied by the excess (x similar to 0.5) oxide ions. The highest oxide ion conductivity was found in Ln(5)Mo(3)O(16.5-y) ranging from 10(-6) (Omega cm)(-1) 11at 275 degrees C to similar to 10(-2) (Omega cm)(-1) at 670 degrees C with E-a similar or equal to 0.84 eV. The transition from the reduced (Ln(5)Mo(3)O(16)) to the oxidized (Ln(6)Mo(3)O(16.5)) phases and the reverse reactions were studied by powder X-ray diffraction, ac impedance, electron spin resonance and differential thermal analysis/thermogravimetric analysis measurements. The conductivity is highly sensitive to oxygen content.