High oxide ion conductivity has been observed in the Ln5Mo3O16+xsystem with Ln = La, Pr, Nd, Sm, and Gd, x ~ 0.5, by ac complex impedance methods in the temperature range 250–700 °C. The Ln5Mo3O16+xphases form in a fluorite-related cubic structure. The oxygen content varies in the range 0 ≤ x ≤ 0.5. In the reduced phases (Ln5Mo3O16+ x)x ≃ 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 (Ln5Mo3O16.5y'y ≃ 0.0). The conductivity in Ln5Mo3O16.5-y(y ~ 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 ~ 0.5) oxide ions. The highest oxide ion conductivity was found in La5Mo3O16.5-yranging from 10-6 (Ω cm)-1 at 275 °C to ~10-2(Ω cm)-1at 670 °C with Ea ≃ 0.84 eV. The transition from the reduced (Ln5Mo3016) to the oxidized (Ln5Mo3016.5) 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.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Materials Chemistry