Stability against reduction of fluorite-like rhombohedral La5.5MoO11.25 and Ho5.4Zr0.6MoO12.3 fluorite: conductivity and neutron diffraction study

Savvin, SN; Avdeev, M; Kolbanev, IV; Kharitonova, EP; Shcherbakova, LG; Shlyakhtina, AV; Nuñez, P

Stability against reduction of fluorite-like rhombohedral La5.5MoO11.25 and Ho5.4Zr0.6MoO12.3 fluorite: conductivity and neutron diffraction study

Date

2018-06-01

Authors

Publisher

Elsevier

Abstract

Zr-substituted rare-earth molybdates Ho5.4Zr0.6MoO12.3 and unsubstituted La5.5MoO11.25 demonstrate appreciable mixed electron-proton conductivity in the 200–470 °C and 145–730 °C temperature range, respectively, under wet oxidizing and mild reducing conditions (air, Ar, Ar-5% H2). Rhombohedral fluorite-like La5.5MoO11.25 showed the highest bulk conductivity of about 1 × 10−5 S/cm at 400 °C in wet air and Ar. Its impedance spectra did not provide any evidence of the grain boundary contribution in wet atmosphere. Total conductivity of Ho5.4Zr0.6MoO12.3 fluorite is much lower and is 3 × 10−7 S/cm at 400 °C in wet air. Besides, it should be noted that there is a grain-boundary contribution of Ho5.4Zr0.6MoO12.3 in wet air and Ar. Thermogravimetry data demonstrate that the fraction of strong structurally bound water and interstitial protons in La5.5MoO11.25 and Ho5.4Zr0.6MoO12.3 is ~0.02–0.03% in the range ~600–900 °C. The stability of Ho5.4Zr0.6MoO12.3 fluorite structure and fluorite-like rhombohedral La5.5MoO11.25 structure in extremely dry conditions under dynamic vacuum of 10−6–10−7 mbar was investigated by in situ variable temperature neutron diffraction between 800 and 1400 °C to understand phase behaviour under mild reducing conditions in a wide temperature range. Rhombohedral fluorite-like La5.5MoO11.25 has been shown to be more resistant to reduction under vacuum below 1100 °С in heating–cooling cycles than is fluorite Ho5.4Zr0.6MoO12.3. Given the higher proton conductivity of Ln5.5MoO11.25, this suggests that rhombohedral fluorite-like La6MoO12 - based molybdates are suitable for long-term use under mild reducing conditions and 600–800 °С. © 2018 Elsevier B.V.

Keywords

Fluorite, Trigonal lattices, Proton conductivity, Reduction, Rare earths, Grain boundaries, Thermal gravimetric analysis, Neutron diffraction

Citation

Savvin, S. N., Avdeev, M., Kolbanev, I. V., Kharitonova, E. P., Shcherbakova, L. G., Shlyakhtina, A. V., & Nuñez, P. (2018). Stability against reduction of fluorite-like rhombohedral La5.5MoO11.25 and Ho5.4Zr0.6MoO12.3 fluorite: conductivity and neutron diffraction study. Solid State Ionics, 319, 148-155, doi:10.1016/j.ssi.2018.02.001