Browsing by Author "Kharitonova, EP"

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    Stability against reduction of fluorite-like rhombohedral La5.5MoO11.25 and Ho5.4Zr0.6MoO12.3 fluorite: conductivity and neutron diffraction study
    (Elsevier, 2018-06-01) Savvin, SN; Avdeev, M; Kolbanev, IV; Kharitonova, EP; Shcherbakova, LG; Shlyakhtina, AV; Nuñez, P
    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.
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    Structure and conductivity of Nd6MoO12-based potential electron–proton conductors under dry and wet redox conditions
    (Royal Society of Chemistry, 2019-01-09) Shlyakhtina, AV; Avdeev, M; Abrantes, JCC; Gomes, ED; Lyskov, NV; Kharitonova, EP; Kolbanev, IV; Shcherbakova, LG
    Nominal compositions Nd6MoO12 (3[thin space (1/6-em)]:[thin space (1/6-em)]1) and Nd10Mo2O21 (5[thin space (1/6-em)]:[thin space (1/6-em)]2) were prepared by high-temperature synthesis in air from a mechanically activated mixture of Nd2O3 and MoO3. Phase-pure Nd6MoO12−δ and Nd6MoO12−α have been obtained at 1600 and 1650 °C, respectively. They are both slightly rhombohedrally distorted derivatives from the cubic fluorite structure. Nd10Mo2O21 with a lower content of Nd2O3 was shown to be a more complex phase based on the rhombohedral phase (R[3 with combining macron]) in the 1600–1650 °C temperature range. As a result of the formation of a more complex rhombohedral phase, the conductivity of Nd10Mo2O21 changes dramatically in comparison with Nd6MoO12−δ. In wet air Nd6MoO12−δ is a p-type electronic conductor, whereas proton conductivity dominates in Nd10Mo2O21 over the entire temperature range studied. The electrical conductivity dependence of Nd6MoO12−δ on the oxygen partial pressure shows a V-type behaviour typical of a transition from a p-type to n-type conductivity mechanism at 800 ≤ T ≤ 1000 °C. There is no p-type conductivity contribution in Nd10Mo2O21 in the same temperature range. The prevalence of electronic conductivity in the samples with nominal composition Nd6MoO12 in a wide temperature range is due to the fact that Nd and Mo in the fluorite materials are readily reduced. Predominantly Nd3+ and Mo6+ forms exist in more complex rhombohedral phase Nd10Mo2O21 and it has proton conductivity ∼8.5 × 10−3 S cm−1 at 800 °C. Thus, the loss of dimensional stability is more characteristic of fluorites and rhombohedral phases with small rhombohedral distortion (Nd5.4Zr0.6MoO12.3, Nd6MoO12−δ) than more complex rhombohedral phases based on (R[3 with combining macron]) (Nd10Mo2O21). A comparative high-temperature in situ neutron diffraction study under high vacuum of rhombohedral Nd6MoO12−δ and cubic fluorite Nd5.4Zr0.6MoO12.3 showed that the former transforms to the high-temperature cubic fluorite type above ∼1140 °C while the latter retains its cubic structure in the studied range up to 1350 °C. © the Partner Organisations 2019