Browsing by Author "Abrantes, JCC"

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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
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    Structure, conductivity and magnetism of orthorhombic and fluorite polymorphs in MoO3–Ln2O3 (Ln = Gd, Dy, Ho) systems
    (Royal Society of Chemistry, 2020-01-24) Shlyakhtina, AV; Avdeev, M; Lyskov, NV; Abrantes, JCC; Gomes, ED; Denisova, KN; Kolbanev, IV; Chernyak, SA; Volkova, OS; Vasiliev, AN
    Phase-pure orthorhombic compositions at a Ln/Mo ratio ∼ 5.2–5.7 (Ln = Gd, Dy, Ho) have been obtained for the first time by prolonged (40–160 h) heat treatment of mechanically activated 5Ln2O3 + 2MoO3 (Ln = Gd, Dy, Ho) oxide mixtures at 1200 °C. Although the starting Ln[thin space (1/6-em)]:[thin space (1/6-em)]Mo ratio was 5[thin space (1/6-em)]:[thin space (1/6-em)]1 (Ln10Mo2O21 (Ln = Dy, Ho)), it changed slightly in the final product due to the volatility of molybdenum oxide at 1200 °C (40–160 h) (ICP-MS analysis). Brief high-temperature firing (1600 °C, 3 h) of 5Ln2O3 + 2MoO3 (Ln = Gd, Dy, Ho) oxide mixtures leads to the formation of phase-pure fluorites with compositions close to Ln10Mo2O21 (Ln = Gd, Dy, Ho). Gd10Mo2O21 molybdate seems to undergo an order–disorder (orthorhombic–fluorite) phase transition in the range of 1200–1600 °C. For the first time, using the neutron diffraction method, it was shown that low-temperature phases with a Ln/Mo ratio ∼ 5.2–5.7 (Ln = Gd, Dy, Ho) have an orthorhombic structure rather than a tetragonal structure. Proton contribution to the total conductivity of Ln10Mo2O21 (Ln = Gd, Dy, Ho) fluorites and gadolinium and dysprosium orthorhombic phases in a wet atmosphere was observed for the first time. In both orthorhombic and fluorite phases, the total conductivity in wet air decreases with decreasing lanthanide ionic radii. In a wide temperature range, the compounds under study exhibit paramagnetic behaviour. However, the orthorhombic phases of Dy and Ho compounds reach the antiferromagnetic state at 2.4 K and 2.6 K, respectively. © The Royal Society of Chemistry 2020