Browsing by Author "Gomes, ED"

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    On the use of crystal vibrational modes in the estimation of the anisotropic displacement parameters of hydrogen atoms in molecular crystals: para-Nitroaniline as a test case
    (Elsevier Science BV., 2013-09-23) Pozzi, CG; Fantoni, AC; Goeta, AE; Gomes, ED; McIntyre, GJ; Punte, G
    The use of crystal vibrational modes and frequencies calculated for the periodic system to complement a Translation Libration Screw (TLS) fit in the estimation of anisotropic displacement parameters (ADPs) of hydrogen atoms in molecular crystals is proposed. As a test case we have used the para-nitroaniline crystal, for which a reference set of ADPs has been obtained by performing a single crystal neutron diffraction study at 100 K. Although the largest difference between estimated and experimental reference values amounts to 0.06 angstrom(2), this value being about six times the experimental uncertainty, the agreement is better than three experimental uncertainties for 33 out of the total of 36 ADPs. The performance of the suggested method, particularly for the amino atoms, is thoroughly analyzed. © 2013, Elsevier Ltd.
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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