Browsing by Author "Lyskov, NV"

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    Comparative study of electrical conduction and oxygen diffusion in the rhombohedral and bixbyite Ln6MoO12 (Ln = Er, Tm, Yb) polymorphs
    (American Chemical Society, 2019-03-19) Shlyakhtina, AV; Lyskov, NV; Avdeev, M; Goffman, VG; Gorshkov, NV; Knotko, AV; Kolbanev, IV; Karyagina, OK; Maslakov, KI; Shcherbakova, LG; Sadovskaya, EM; Sadykov, VA; Eremeev, NF
    Electrical conduction and oxygen diffusion mobility in the bixbyite (Ia3̅) and rhombohedral (R3̅) polymorphs of the Ln6MoO12−Δ (Ln = Er, Tm, Yb; Δ = δ, δ1, δ2; δ1 > δ2) heavy lanthanide molybdates, belonging to new, previously unexplored classes of potential mixed (ionic–electronic) conductors, have been studied in the range of 200–900 °C. The oxygen self-diffusion coefficient in bixbyite (Ia3̅) Yb6MoO12−δ phase estimated by the temperature-programmed heteroexchange with C18O2 was shown to be much higher than that for rhombohedral (R3̅) RI (with large oxygen deficiency) and (R3̅) RII (with small oxygen deficiency) Ln6MoO12−Δ (Ln = Tm, Yb; Δ = δ1; δ1 > δ2) oxides. According to the activation energy for total conduction in ambient air, 0.99, 0.93, and 1.01 eV in Er6MoO12−δ, Tm6MoO12−δ, and Yb6MoO12−δ bixbyites, respectively, oxygen ion conductivity prevails in the range ∼200–500 °C. Oxygen mobility data for the rhombohedral Ln6MoO12−Δ (Ln = Er, Tm, Yb; Δ = δ1, δ2) phases RI and RII indicate that the oxygen in these phases exhibits mobility at much higher temperatures, such as those above 600–700 °C. Accordingly, below 600–700 °C they have predominantly electronic conductivity. As shown by total conductivity study of Ln6MoO12−δ (Ln = Er, Tm, Yb) bixbyites (Ia3̅) and rhombohedral phases Ln6MoO12−Δ (Ln = Er, Tm, Yb; Δ = δ1, δ2) (R3̅) in dry and wet air, the proton conductivity contribution exists only in Ln6MoO12−δ (Ln = Er, Tm, Yb) bixbyites up to 450–600 °C and decreases with a decreasing of the lanthanide ionic radius. The obtained data on the mobility of oxygen and the presence of proton contribution in bixbyites in the 300–600 °C temperature range make it possible to confirm unequivocally that Ln6MoO12−δ (Ln = Er, Tm, Yb) bixbyites are mixed electron–proton conductors at these temperatures. © 2019 American Chemical Society
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    Polymorphs of rare-earth molybdates Ln10Mo2O21 (Ln= Gd, Dy, Ho): structure, conductivity and magnetism
    (e.LIBRARY.RU, 2019-07-05) Shlyakhtina, AV; Avdeev, M; Lyskov, NV; Denisova, KN; Kolbanev, IV; Chernyak, S; Shcherbakova, LG; Volkova, OS; Vasiliev, AN
    The polymorphism of Ln6MO12(M = Mo, W) rare-earth molybdates and tungstates has been studied for a rather long time [Bartram, 1966].Nevertheless, many questions related to the structure of REE molybdates remain open. The purpose of this work was to find conditions for the synthesis of phase-pure Ln10Mo2O21(Ln = Gd, Dy, Ho) intermediate rare-earth molybdates; to synthesize high-temperature (fluorite) and low-temperature polymorphs of the Ln10Mo2O21(Ln = Gd, Dy, Ho) molybdates and study their structure by XRD, to determine the structure of the low-temperature polymorph phase (orthorhombic or tetragonal) by neutron diffraction, using Ho10Mo2O21as an example, to study the microstructure and conductivity of the synthesized compounds in dry and wet air, and to study the magnetic properties of the Ln10Mo2O21( Ln = Gd, Dy, Ho) polymorphs.Orthorhombic and fluorite Ln10Mo2O21 (Ln = Gd, Dy, Ho) polymorphs have been synthesized by firing mechanically activated 5Ln2O3+ 2MoO3(Ln = Gd, Dy, Ho) oxide mixtures at 1200 and 1600 °C, respectively. Stable, phase-pure orthorhombic Ln10Mo2O21 (Ln = Gd, Dy, Ho) molybdates have been obtained for the first time by prolonged (40–160 h) heat treatment at 1200 °C. The preparation of orthorhombic Gd10Mo2O21requires the shortest time (40 h), the synthesis of orthorhombic Dy10Mo2O21takes double this time (80 h), and 160 h are needed to obtain orthorhombic Ho10Mo2O21. Higher temperature firing (1600 °C, 3 h) leads to the formation of phase-pure fluorite Ln10Mo2O21 (Ln = Gd, Dy, Ho). Summarizing the results of the present and earlier works [Bevan et al., 1982; Lashtaberg et al., 2010] it can be stated with all certainty that the structure of the low-temperature R10Mo2O21(R = Gd-Er, Y) polymorphs is orthorhombic and not tetragonal. Proton conduction of orthorhombic and fluorite polymorphs Gd10Mo2O21, Dy10Mo2O21, Ho10Mo2O21has been investigated.Down to liquid helium temperatures, the title compounds exhibit paramagnetic properties due to weakly interacting localized magnetic moments of rare-earths.This work was supported by the Russian Foundation for Basic Research (grants no. 16-03-00143, 19-03-00358
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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