Browsing by Author "Denisova, KN"

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    Low temperature thermodynamics of Yb6MoO12 and Lu6MoO12
    (Elsevier, 2019-03-25) Denisova, KN; Shlyakhtina, AV; Yumashev, O; Avdeev, M; Abdel-Hafiez, M; Volkova, OS; Vasiliev, AN
    We report on thermodynamic properties of both rhombohedral and cubic phases of Yb6MoO12-δ supplemented by measurements of Lu6MoO12-δ() magnetic properties. Among these series, Yb6MoO12-δ() only exhibits signatures of magnetic long-range order at TN∼2.5 K which is confirmed by both physical property measurements and neutron powder diffraction. Low temperature specific heat measured down to 100 mK suggests the rearrangement of magnetic subsystem at T* = 500 mK. © 2018 Elsevier B.V.
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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, 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