Polymorphs of rare-earth molybdates Ln10Mo2O21 (Ln= Gd, Dy, Ho): structure, conductivity and magnetism
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Date
2019-07-05
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e.LIBRARY.RU
Abstract
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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Keywords
Tungstates, Molybdenum compounds, Rare earths, X-ray diffraction, Neutron diffraction, Microstructure, Magnetic properties, Orthorhombic lattices
Citation
Shlyakhtina, A., Avdeev, M., Lyskov, N., Denisova, K., Kolbanev, I., Chernyak, S., Shcherbakova, L., Volkova, O., Vasiliev, A. (2019). Polymorphs of rare-earth molybdates Ln10Mo2O21 (Ln= Gd, Dy, Ho): structure, conductivity and magnetism. Paper presented at XIX International Meeting of Crystal Chemistry, X-ray Diffraction and Spectroscopy of Minerals, 2–5 July, 2019, Apatity, Kola Peninsula, Russia. In Book of Abstracts, (pp. 157). Retrieved from: https://elibrary.ru/item.asp?id=42697551