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Structure, conductivity and magnetism of orthorhombic and fluorite polymorphs in MoO3-Ln2O3 (Ln=Gd, Dy, Ho) system

Abstract

Phase-pure orthorhombic compositions with Ln/Mo ~ 5.2-5.7 (Ln = Gd, Dy, Ho) ratio 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:Mo ratio was 5:1 (Ln10Mo2O21 (Ln = Dy, Ho)), it changed slightly in the final product due to 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 near Ln10Mo2O21 (Ln = Gd, Dy, Ho). Gd10Mo2O21 molybdate seems to undergo an order–disorder (orthorhombic–fluorite) phase transition in the range 1200-1600ºC. For the first time, using the neutron diffraction method, it was shown that low-temperature phases with Ln/Mo ~ 5.2-5.7 (Ln = Gd, Dy, Ho) have orthorhombic rather than tetragonal structure. The proton contribution to the total conductivity of the 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, orthorhombic phases of Dy and Ho compounds reach antiferromagnetic state at 2.4K and 2.6K, respectively.

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Article information


Submitted
12 Dec 2019
Accepted
24 Jan 2020
First published
24 Jan 2020

Dalton Trans., 2020, Accepted Manuscript
Article type
Paper

Structure, conductivity and magnetism of orthorhombic and fluorite polymorphs in MoO3-Ln2O3 (Ln=Gd, Dy, Ho) system

A. V. Shlyakhtina, M. Avdeev, N. V. Lyskov, J. C.C. Abrantes, E. Gomes, K. Denisova, I. V. Kolbanev, S. A. Cheryak, O. S. Volkova and A. Vasiliev, Dalton Trans., 2020, Accepted Manuscript , DOI: 10.1039/C9DT04724G

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