Structure, conductivity and magnetism of orthorhombic and fluorite polymorphs in MoO3-Ln(2)O(3) (Ln = Gd, Dy, Ho) systems


Phase-pure orthorhombic compositions at a Ln/Mo ratio similar to 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 5Ln(2)O(3) + 2MoO(3) (Ln = Gd, Dy, Ho) oxide mixtures at 1200 degrees C. Although the starting Ln:Mo ratio was 5:1 (Ln(10)Mo(2)O(21) (Ln = Dy, Ho)), it changed slightly in the final product due to the volatility of molybdenum oxide at 1200 degrees C (40-160 h) (ICP-MS analysis). Brief high-temperature firing (1600 degrees C, 3 h) of 5Ln(2)O(3) + 2MoO(3) (Ln = Gd, Dy, Ho) oxide mixtures leads to the formation of phase-pure fluorites with compositions close to Ln(10)Mo(2)O(21) (Ln = Gd, Dy, Ho). Gd10Mo2O21 molybdate seems to undergo an order-disorder (orthorhombic-fluorite) phase transition in the range of 1200-1600 degrees C. For the first time, using the neutron diffraction method, it was shown that low-temperature phases with a Ln/Mo ratio similar to 5.2-5.7 (Ln = Gd, Dy, Ho) have an orthorhombic structure rather than a tetragonal structure. Proton contribution to the total conductivity of Ln(10)Mo(2)O(21) (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.






Shlyakhtina, AV; Avdeev, M; Lyskov, NV; Abrantes, JCC; Gomes, E; Denisova, KN; Kolbanev, IV; Chernyak, SA; Volkova, OS; Vasiliev, AN

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This work was supported by the Russian Foundation for Basic Research (grant no. 19-03-00358, 20-03-00399) and was supported within frameworks of the state task for FRCCP (state registration number AAA-A20-120013190076-0). Conductivity measurements of the samples were performed in accordance with the state task of IPCP RAS, state registration no. AAAA-A19-119061890019-5. This work has also been supported by the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST MISiS Grant No. K2-2017-084; by Act 211 of the Government of Russian Federation, Contract No. 02.A03.21.0004 and 02.A03.21.0011.

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