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