Nd2Gd2O3F6was prepared by solid-solid reaction between 1 mol Nd2O3and 2 mol GdF3at 1100°C for 3 h in an argon flow. X-ray powder diffraction-Rietveld analysis revealed that the crystal system of Nd2Gd2O3F6was assigned to the monoclinic structure with the cell parameters; a0= 0.3973 nm, b0= 1.123 nm, c0= 0.5595 nm and β = 134.75°. The ionic arrangement was suggested to be slightly disordered in the Nd2Gd2O3F6crystal lattice in contrast to in the Nd2Eu2O3F6with the high ordered ionic arrangement. It was assumed that the slightly disordered ionic arrangement in Nd2Gd2O3F6resulted in the smaller electrical conductivity of Nd2Gd2O3F6(0.2 S m-1at 600°C) than that of Nd2Eu2O3F6(5.4 S m-1at 600°C). Nd2Gd2O3F6was confirmed to be stable up to ca. 650°C in air, and converted into NdGdO3(monoclinic) at ca. 1350°C through the metastable state of NdGdO2F2(rhombohedral) at ca. 850°C by the pyro-hydrolysis. The rate of pyro-hydrolysis was analyzed as the tarnishing reaction represented by the surface reaction control. The electrical conductivity largely declined with decreasing fluorine content due to the pyro-hydrolysis. This cause was considered due to disappearance of the vacant sites for oxide ion migration according to change in the crystal structure. (C) 2000 Editions scientifiques et medicales Elsevier SAS.
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