Oxide ion conductive structure and chemical stability of Nd2Gd2O3F6

Abstract

Abstract Nd2Gd2O3F6 was prepared by solid-solid reaction between 1 mol Nd2O3 and 2 mol GdF3 at 1100°C for 3 h in an argon flow. X-ray powder diffraction-Rietveld analysis revealed that the crystal system of Nd2Gd2O3F6 was 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 Nd2Gd2O3F6 crystal lattice in contrast to in the Nd2Eu2O3F6 with the high ordered ionic arrangement. It was assumed that the slightly disordered ionic arrangement in Nd2Gd2O3F6 resulted in the smaller electrical conductivity of Nd2Gd2O3F6 (0.2 S m−1 at 600°C) than that of Nd2Eu2O3F6 (5.4 S m−1 at 600°C). Nd2Gd2O3F6 was 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.