Oxide ion conductive structure and chemical stability of Nd_2Gd_2O_3F_6

摘要

Nd_2Gd_2O_3F_6 was prepared by solid-solidreaction between 1 mol Nd_2O_3 and 2 mol GdF_3 at1100deg C for 3 h in an argon flow. X-ray powder diffraction-Rietveld analysis revealed that the crystal system ofNd_2Gd_2O_3F_6 was assigned to the monoclinic structurewith the cell parameters; a_0=0.3973 nm, b_0=1.123 nm,c_0=0.5595 nm and #beta#=134.75deg. the ionic arrangementwas suggested to be slightly disordered in theNd_2Gd_2O_3F_6 crystal lattice in contrast to in theNd_2Eu_2O_3F_6with the high ordered ionic arrangement. Itwas assumed that the slightly disordered ionic arrangement inNd_2Gd_2O_3F_6 resulted in the smaller electricalconductivity of Nd_2Gd_2O_3F_6 (0.2 S m~(-1) at 600deg C)than that of Nd_2Eu_2O_3F_6 (5.4 S m~(-1) at 600deg C).Nd_2Gd_2O_3F_6 was confirmed to be stable up to ca.650deg C in air, and converted into NdGdO_3 (monoclinic) atca. 1350deg C through the metastable state of NdGdO_2F_2(rhombohedral) at ca. 850deg C by the pyro-hydrolysis. Therate of pyro-hydrolysis was analyzed as the tarnishing reactionrepresented by the surface reaction represented by the surfacereaction control. The electrical conductivity largely declinedwith decreasing fluorine content due to the pyro-hydrolysis.This cause was considered due to disappearance of the vacantsites for oxide ion migration according to change in the crystal structure.