Structures and Stabilities of Trivalent and Tetravalent Rare Earth Ions in Sevenfold Andeightfold Coordination in Fluorite-Related Complex Oxides

Abstract

ABSTRACTThis paper reports the preparation and characterization of a series of oxides containing3+ or 4+ lanthanide (M = Ce, Pr, or Tb) ions, with different ionic sizes and varying M4+/M3+ reduction potentials, in nearly cubic coordination. The objective of the study was to demonstrate how oxidation-reduction characteristics and ionic-size trends explain the properties of these oxides and to compare the oxidation-reduction stability of M3+ and M4+ lanthanide ions in high (CN 7 or 8) coordination in fluorite-related oxides versus low (CN 6) coordination in perovskite oxies. Efficient preparative methods are reported, as well as powder diffraction and thermogravimetric measurements for oxides CaMTi2O7-x and CaMZr2O7-x. These oxides were characterized by X-ray powder diffraction and by thermogravimetric analysis. CaCeTi2O7 is a pyrochlore, a = 10.142(4) Å with Ce4+ much more easily reducible than in the perovskite BaCeO3. By contrast, a preparation with the stoichiometry “CaPrTi2O7-x” is a two-phase mixture of perovskite CaTiO3 and a presumably Pr3+-rich pyrochlore Pr2Ti2O7(?). CaTbTi2O7-x appears to be a Tb3+ pyrochlore, a = 10.149(2) Å. CaCeZr2O7 is a pyrochlore, a = 10.524(1) sÅ. A preparation of “CaPrZr2O7-x” also appeared to yield a two-phase mixture, perovskite CaZrO3 and pyrochlore Pr2Zr2O7. In this paper, the structures, f-element ion sites, and M(IV)- MM1I stability trends in the CaMTi2O7-x and CaMZr2O7-x oxides are compared with the structural and stability trends in the perovskites BaMO3 which have M4+ ions in sixfold (tilted octahedra) coordination.