Structures and stabilities of trivalent and tetravalent rare earth ions in sevenfold and eightfold coordination in fluorite-related complex oxides

摘要

This paper reports the preparation and characterization of a series of oxides containing 3+ or 4+ lanthanide (M = Ce, Pr, or Tb) ions, with different ionic sizes and varying M(sup 4+)/M(sup 3+) 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 M(sup 3+) and M(sup 4+) lanthanide ions in high (CN 7 or 8) coordination in fluorite-related oxides versus low (CN 6) coordination in perovskite oxides. Efficient preparative methods are reported, as well as powder diffraction and thermogravimetric measurements for oxides CaMTi(sub 2)O(sub 7-x) and CaMZr(sub 2)O(sub 7-x). These oxides were characterized by X-ray powder diffraction and by thermogravimetric analysis. CaCeTi(sub 2)O(sub 7) is a pyrochlore, a = 10.142(4) (Angstrom), with Ce(sup 4+) much more easily reducible than in the perovskite BaCeO(sub 3). By contrast, a preparation with the stoichiometry ''CaPbTi(sub 2)O(sub 7-x)'' is a two-phase mixture-of perovskite CaTiCo(sub 3) and a presumably Pr(sup 3+)-rich pyrochlore Pr(sub 2)Ti(sub 2)O(sub 7)(.). CaTbTi(sub 2)O(sub 7-x) appears to be a Tb(sup 3+) pyrochlore, a = 10.149(2) (Angstrom). CaCeZr(sub 2)O(sub 7) is a pyrochlore, a = 10.524(1) (Angstrom). A preparation of ''CaPbZr(sub 2)O(sub 7-x)'' also appeared to yield a two-phase mixture, perovskite CaZrO(sub 3) and pyrochlore Pr(sub 2)Zr(sub 2)O(sub 7). In this paper, the structures, f-element ion sites, and M(4)-M(3) stability trends in the CaMTi(sub 2)O(sub 7-x) and CaMZr(sub 2)O(sub 7-x) oxides are compared with the structural and stability trends in the perovskites BaMO(sub 3) which have M(sup 4+) ions in sixfold (tilted octahedra) coordination. (ERA citation 17:007063)