Accommodation of niobium and cerium variable valence cations in the crystal lattice of the Ln 2 3+ Ti 2 4+ O7 (Ln = Dy, Yb) pyrochlores

Abstract

We have studied the accommodation of niobium and cerium variable valence cations in the Ln 2 3+ Ti 2 4+ O7 (3+/4+) (Ln = Dy, Yb) pyrochlores in air and in a 5% H2 + 95% Ar reducing atmosphere. The (Dy0.8Ca0.1Ce0.1)2Ti2O7 − δ and (Yb0.8Tb0.1Ca0.1)2[Ti1−x Nb x ]2O7 − δ (x = 0–0.3) oxygen ion conducting solid solutions were synthesized through coprecipitation followed by heat treatment in air for 4 h at temperatures of 1400 and 1550°C, respectively. According to X-ray photoelectron spectroscopy (XPS) data, the valence state of the niobium in the (Yb0.8Tb0.1Ca0.1)2[Ti1−x Nb x ]2O7 − δ (x = 0.05, 0.1) solid solutions is 4+ and, hence, isovalent niobium substitution for titanium takes place. During synthesis in air, the titanium sublattice of the (Yb0.8Tb0.1Ca0.1)2[Ti1−x Nb x ]2O7 − δ pyrochlores is stable when the Nb4+ content does not exceed 10% (x = 0.1). According to conductivity data for the high-conductivity material (Yb0.8Tb0.1Ca0.1)2[Ti0.95Nb0.05]2O6.9 at low oxygen partial pressures, there is no reduction of pentavalent niobium and no increase in electronic conductivity, which is supported by XPS results that only Nb4+ is present in the solid solution. In pyrochlore-like (Dy0.8Ca0.1Ce0.1)2Ti2O7 − δ synthesized in air at 1400°C, the dysprosium site contains both tri- and tetravalent cerium, as supported by conductivity measurements under reducing conditions. The Ce4+ + e → Ce3+ reduction process led to a change in the color and disintegration of the sample when the oxygen partial pressure was reduced. In the 3+/4+ A2B2O7 pyrochlores, variable valence cations on the A site (eightfold coordination) more readily participate in redox processes than do those on the B site (sixfold coordination).