Phase formation and dielectric properties of Ln_2(Ln'_(0.5)Nb_(0.5)_2O_7 (Ln = rare earth element)

摘要

Weberites and pyrochlores (A_2B_2O_7), both fluorite-related superstructures, are attractive dielectric ceramics due to their ability to accommodate diverse cations, thus allowing their properties to be tailored. This study focuses on the fundamental understanding of the structure-dielectric property relationships in fluorite-related oxides. Specifically, Ln_3NbO_7 and Ln_2(Ln'_(0.5)Nb_(0.5))_2O_7 (where the ionic radius of Ln' is smaller than that of Ln) compounds are investigated. It has been previously shown that weberite-type Ln3NbO7 exhibits a composition dependent dielectric relaxation above room temperature. It is here shown that a dielectric relaxation also occurs in Ln_2(Ln'_(0.5)Nb_(0.5))_2O_7 (Ln' = Yb~(3+), Er~(3+), and Dy~(3+)) compounds near or below —158 deg C. The temperature, at which the maximum permittivity occurs, is different for different compositions (—132 deg C for La_2(Yb_(0.5)Nb_(0.5))_2O_7, -197 deg C for La_2(Er_(0.5)Nb_(0.5))2O_7, and -187 deg C for La_2(Dy_(0.5)Nb_(0.5))_2O_7 at 1 MHz) and is correlated with the distortion of the NbO_6 octahedra. The room temperature dielectric permittivity of all three compounds was measured to be between 40 and 50 at 1 MHz.