Phase Formation and Dielectric Properties of Ln3NbO7 (Ln = Rare Earth Elements)
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Phase Formation and Dielectric Properties of Ln3NbO7 (Ln = Rare Earth Elements) Lu Cai, Julián Guzmán, Louis A. Pérez, and Juan C. Nino Materials Science and Engineering, University of Florida, 172 Rhines Hall, MSE Dept. 116400, Gainesville, FL, 32611-6400
ABSTRACT The structure and dielectric properties of rare earth niobate compounds within the Ln3NbO7 (Ln = Nd, Gd, Dy, Er, Yb and Y) and Ln2(Ln’,Nb)O7 (Ln = Nd, Sm and Ln’ = Yb) series are investigated. The crystal structure of the all the studied materials is found to be fluorite-related including webertite-type, pyrochlore, and defect fluorite structures. It is observed that the relative permittivity of the defect fluorite Ln3NbO7 (Ln = Dy, Er, Yb and Y) increases with the increase in temperature and exhibits low dielectric loss up to approximately 350 K. Above 350 K, the dielectric loss increases rapidly with increasing temperature as the onset of electrical conductivity takes place. Of particular interests are Gd3NbO7 and Nd3NbO7, which exhibit a frequency and temperature dependent dielectric relaxation behavior. At 1 MHz Gd3NbO7 reaches its maximum relative permittivity of ~34 at about 330K, while at the same frequency, the maximum relative permittivity of Nd3NbO7 is attained at about 500 K. By contrast, Nd2(Yb,Nb)O7 and Sm2(Yb,Nb)O7, which crystallize in a pyrochlore-type structure, do not show dielectric relaxation and, comparatively, exhibit a more temperature-stable dielectric permittivity response.
INTRODUCTION Weberite and pyrochlore structures, expressed as A2B2O7, are anion-deficient superstructure derivatives of the fluorite (AX2) crystal structure. Compared to fluorite, the reduction in the number of anions leads to a decrease in the coordination number of B cations (VI coordination) with respect to the A cations (VIII coordination).[1-3] The crystallographic relationship between the weberite and pyrochlore structures is clearly observed when analyzing the structures along their similar (010) planes. These planes can be seen as layers formed by alternate lines of BO6 octahedra and AO8 cubes in [101] direction for pyrochlores and [100] for weberites. Between the layers, there are equal numbers of A-type atoms, which reside in oxygen cubes for pyrochlores and hexagonal bi-pyramids for weberites, and B-type atoms located in oxygen octahedra[4] Compounds with these structures exhibit a wide variety of physical properties because of their ability to accommodate various metal atoms with different chemical properties at both the A- and B-sites.[1, 3] Thus, they can be used in different electroceramic applications such as capacitors, cathodes, ionic conductors, high frequency filters, tunable dielectrics, and semiconductors.[1, 2, 5-7]
In the past, it has been shown that the crystal structure of Ln2(Ln,Nb)O7 (Ln3NbO7) shifts from orthorhombic weberite-type structures to a cubic defect fluorite with decreasing Ln3+ ionic radius.[4, 8-15] The weberite-type Ln3NbO7 compounds tend to crystallize in the same arrangement of NbO6-LnO8 layers as we
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