Crystal Structural Characterization and Microwave Dielectric Properties of Ca(Ta2-xNbx)O6 Ceramics

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The effects of Nb substitution for Ta on crystal structures and the microwave dielectric properties of Ca(Ta2-xNbx)O6 (CTN) ceramics were investigated in this study. The X-ray powder diffraction (XRPD) patterns of CTN ceramics with the composition $x$ ranging from 0 to 1 show a single phase, which corresponds to an orthorhombic phase with an aeschynite-type structure (space group: Pnma), whereas those of the samples with compositions higher than $x=1.5$ are indexable to an orthorhombic phase with a columbite-type structure (space group: Pbcn). In the composition range of 1.1–1.4, the two phases coexist in the CTN ceramics. As for the covalencies of CTN ceramics, those of the Ca–O and Ta/Nb–O bonds strongly depend on the crystal structure. In addition, the microwave dielectric properties of CTN ceramics significantly varied at compositions ranging from 1 to 1.5, depending on the weight fraction of the aeschynite and columbite phases. As a result, the microwave dielectric properties of CTN ceramics at $x=1.5$ showed a $Q\cdot f$ of 102,500 GHz with a $\varepsilon_{\text{r}}$ of 16 and a $\tau_{\text{f}}$ of $-56$ ppm/°C. Moreover, a near zero $\tau_{\text{f}}$ value was obtained at $x=1.25$ because of the coexistence of the aeschynite and columbite phases.
2008-09-25