Structural, dielectric, and optical properties of Ni-doped barium cadmium tantalate ceramics

We report the effect of Ni-doping on the structural, dielectric and optical properties of Ba(Cd_1/3Ta_2/3)O₃ (BCT) ceramics. Rietveld analysis of the X-ray diffraction (XRD) data indicates that the BCT structure is similar to other Ba(Bi_1/3^&primeB _2/3^&Prime)O₃ perovskites, although the Ta-P-Cd is distorted to an angle of ∼173°; very close to our earlier theoretical prediction of 172°. The XRD analysis also indicates that Ni doping significantly enhances the extent of Cd-Ta ordering in BCT. The temperature coefficient of resonant frequency decreases with Ni concentration up to 2wt%. While the loss tangent of BCT is reduced at small levels of Ni doping (up to 0.5 wt%), it increases abruptly at higher concentrations. We found a correlation between the loss tangent of Ni-doped BCT samples and the intensity of a continuous absorption background in the optical spectra. This optical activity results from the presence of optically active point defects and is suggestive that these defects play an important role in the microwave loss in BCT ceramics.