Crystal structure and microwave dielectric characteristics of Co-substituted Zn1−xCoxZrNb2O8 (0 ≤ x ≤ 0.1) ceramics

Abstract

Wolframite structure Zn_1−xCo_xZrNb₂O₈ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.10) ceramics were prepared by the conventional solid state method. The crystal structures were studied via X-ray diffraction, and lattice vibrational modes were obtained by Raman spectroscopy. It could be found that all the A-site (Zn²⁺/Co²⁺, Zr⁴⁺) and B-site (Nb⁵⁺) cations were octahedrally coordinated with oxygen anions from the diagrammatic sketch of the crystal structure. Rietveld refinement was used to analyze the crystal structure and the lattice parameters were obtained. Based on the complex chemical bond theory and lattice parameters, intrinsic factors such as the bond ionicity, lattice energy, bond energy and coefficient of thermal expansion were calculated. Based on the results, Nb–O bonds played an important role in affecting the microwave dielectric proprieties of Zn_1−xCo_xZrNb₂O₈ ceramics. Co²⁺ substitution would affect the crystal structure and influence the microwave dielectric properties. The tendency of the dielectric constant (ε_r) could be predicted by the polarizability and bond ionicity of Nb–O bonds, and the ε_r showed the same tendency with Raman shifts of Nb–O vibration modes. The variation of the quality factor (Qf) could be explained by the change of the lattice energy of Nb–O bonds and the full width at half maximum (FWHM) values of Nb-site Raman modes. The temperature coefficient of resonant frequency (τ_f) showed the same tendency with the bond energy, octahedral distortion and bond valence of Nb–O bonds, and the opposite tendency with the thermal expansion coefficient.