Influence of cobalt ions non-stoichiometry on the microstructure and microwave properties of Ca5Co4(VO4)6 ceramics

Abstract

Abstract The influence of cobalt ions non-stoichiometry on the crystal structure, microstructure, and microwave properties of Ca 5 Co 4 (VO 4 ) 6 was investigated. The non-stoichiometric Ca 5 Co x (VO 4 ) 6 (x = 3.9–4.1) ceramics were prepared by the solid-state reaction. XRD patterns showed that a slight excess and deficiency of cobalt ions could not alter the crystal phase but change the lattice parameters. From Raman spectra of Ca 5 Co x (VO 4 ) 6 , the vibrational models could be classified into two parts: internal and external models, and seven emission bands were detected. SEM revealed that the superfluous cobalt ions could hinder the growth of grains and also worsen the density of ceramics. The microwave properties as a function of the cobalt content were strongly correlated with the density, ionic polarizability, average grain size, lattice parameters, and unit cell volume. As a result, Ca 5 Co 3.95 (VO 4 ) 6 ceramic sintered at 875 °C had best microwave properties: e r = 10.14, Q × f = 54,100 GHz, and τ f = −68.0 ppm/°C. Furthermore, Ca 5 Co 3.95 (VO 4 ) 6 ceramic with 10 wt% TiO 2 sintered at 925 °C exhibited good performances: e r = 12.4, Q × f = 11,304 GHz, and τ f = +9.5 ppm/°C.