Microwave dielectric properties of Mg4Nb2O9-based ceramics with (BxW1−x)5+ substitutions at Nb5+ sites (B = Li, Mg, Al, Ti)

The dependences of the microwave dielectric properties on the isovalent substitution of (Li1+1/5W6+4/5)5+, (Mg2+1/4W6+3/4)5+, (Al3+1/3W6+2/3)5+ and (Ti4+1/2W6+1/2)5+ at the Nb5+-sites of Mg4Nb2O9 ceramics were investigated. For the samples sintered at 1350 °C for 10 h, a single phase of corundum structure with hexagonal symmetry was observed through the entire range of compositions. The structural characteristics of the Mg4Nb1.95(BxW1-x)0.05O9 ceramics were quantitatively evaluated by Rietveld refinement method from the X-ray diffraction data. The specimens containing (Ti4+1/2W6+1/2)5+ substitutions had the highest quality factor (Qf) of 233,000 GHz of all the samples. The dielectric constants (K) of the samples were similar for all compositions. The temperature coefficient of the resonant frequency (TCF) of the samples was dependent on the distortion of the oxygen octahedra (Δavg × 103). As the distortion of the oxygen octahedra increased, the TCF values decreased and eventually become negative.