Microwave dielectric properties of Mg 4 Nb 2 O 9 -based ceramics with (B x W 1−x ) 5+ substitutions at Nb 5+ sites (B = Li, Mg, Al, Ti)

Abstract The dependences of the microwave dielectric properties on the isovalent substitution of (Li 1+ 1/5 W 6+ 4/5 ) 5+ , (Mg 2+ 1/4 W 6+ 3/4 ) 5+ , (Al 3+ 1/3 W 6+ 2/3 ) 5+ and (Ti 4+ 1/2 W 6+ 1/2 ) 5+ at the Nb 5+ -sites of Mg 4 Nb 2 O 9 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 Mg 4 Nb 1.95 (B x W 1- x ) 0.05 O 9 ceramics were quantitatively evaluated by Rietveld refinement method from the X-ray diffraction data. The specimens containing (Ti 4+ 1/2 W 6+ 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 (Δ avg × 10 3 ). As the distortion of the oxygen octahedra increased, the TCF values decreased and eventually become negative.