Crystal structure evolution, bond characteristics and tunable microwave dielectric properties of (Ce1-xCax)(Nb1-xWx)O4 ceramics

Co-substitution of Ce 3+ at the A-site and Nb 5+ at the B-site by smaller Ca 2+ and W 6+ in (Ce 1- x Ca x )(Nb 1- x W x )O 4 ceramics drives the structure from monoclinic fergusonite ( x  = 0.1 - 0.3) to tetragonal scheelite ( x  = 0.4). Adjusting the A-O and B-O bond characteristics could drastically tune the microwave dielectric properties . The ε r decreased from 20.7 to 16.2, and the negative deviations between ε r and ε r (C-M) resulted from the rattling effect of Nb 5+ /W 6+ cations in the octahedron along the b -axis direction. The τ f varied from negative (−11 ppm/°C at x  = 0.1) to positive (+87.8 ppm/°C at x  = 0.4), mainly attributed to the rattling effect of Nb 5+ /W 6+ . Q×f increased from 21,006 to 45,025 GHz ( x  = 0.3) and then decreased to 36,563 GHZ ( x  = 0.4), accompanied by increasing packing fraction and decreasing Raman FWHM. The ceramic at x  = 0.15 exhibited the optimum microwave dielectric properties of ε r = 19.7, Q×f = 35,450 GHz, and τ f = 0.8 ppm/°C.