Crystal structure evolution, bond characteristics and tunable microwave dielectric properties of (Ce_1-xCax)(Nb_1-xW_x)O₄ ceramics

Co-substitution of Ce3+ at the A-site and Nb5+ at the B-site by smaller Ca2+ and W6+ in (Ce1-xCax)(Nb1-xWx)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 epsilon(r) decreased from 20.7 to 16.2, and the negative deviations between epsilon(r) and epsilon(r(C-M)) resulted from the rattling effect of Nb5+/W6+ cations in the octahedron along the b-axis direction. The tau(f) varied from negative (-11 ppm/degrees C at x = 0.1) to positive (+87.8 ppm/degrees C at x = 0.4), mainly attributed to the rattling effect of Nb5+/W6+. Qxf 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 epsilon(r) = 19.7, Qxf = 35,450 GHz, and tau(f) = 0.8 ppm/degrees C.