Sintering characteristics, phase transitions, and microwave dielectric properties of low-firing [(Na_0.5Bi_0.5)_xBi_1−x](W_xV_1−x)O₄ solid solution ceramics

A series of high-k [(Na_0.5Bi_0.5)_xBi_1−x](W_xV_1−x)O₄ (abbreviated as NBWV(x value)) solid solution ceramics with a scheelite-like structure are synthesized by a modified solid-state reaction method at the temperature range of 680–760 ℃. A monoclinic (0 ≤ x < 0.09) to tetragonal scheelite (0.09 ≤ x ≤ 1.0) structural phase transition is confirmed by X-ray diffraction (XRD), Raman, and infrared (IR) analyses. The effect of structural deformation and order–disorder caused by Na⁺/Bi³⁺/W⁶⁺ complex substitution on microwave dielectric properties is investigated in detail. The compositional series possess a wide range of variable relative permittivity (ε_r = 24.8–80) and temperature coefficient of resonant frequency (TCF value, −271.9–188.9 ppm/℃). The maximum permittivity of 80 and a high Q×f value of ~10,000 GHz are obtained near the phase boundary at x = 0.09. Furthermore, the temperature-stable dielectric ceramics sintered at 680 ℃ with excellent microwave dielectric properties of ε_r = 80.7, Q×f = 9400 GHz (at 4.1 GHz), and TCF value = −3.8 ppm/℃ are designed by mixing the components of x = 0.07 and 0.08. In summary, similar sinterability and structural compatibility of scheelite-like solid solution systems make it potential for low-temperature co-fired ceramic (LTCC) applications.