Mechanism of tuning thermal stability and enhancing quality factor in CaWO₄-based ceramics with scheelite–fergusonite structure evolution

To modify the near-zero temperature coefficient of resonant frequency (τ_f) of CaWO_4-based materials and explore their structure-property relationships, Ca_1−xNd_xW_1−xNb_xO₄ (CWNNx) ceramics were prepared. Phase evolution occurred in the system from tetragonal-scheelite to monoclinic-fergusonite at x = 0.7. Relative permittivity (ε_r) increased from 11.28 to 16.84, influenced by the polarizability per unit molar volume (α /V_m), whereas the negative deviations between _εr,corr and _{εr,C - M} (Δ_εr≈ -3.39 to -12.50) resulted from the compression effect of Ca² + and Nd³⁺ coupled with Nb⁵⁺ at the B site. The quality factor (Q) rose by 52, attributed to the decreased Raman full width at half-maximum and increased lattice energy (U). The τ_f values changed from -27.6× 10^-6 to 72.1× 10^-6°C^-1, governed by the increased temperature coefficient of ionic polarizability (τ_um), decreased coefficient of thermal expansion (α_L), and weakened A-site compression. CWNN0.3 ceramic demonstrated superior properties (ε_r = 13.00, Q× f = 60170GHz, and τ_f = -1.9× 10^-6°C^-1), suggesting potential applications in sub-6 GHz antennas.