Achieving near-zero τf in scheelite Ca1-xLaxW1-xTaxO4 ceramic via A- and B-site co-substitution

A series of tetragonal scheelite-type Ca 1- x La x W 1- x Ta x O 4 (CLWT x , x  = 0.1–0.4) ceramics were successfully fabricated using the traditional solid-state sintering method. The effects of A- and B-site dual substitution on phase formation, microstructural morphology, and microwave dielectric performance were comprehensively characterized using X-ray diffraction, Rietveld refinement, Raman spectroscopy, and scanning electron microscopy. The results revealed that single-phase scheelite ceramics were obtained over the compositional range x  = 0.1–0.3, whereas a secondary Ca 0.5 TaO 3 phase emerged at x  = 0.4. Within the single-phase scheelite solid-solution region (0.1 ≤  x  ≤ 0.3), the unit cell volume expanded linearly from 315.07 Å 3 to 321.15 Å 3 with increasing substitution level, accompanied by elongation of A–O bonds and contraction of B–O bonds. The bond valence analysis indicated that the co-substitution strategy effectively mitigated the rattling effect of the overall crystal structure. Consequently, the τ f increased monotonically from −41.10 ppm/°C to +8.12 ppm/°C, with a near-zero τ f of −7.22 ppm/°C achieved at x  = 0.3. The Q × f decreased gradually from 69,172 GHz to 61,630 GHz for x  = 0.1–0.3, primarily attributed to reduced packing fraction, decreased lattice energy, and enhanced Raman FWHM. The optimal microwave dielectric properties (εr = 16.4, Q × f = 61,630 GHz, τ f = - 7.22 ppm/°C) were obtained for CLWT 0.3 ceramic sintered at 1400 °C. Furthermore, a cylindrical dielectric resonator antenna was designed and simulated, which exhibited a reflection coefficient S 11 of −47.90 dB, an impedance bandwidth of 200 MHz, and a radiation efficiency exceeding 93% at 3.99 GHz, demonstrating the potential of this material for S-band wireless communication applications.