Enhanced microwave dielectric properties and reduced bulk conductivity in scheelite CaWO4 via rare-earth doping

This work investigates the effect of rare-earth substitution on the structural, electrical and microwave dielectric properties of scheelite-type CaWO 4 ceramics. Ca 0.98 RE 0.02 WO 4+ δ ( RE = La–Yb) samples were prepared and uniformly sintered at 1150 °C. Ca 0.98 Yb 0.02 WO 4+ δ exhibits the highest ε r and Q ×  f values (ε r ∼10.62, Q × f ∼ 65,868 GHz), albeit with a negative τ f (-56.7 ppm/°C). In contrast, bulk conductivity varies non-monotonically with ionic radius and reaches a local maximum near A-site dopant/host radius matching, consistent with a lower migration barrier. Structural analysis indicates that nominal A-site cation radius mismatch modifies local bonding and lattice dynamics, leading to different responses in bulk transport and microwave dielectric behavior. These results establish a clear radius-dependent structure–property relationship in dilute RE -doped CaWO 4 and identify nominal A-site cation radius mismatch as an effective compositional parameter for balancing transport and microwave dielectric performance.