Abstract

Phase transition–structure–dielectric properties in microwave band correlations were determined for the (Sm_1–xCa_x) (Nb_1–xMo_x)O₄ (SNCMo@x) system. X-ray and Raman analyses along with selected-area electron diffraction indicated that SNCMo@x (0.15 ≤ x < 0.375) ceramics crystallize in the I2/a space group (monoclinic fergusonite), whereas the I4₁/a space group (tetragonal scheelite) best describes SNCMo@x (0.375 ≤ x ≤ 0.7), suggesting that the increased ionic radius of the A-site effectively contributed to the ferroelastic phase transition and ensures the stability of the scheelite phase. The SNCMo@x ceramic materials exhibit composition-dependent permittivity (εr) with a distribution between 12.0 and 17.7. The distortion and deformation of the [BO] polyhedra should be responsible for the shift from negative to positive temperature coefficient of resonant frequency (TCF) and the irregular behavior of the quality factor (Q × f). An optimum microwave dielectric performance was achieved for SNCMo@0.18 (εr ∼ 17.1, Q × f ∼ 52, 800 GHz at ∼8.80 GHz, and TCF ∼ −1.4 ppm/°C). This work demonstrates the important role of simultaneous substitution of A/B cations on [BO] polyhedral distortion and deformation in RENbO₄ materials and its significant effect on the microwave dielectric properties. Also, the SNCMo@0.18 ceramic has been designed as a cylindrical dielectric resonator antenna with a high simulated radiation efficiency (97.1%) and gain (5.96 dBi) at the center frequency (7.75 GHz), indicating its promising application in X-band satellite communication (7.62–7.89 GHz) because of its adjustable permittivity, low loss, and good temperature stability.