Near-Zero Temperature Coefficient of Resonance Frequency in Rare-Earth Titanates via the Phase Transition Strategy

This study presents an approach to achieve a near-zero temperature coefficient of resonance frequency (τ_f) in rare-earth titanate microwave dielectric ceramics (MWDCs) by inducing a phase transition. By Zr⁴⁺ substitution at the B site, a series of Sm₂Ti_1-<i>x</i>Zr_<i>x</i>O₅ (0.02 ≤ <i>x</i> ≤ 0.55) ceramics are synthesized using the solid-state method to intentionally alter the radius ratio of the A/B sites, realizing in a controlled phase transition from orthorhombic (<i>Pnma</i>) to biphasic coexistence and ultimately to cubic (Fd<b>3</b>̅m) structure. The phase composition is rigorously identified through X-ray diffraction (XRD) Rietveld refinement, high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), and Raman spectroscopy. A comprehensive analysis is conducted to elucidate the relationships between factors such as ionic polarizability, packing fraction, bond valence, complex chemical bonding, and far-infrared reflectivity spectra with microwave dielectric properties. The results demonstrate that these ceramics exhibit a broad range of permittivity (14.30-23.18), high-quality factors (14,828-22,300 GHz), opposite temperature coefficient of resonance frequency (-16.0 to + 22.4 ppm/°C), and nice thermal conductivity (1.81-2.76 W·m^-1·K^-1), particularly at <i>x</i> = 0.30 with a near-zero τ_f value of +1.6 ppm/°C. The findings not only provide insights into designing MWDCs with a near-zero τ_f but also offer a promising route for developing advanced microwave materials with improved performance and reliability.