Improving the Temperature Stability of MgZrNb2O8 through the Synergistic Effect of Ta5 + Substitution and High-Entropy Strategy

High-entropy Mg(ZrHfTiSnGe)0.2(Nb1–xTax)2O8 (x = 0–1) microwave dielectric ceramics were successfully synthesized via a solid-state reaction method. The effects of Ta5+ substitution and high entropy on sintering behavior, crystal structure, and microwave dielectric properties were systematically investigated. XRD confirms that Hf4+, Ti4+, Sn4+, Ge4+, and Ta5+ successfully entered the crystal lattice, forming a single-phase solid solution with a wolframite structure (P2/c space group). SEM images show that grains grow well, the grain size is uniform, and there are few pores. HRTEM images show that the sample has both structural long-range order and local lattice distortion. The dielectric constant (εr) increases with Ta5+ substitution, which is primarily attributed to the higher ionic polarizability of Ta5+ and the cation “under-bonding” effect. The continuous decline in the quality factor (Q × f) value is closely related to the reductions in both relative density and lattice energy. This is further supported by the broadening of the Nb/Ta–O symmetric stretching vibration peak in Raman spectra, indicating enhanced phonon interactions. Most importantly, the temperature coefficient of resonant frequency (τf) improves to −10 ppm/°C, benefiting from the increase in bond energy and the weakening of [AO6] (A = Mg, Zr, Hf, Ti, Sn, and Ge) octahedral distortion. Compared with single Ta5+ substitution (MgZrTa2O8: τf = −34 ppm/°C) and single high-entropy (Mg(ZrHfTiSnGe)0.2Nb2O8: τf = −31 ppm/°C) ceramics, the τf value has been greatly improved, indicating that the combined action of Ta5+ substitution and high-entropy strategy has improved the temperature stability of MgZrNb2O8 ceramics.