Tailoring microwave dielectric properties through high-entropy strategy in spinel-type titanium ceramics

Abstract

In this work, high-entropy [(Mg_1/3Zn_1/3Li_1/3)_1−x(Co_1/2Ni_1/2)_x]₂TiO_4−δ (MZLCNT) ceramics (x = 0.0–0.4) were synthesized via a solid-state reaction method. The introduction of low concentrations of Co and Ni significantly increased the configurational entropy (ΔS_config) of the ceramics, inducing cation disorder and promoting the system transformation from the stable Mg₂TiO₄ phase to an entropy-stabilized Zn₂TiO₄-based high-entropy solid solution in the high-entropy MZLCNT ceramic, as confirmed by Raman spectroscopy. However, excessive Co and Ni contents reduce the cation disorder due to their strong preference for octahedral sites. Meanwhile, reducing the Li⁺ content effectively suppresses the formation of oxygen vacancies. Based on the complex chemical bond theory (P–V–L theory), the Ti–O bond exhibits significantly higher ionicity and lattice energy than the A–O bond (A = Mg/Zn//Li/Co/Ni), contributing more substantially to the dielectric constant (ε_r) and quality factor (Qf) of ceramics. Furthermore, the increase in ΔS_config significantly reduces lattice distortion, thereby enhancing the bond strength of A(1)–O bonds in tetrahedra and further improving the temperature coefficient of the resonant frequency (τ_f) of ceramics. As ΔS_config changes, excellent microwave dielectric properties were obtained at 1300 °C: ε_r values of 14.81–16.24, Qf values of 36 700–74 600 GHz, and τ_f values of −42 to −18 ppm °C⁻¹. The research findings offer promising candidate systems for the development of high-performance and highly stable microwave dielectric materials, while further enhancing the understanding of the high-entropy effect mechanism in ceramics.