Tunable Microwave Dielectric Properties in Rare-Earth Niobates via a High-Entropy Configuration Strategy To Induce Ferroelastic Phase Transition

In this study, (La0.2Nd0.2Sm0.2Ho0.2Y0.2)(Nb1-xVx)O4 (0.1 ≤ x ≤ 0.4) ceramics were prepared using a high-entropy strategy via the solid-phase method. The crystal structure, microstructure, vibration modes, and phase transition were studied by X-ray diffraction, scanning electron microscopy/transmission electron microscopy (SEM/TEM), and Raman spectroscopy techniques. The phase of ceramics was confirmed to be a monoclinic fergusonite in the range of x ≤ 0.28, a tetragonal scheelite was in the range of 0.3 ≤ x ≤ 0.32, a complex phase of tetragonal scheelite, and zircon was observed in the ceramics when x ≥ 0.35. A zircon phase was also detected by TEM at x = 0.4. The ceramic at x = 0.25 exhibited outstanding temperature stabilization with εr = 18.06, Q × f = 56,300 GHz, and τf = -1.52 ppm/°C, while the x = 0.2 ceramic exhibited a low dielectric loss with εr = 18.14, Q × f = 65,200 GHz, and τf = -7.96 ppm/°C. Moreover, the permittivity, quality factor, and the temperature coefficient of resonance frequency were related to the polarizability, packing fraction, density, and the temperature coefficient of permittivity caused by phase transition. This is an effective method to regulate near-zero τf by the synergism of the high-entropy strategy and substituting Nb with V in LnNbO4 ceramics.