Structure-property relationships of AWO₄(A = Na, Li, Ca, Sr, Bi, Sm) microwave dielectric ceramics with different entropy designs

Microwave dielectric ceramics are critical components in wireless communications. However, it is still a challenge to develop microwave dielectric ceramics with reduced sintering temperatures, high thermal stabilities and good dielectric properties at the same time. Herein, the entropy value, which is able to modulate the grain boundary energy and adjust the inherent sintering temperature, is engineered to develop high-performance microwave dielectric ceramics with low sintering temperatures. The structure-property relationships of AWO₄ ceramics, including (NaCaBi)_1/3WO₄, (NaCaSrBi)_1/4WO₄, (NaLiCaBiSm)_1/5WO₄ and (NaLiCaSrBiSm)_1/6WO₄ with increased entropy values are investigated systematically. X-ray diffraction and high-resolution TEM analyses confirm the formation of single-phase solid solutions with the tetragonal scheelite structure in AWO₄ ceramics. As the number of elements in the A-site varies, the relative permittivities, Q × f values and TCF values shift in the range of 12.2–16.5, 8,000–25,000 GHz and −32.3–31.6 ppm/°C, respectively. Simultaneously, the high-entropy samples demonstrate the lowest sintering temperature of 750 °C among the four formulations. Phillips-Van Vechten-Levine (PVL) complex chemical bond theory reveals that the covalency, lattice energy and bond energy affect the relative permittivity, Q × f value and TCF value, respectively. Raman spectra suggest that high entropy value leads to increased disorder, and infrared reflectivity spectra illustrate that the dielectric response is dominated by polarized optical phonons of [WO₄] tetrahedron. It is anticipated that entropy design will facilitate the development of microwave dielectric ceramics in low temperature co-fired ceramic applications.