The rapid advancement of 5G and emerging 6G communication technologies has placed unprecedented demands on the microwave dielectric ceramics that form the backbone of bandpass filters, dielectric resonator antennas, and low-loss substrates. These applications require an unusually difficult combination of properties: a low dielectric constant (εr), an ultra-high quality factor (Q×f), and excellent temperature stability (τf), ideally combined with compatibility with low-temperature co-fired ceramic (LTCC) technology. Unfortunately, these three parameters are locked in a delicate trade-off, making their simultaneous optimization one of the most formidable challenges in the field.
Among candidate material systems, vanadate-based zircon-type AVO4 ceramics (A = trivalent rare-earth cation) are attractive because their distorted [AO8] dodecahedra and rigid [VO4] tetrahedra yield intrinsically low permittivity. However, conventional single-cation AVO4 ceramics suffer from two critical drawbacks: large negative τf values (< –30 ppm/°C) arising from strong dielectric anharmonicity, and modest Q×f values (< 50,000 GHz) caused by A-site lattice distortion and phonon damping. High-entropy strategies have recently pushed Q×f dramatically higher—the (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)VO4 host matrix used in this work reaches 107,200 GHz—but its τf remains stuck at –32.7 ppm/°C.
Recently, a team led by Professor Huaicheng Xiang at Guilin University of Technology, China, has now combined configurational entropy engineering with targeted Bi3+ doping to break this deadlock. Using a conventional solid-state reaction route, the researchers synthesized a compositional series (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)1–xBixVO4 (x = 0.05–0.9) in which Bi3+ progressively replaces the five rare-earth cations at the A-site, driving the configurational entropy from the high-entropy regime (>1.61R) through the medium-entropy zone to a low-entropy state of only 0.49R.
The microwave dielectric properties validate this entropy engineering design. The relative permittivity rises steadily from 12.53 to 60.91 with increasing highly polarizable Bi3+ content, while the Q×f decreases from 96,870 GHz (x = 0.05) to 18,768 GHz (x = 0.9), remaining far above pure scheelite BiVO₄ (~6,500 GHz) owing to entropy-stabilized lattice rigidity. Most strikingly, τf varies non-monotonically—climbing from –11.9 ppm/°C (x = 0.05) to a peak of +132.0 ppm/°C (x = 0.7) before falling to –195.7 ppm/°C (x = 0.9)—a behavior traced to competing thermal expansion, ionic polarizability, and dielectric anharmonicity effects. Consequently, two compositions achieve near-zero τf
