High-Entropy Design of Perovskite Quantum Paraelectrics with Improved Dielectric Properties in GHz and THz Bands

High-entropy engineering offers a promising pathway to enhance dielectric permittivity at elevated frequencies in perovskite oxides by increasing compositional complexity. Here, two titanate-based high-entropy quantum paraelectrics, (Ba_0.2Sr_0.2Ca_0.2La_0.2Na_0.2)TiO₃ (BSCLN) and (Ba_0.2Sr_0.2Ca_0.2La_0.2K_0.2)TiO₃ (BSCLK) were developed via entropy-driven phase stabilization, and their dielectric responses were systematically investigated from radiofrequency to terahertz regimes. Both materials exhibit spatially distributed local polar modes arising from entropy-induced configurational local polar disorder (LPD) and entropy-enhanced lattice-coupled local polar fluctuation (LPF). In the radiofrequency range, weak- and strong-field permittivity are nearly identical, with the strong-field permittivity remaining ∼ 250, indicating a dominant ionic polarization contribution and excellent dielectric stability. Negative extrapolated Curie temperatures (<0 K), together with the temperature-dependent permittivity behavior, confirm the quantum paraelectric nature of both compositions. In the microwave regime, the high-entropy quantum paraelectrics show significantly higher dielectric permittivity (∼155-189) and lower dielectric loss (∼0.002) compared to BaTiO₃ and SrTiO₃, while their terahertz response is governed by entropy-enhanced local polar fluctuations. These results demonstrate that combining high-entropy design with quantum paraelectric functionality provides an effective strategy for developing high-frequency dielectric materials for next-generation electronic devices.