Balancing Temperature Stability and Dielectric Loss in Microwave Ceramics via Embedded Polar Nanoregions

The deployment of 5G/6G communication systems demands microwave dielectric ceramics that simultaneously possess a near‐zero temperature coefficient of resonant frequency and an ultra‐low dielectric loss (tan δ ), which represents a long‑standing challenge that demands effective optimization strategies. Herein, the fundamental limitation is substantially alleviated by a strategy integrating interpretable machine learning with entropy engineering, enabling the reverse design of a medium‐entropy perovskite microwave ceramic composition (Ca 0.7 Sm 0.27 Nd 0.03 )[Ti 0.686 (Mg 1/3 Nb 2/3 ) 0.014 Al 0.3 ]O 3 . This ceramic material delivers a desired combination perfomance of τ f = −0.18 ppm/°C and tan δ = 1.19 × 10 −4 at 4.6 GHz. Multiscale characterization reveals that entropy‐induced chemical disorder creates isolated polar nanoregions that pin charged defects, transforming them from a source of relaxation loss into static structural units, which is different from the conventional defect–loss correlation mechanism. The optimized ceramic is implemented in a dielectric resonator antenna, which demonstrates superior performance: a radiation efficiency of 96.2% and a gain of 6.06 dBi. The findings establish a generalizable framework for navigating complex compositional spaces and resolving conflicting properties in perovskite materials.