Simultaneous Realization of High Dielectric Constant and Ultrahigh Quality Factor in SrTiO₃ via a Tunable Grain Growth Strategy

In high-performance dielectric ceramics, grain boundaries are the primary reason attributed to microwave dielectric losses; therefore, increasing grain size to reduce their density effectively enhances dielectric properties. Although ion doping shows promising modification effects, how to control the grain size by ion codoping has been barely investigated due to the unclear nucleation and growth mechanism. Previous ion doping did not precisely control the grain size of the dielectric ceramic owing to its high nucleation density and low grain boundary mobility, typically resulting in sub-20 μm grains. In this study, combining the effect of Al/Ta codoped SrTiO₃ (ST-<i>x</i>AT, <i>x</i> = 0-0.1) microwave dielectric ceramics, anomalous crystal nucleation and growth were observed. To confirm the grain size effect on optimizing the dielectric properties, the prolonged sintering process was implemented to further increase the grain size of the Al/Ta codoped ceramics, which other doping strategies fail to replicate. Consequently, the average grain size of the obtained Al/Ta codoped SrTiO₃ is 4 times greater than existing cases. When ST-0.02AT was sintered at 1550 °C for 8 h, the outstanding dielectric properties of <i>Q</i> × <i>f</i> ∼ 10,681 GHz (at 3.3 GHz) were achieved, representing a 268% improvement compared to the undoped sample (<i>Q</i> × <i>f</i> ∼ 2901 GHz) while maintaining a high dielectric constant ε_r ∼ 258.01. Meanwhile, the intrinsic factors and growth mechanism of dielectric properties were explained by applying the P-V-L theory and density functional theory calculations to analyze chemical bond characteristics and electronic structures. These results not only provide a promising avenue for achieving tunable properties of high dielectric constant ceramics but also shed light on understanding the crystal nucleation and growth control mechanism of codoping dielectric ceramics.