Unraveling Hierarchical B‐Site Ordering: Mechanism of Microwave Dielectric Enhancement in Complex Perovskite Oxide Ceramics

Complex perovskite oxides with mixed cation occupancy are important class of functional materials, offering tunability for microwave dielectric properties, ferroelectricity, and magnetism. In despite of their significant utility, the correlation between different hierarchical levels of cation ordering, ranging from macroscopic domains to atomic‐scale clusters, remains elusive, impeding optimization and rational design of high‐performance ceramics. Here, a comprehensive multiscale characterization route is established to quantify ordering in a model 1:2 B‐site‐ordered microwave dielectric system, Ba[(Co 0.6‐ x /2 Zn 0.4‐ x /2 Mg x ) 1/3 Nb 2/3 ]O 3 . By integrating multiscale characterization techniques, the evolution of ordering is tracked from the micrometer to the sub‐angstrom level. Significant lattice strain is observed within the ordered domains, which is strongly correlated with the specific arrangement of B‐site cations. The concentration of oxygen vacancies increases with increasing annealing temperature. Local chemical fluctuations, specifically the compositional variability of multiple cations and Mg disorder, are also identified. Collectively, these three factors disrupt the translational symmetry of the lattice. They act as exceptionally strong phonon scattering centers that significantly shorten the phonon lifetime, ultimately affecting the performance. This work provides a possible path for future optimization and design of dielectric properties through manipulating chemical ordering, point defects, and lattice strain within ordered domains in complex perovskite microwave dielectric ceramics systems.