Pressure-induced cation and vacancy disorder-order transition and near-zero τ_f in deficient hexagonal perovskite Ba₈ZnTa₆O₂₄ dielectrics

Pressure applications can enable the tuning of atomic/defect ordering and provide access to new functional materials. Here, we report that pressure-induced structural transformation featuring disorder-order transition of both cations and vacancies in the 8-layer deficient hexagonal perovskite tantalate dielectrics Ba₈ZnTa₆O₂₄, which transformed the structure from twin to shift and remarkably lowered the temperature coefficient of resonant frequency τ_f down to near zero (∼0.56 ppm/°C) from 38 ppm/°C for the twinned precursor. The atomic scale STEM-HAADF and EDS results confirm the ordering of Zn in the Ta host at the nanometer scale in the shifted material featuring well-ordered Ba₈ZnTa₆O₂₄ slabs intergrown with Ba₃ZnTa₂O₉ and Ba₅Ta₄O₁₅ monolayers and anti-phase grain boundaries as planar defects. The pressure-induced twin-shift structural transformation of Ba₈ZnTa₆O₂₄ features the rare constant concentration of the hexagonal stacked layers, which is allowed by the vacancy ordering at the central layers of face-shared octahedral (FSO) trimers avoiding the FSO B-B repulsion, and remarkably the faster cationic ordering kinetics compared with the 2:1 ordered complex perovskites. Although the inclusion of numerous planar defects and the oxidizable atomic defects led to significant p-type conduction and inhomogeneous electrical microstructures, resulting in an extraordinarily high extrinsic dielectric loss for the high-pressure shifted Ba₈ZnTa₆O₂₄ pellet, the intrinsically near-zero τ_f could make the shifted Ba₈ZnTa₆O₂₄ perovskite an ideal microwave dielectric resonator candidate if the defects could be eliminated.