The effect of cation ordering and domain boundaries on low loss Ba(B~I_(1/3)B~(II)_(2/3))O_3 perovskite dielectrics revealed by high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM)

摘要

High quality ceramics of Ba((Co_(0.7)Zn_(0.3))_(1/3)Nb_(2/3))O_3 (BCZN), Ba(Mg_(1/3)Nb_(2/3))O_3 (BMN) and Ba(Mg_(1/3)Ta_(2/3))O_3 (BMT) were prepared by the mixed oxide route using sintering temperatures up to 1620 ℃. Products with a high degree of cation ordering exhibited dielectric Q x f values from 83,000 GHz (BCZN) to 360,000 GHz (BMT). High Resolution TEM and aberration-corrected scanning transmission electron microscopy (STEM) revealed ordering domains and type I, II and in boundary structures. High-Angle Angular Dark Field (HAADF) STEM images provided direct evidence of 1:2 ordering and stacking sequences, and the presence of disordered regions within domain boundaries. The exceptionally high Q x f values for BMT are associated with a high degree of B-site ordering and the removal of domain boundaries in large, single domain grains. The catastrophic degradation of Q x f values in BMN after prolonged sintering is associated with formation of a lossy ferroelectric secondary phase (Ba_3Nb_2O_8), and changes to composition and stoichiometry of BMN grains.