铅基复合钙钛矿型弛豫铁电单晶的生长基元与生长机理Ⅰ.PMNT单晶的表面形貌、负晶结构及生长基元的组装与拆分

铅基复合钙钛矿型弛豫铁电单晶体PMNT的生长基元为多种[BO_6]配位八面体,晶体生长过程可视为多种八面体基元与Pb~(2 )的组装过程.这些生长基元向{111}面叠合时易采取法向生长机制,向{001}面叠合时易采取层状生长机制,由此决定了晶体生长速度的各向异性与晶体的形貌.Bridgman法生长的PMNT晶体在生长过程中由内向生长机制形成规则的负晶结构;在晶体生长过程中,在其自然表面上可形成正形与负形两种形貌;在高温退火过程中,由于PbO的分解,晶体表面上可形成类似“蚀象”的构型,这些可从[BO_6]八面体生长基元的组装或拆分方面获得解释.     

Defect‐Driven Structural Distortions at the Surface of Relaxor Ferroelectrics

Relaxor‐ferroelectric materials find application in a broad range of technological devices, including ultrasonic imaging transducers, nanopositioning, and high‐performance capacitors. They generally exhibit occupationally disordered structures creating local polar fluctuations that are highly sensitive to applied electric or stress fields. The sensitivity of the material structure to external field and stress conditions also makes them likely to develop skin or surface phases that are unique from the bulk. Surface layers can adjust the material response and also lead to ambiguity in structural characterization. Here, using a combination of X‐ray diffraction methods, it is shown that a ≈20 µm skin structure commonly exists in the lead‐free relaxor‐ferroelectric ceramic (Na_1/2Bi_1/2)TiO₃–BaTiO₃. Using experiments and density functional theory calculations, it is shown that the combined action of oxygen vacancies providing internal chemical pressure and the surface plane stress state dictates the stability and structure of the skin layer. This work can be extended to all perovskite relaxor ferroelectrics and provides new insights into the origin of skin layers in these materials. The opportunity exists to further enhance the functionality of these materials through engineering of surface structures using the methods outlined here.