Design for Lead-free Ferroelectric Functional Materials using Layered Crystal Lattices

利用无铅铋层状结构氧化物的层状晶格进行了实现新型铁电和集成功能的材料设计，得到了以下结果：利用缺陷工程改善铁电性能在Sr缺乏和Bi过剩的锶铋钽单晶中，获得了28μC/cm^2的大永久极化。这是由于Sr位的空位导致了晶格畸变的增加。通过稀土元素的可控掺杂，实现了稀土的软硬极化特性。在钛酸铋单晶中掺杂了W和Nb等高价离子，观察到非常大的永久极化。这是由于氧空位和泄漏电流的减少。空位形成能的估计表明，掺杂导致钙钛矿层缺陷浓度的降低和稳定性的提高。钛酸铋和钛酸铋钡的生长间超晶格显示出52μC/cm^2的残余极化，是无铅钙钛矿材料中最高的。集成函数的建立锰取代钛酸铋单晶沿a轴表现出半导体性，并发现了极化态变化引起的半导体性的记忆效应。这种效应在含铅铁电体中也得到了证实，因此有望应用于新的功能器件。

Materials Design to realize novel ferroelectric and integrated functions was conducted using layered crystal lattices of Lead-free bismuth layer-structured oxides, and following results were obtained.1.Improvement of ferroelectric properties by defect engineeringA large permanent polarization of 28μC/cm^2 was confirmed in strontium bismuth tantalite single crystals with Sr deficiency and excess Bi. This was attributed to vacancies at the Sr sites leading to an increase in lattice distortion. Soft to hard polarization properties were realized by controlled doping of Rare-earth elements. Very large permanent polarizations were observed in bismuth titanate single crystals doped with higher-valent ions as W and Nb. This was attributed to decreases in oxygen vacancies and leakage current. The estimated vacancy-formation energies suggested that the doping results in a decrease of defect concentration and stabilization of the perovskite layers.2.Formation of intergrowth layered-structures and evaluationThe intergrowth superlattice of bismuth titanate and barium bismuth titanate showed a remanent polarization of 52μC/cm^2, which is the highest in Lead-free perovskite materials.3.Creation of integrated functionsThe Mn-substituted bismuth titanate single crystal showed semiconductivity along a-axis, and a memory-effect of the semiconductivity due to changes in polarized state was discovered. This effect was also confirmed in Lead-containing ferroelectrics, and thus expected for applications to new functional devices.