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

利用无铅铋层状氧化物的层状晶格进行了实现铁电和集成功能的材料设计，得到了以下结果：1.通过缺陷工程改善了铁电性能。在锶缺乏和铋过量的锶铋钽酸锶单晶中，证实了28μC/cm^2的大的永久极化。这是由于锶晶位上的空位导致晶格变形增加所致。通过控制稀土元素的掺杂，实现了从软到硬的极化特性。在掺有W和Nb等高价离子的钛酸铋单晶中观察到了很大的永久极化。这归因于氧空位和漏电流的减少。共生层状结构的形成与评价钛酸铋和钛酸铋共生超晶格的剩余极化强度为52μC/cm2，是无铅钙钛矿材料中最高的。3.复合功能的产生锰代钛酸铋单晶表现出沿a轴的半导性，并且由于极化状态的变化发现了半导性的记忆效应。这一效应在含铅铁电材料中也得到了证实，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.