Search for oxides showing reversible changes in structure upon visible light illumination by Raman scattering and ellucidation of its mechanism

本研究的目的是开发一种方便的方法来发现在光照下显示其结构变化的氧化物。为了从众多的氧化物中选择一种新的候选物，人们必须精确地监测由光照引起的结构的细微变化。为了探索在特定可见光下表现出结构变化的氧化物并阐明其机理，我们利用多种激光（Kr、染料、Ti蓝宝石等）作为拉曼光谱的激发源，进行了原位拉曼散射实验，使我们能够观察到结构的变化。我们能够评估物质与光之间相互作用的大小，从而可以选择显示光诱导结构变化的候选氧化物。用这种方法发现的是略缺氧的YBa_2Cu_3O_x（以下简称Y123）。为了了解Y123体系中光致结构变化的机理，本文阐明了(1)该现象发生在晶体结构的哪个部分，(2)该现象中哪个电子跃迁是必不可少的。由于与特定原子相关的给定振动模式可以通过偏振拉曼散射分别观察到，因此光诱导结构变化发生的位置已被指定。结果表明，活性位点位于一维CuO链内。利用波长为363 ~ 800 nm的光源进行了一系列拉曼散射实验，确定了触发该现象的最小能量为2.15 eV，并证明了电子跃迁的起源对应于类分子Cu-O的电荷转移。

The aim of this study is to develop a convenient means toward the discovery of oxides showing changes in their structure under light illumination. In order to choose a new candidate from a numerous number of oxides, one has to monitor precisely a subtle change in structure induced by light illumination. To explore an oxide showing structural change under a specific visible light and to ellucidate its mechanism, the in-situ Raman scattering experiment, which enabled us to observe the change in structure, has been carried out using a variety of lasers (Kr, dye, Ti sapphire etc.) as an excitation source for the Raman spectra. We were able to evaluate the magnitude of interaction between the substance and light, making possible to choose the candidate oxide showing the light-induced structural change. It was the slightly oxygen-deficient YBa_2Cu_3O_x (hereinafter, referred to Y123) that was discovered in this way.In order to know the mechanism for the light-induced structural change in Y123 systems, it was ellucidated (1) at what part the phenomenon takes place in the crystal structure and (2) which electronic transition is essential to the phenomenon. Since a given vibrational mode associated with a specific atom could be each individually observed by polarized Raman scattering, the place at which the light-induced structural change occurs has been specified. As a result, the active site was ellucidated to be within the one dimensional CuO chain. The minimum energy triggering the phenomenon was determined to be 2.15 eV by a series of Raman scattering experiments using light sources with wave lengths ranging from 363 to 800 nm, and it has turned out that the origin of the electronic transition corresponds to a charge transfer of a molecular-like Cu-O.