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

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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.