Vacancy-ordered perovskite oxides are attracting attention due to their diverse functions such as resistive switching, electrocatalytic activity, oxygen diffusivity, and ferroelectricity. It is important to clarify the chemical lattice strains arising from compositional changes and the associated vacancy order–disorder phase transitions at the atomic scale. Here, we elucidate the intermediate process of a topotactic phase transition in Ca-doped bismuth ferrite films consisting of alternating stacks of oxygen perovskite layers and a brownmillerite-type oxygen vacancy layer. We use Raman spectroscopy and transmission electron microscopy to closely examine the evolution of local strains exerted on the constituent sub-layers by electrochemical oxidation. A negative Raman chemical shift is observed during oxidation, which is linearly correlated with the local negative chemical expansivity of the oxygen layer. It seemingly contradicts with the general trend that oxides undergo lattice contraction upon oxidation. Oxygen vacancies initially confined in the vacancy layers can be understood to diffuse into the oxygen layers during melting of the ordered structure. The finding deepens our understanding of the electro-chemo-mechanical coupling of vacancy-ordered oxides.

中文翻译：

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拉曼显微镜和透射电子显微镜追踪钙钛矿氧化物拓扑规丝形成过程中的氧空位序-无序转变过程


空位有序钙钛矿氧化物因其多种功能（如电阻开关、电催化活性、氧扩散率和铁电性）而备受关注。阐明原子尺度上由成分变化和相关的空位序-无序相变引起的化学晶格应变非常重要。在这里，我们阐明了由交替的氧钙钛矿层和褐米勒石型氧空位层组成的 Ca掺杂铋铁氧体薄膜中拓扑趋相变的中间过程。我们使用拉曼光谱和透射电子显微镜来仔细研究电化学氧化作用在组成亚层上的局部应变的演变。在氧化过程中观察到负的拉曼化学位移，这与氧层的局部负化学膨胀率呈线性相关。这似乎与氧化物在氧化时发生晶格收缩的总体趋势相矛盾。最初局限在空位层中的氧空位可以理解为在有序结构熔化过程中扩散到氧层中。这一发现加深了我们对空位有序氧化物的电化学-机械耦合的理解。