In situ synchrotron powder diffraction for an ionic conductor transition

In situ synchrotron X-ray powder diffraction can be one of the most powerful probes to investigate the structure evolution by a chemical reaction thanks to simultaneity of data collection. It is not, however, with ease to produce a homogeneous chemical reaction in the limited spaces, which is essential to see an atomic-scale structure evolution. We have developed an in situ capillary cell for both high-temperature H₂ reduction and precise humidity control at the SPring-8 BL44B2. We successfully applied this in situ system to an electronic conductor LaSr₃Fe₃O₁₀, which is transformed into an ionic conductor by the two-step chemical treatments [1]. LaSr₃Fe₃O₁₀ has a triple-layer structure with a FeO₆ octahedral unit. One triple layer is bonded with another layer through van der Waals interaction. Structure refinements with in situ synchrotron powder diffraction data revealed that the H₂ reduction at 613 K produced in-plane oxygen vacancies, which resulted in suppression of the interlayer interaction. We found from charge density studies and Raman spectroscopy measurements that the following humidification intercalated H₂O and OH^- into the interlayer and intralayer, respectively. That means that H₂O plays a role for suppression of three-dimensional electronic conductivity, stabilizing the intercalation structure. On the other hand, the OH^- ions behave as carriers for ionic conductivity, maintaining the charge neutrality in the intralayer. Finally we determined the composition of the ionic conductor to be LaSr₃Fe₃O_8.0 (OH) _1.2·2H₂O, which indicates a transformation of LaSr₃Fe₃O₁₀ into an OH^- ionic conductor. In the presentation, I will discuss the OH^- ionic conduction channel based on electrostatic potentials obtained from charge densities.