Structural Basis for Emergence of Superionic Conductivity by an Ion Exchange

Various superionic conductors have been examined in terms of the application to electrolytes for solid fuel cells [1]. Recently we demonstrated by impedance measurements that a simple two-step chemical reaction transformed an electronic conductor Na_xCoO₂ into a superionic one. In the present study, we performed in situ synchrotron X-ray diffraction experiments to investigate a structural mechanism for the superionic conductivity driven by the chemical treatment of the layered oxide Na_xCoO₂. We developed a temperature- and humidity-controllable capillary cell under hydrogen and helium gas flow to install in the Debye-Scherrer camera at BL44B2 of SPring-8. This cell allows us to explore a structural transformation process by reduction and humidification treatments. Structural identifications and refinements with in situ diffraction data proved that Co vacancies formed by a CoO separation suppressed the electronic conductivity. Meanwhile it turned out from charge estimation in the Na layers that the superionic conductor transition originated from an ion exchange of H₃O⁺ for Na⁺, which was confirmed by Raman spectroscopy measurements. In addition, charge densities clearly visualized the H₃O⁺ ions disordering around the Na original sites, suggesting that the H₃O⁺ behave as a carrier source. Finally it was found from electrostatic potentials that the disordering H₃O⁺ sites were coupled through shallow potential barriers to trace a honeycomb-like ion pathway. In the presentation, I will discuss what a carrier is for the superionic-conductive phase from different viewpoints such as activation energies, concentration cell tests, and molecular dynamics simulations using the experimental structure information.

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.