Vibrational Water Dynamics in Sodium-Based Prussian Blue Analogues

The Prussian blue analogues (PBAs) Na_2-xFe[Fe(CN)₆]·z H₂O (x,z = 0–2) exhibit many phase transitions as a function of the sodium and water content, which involves large volume changes that can negatively affect its energy storage performance in a battery. However, the presence of water helps stabilize the PBA framework and thus diminishes these volume changes. To improve the material for its desired applications, a deeper fundamental understanding of the interactions between water, sodium, and the PBA framework is needed. Here, the local structure and vibrational dynamics of water were studied using inelastic neutron scattering, neutron diffraction, and theoretical calculations. When the sodium content is high, the material exhibits well-defined water environments that become less defined when the sodium content is lower. It was shown that the positions of sodium and water are more complex than suggested by previous diffraction and computational studies. Most of the water in the high sodium sample occupies the center of the PBA subcube, while only a small fraction is located close to the window site of the subcube. For the low sodium sample, the results suggest that a large distribution of local water environments is present. These results lay the groundwork for unraveling the ionic transport in PBAs and the development of improved energy storage materials.