New strategies for characterizing Prussian blue analogues

Sammanfattning: Prussian blue analogues (PBAs), AxM[M’(CN)6]1–y·zH2O, are used in many different applications, such as energy storage, due to their tunable composition and structural diversity. To understand the material properties, it is important to accurately determine the composition and atomic structure of these materials. Determining the composition of iron-based PBAs is challenging due to the interdependent relationship between the three compositional parameters namely the sodium (Ax), water (z), and [Fe(CN)6]n– (y) vacancy content. In addition, the atomic structure depends on the composition leading to a rich structural landscape, which further influences the material properties. This thesis presents a comprehensive strategy for characterizing the composition and atomic structure of iron-based PBAs by applying different characterization techniques. Firstly, it was shown that to accurately determine the composition of iron-based PBAs, it is crucial to combine multiple characterization techniques in combination. In particular, Mössbauer spectroscopy proved to be a key technique for accurately determining the vacancy content in iron-based PBAs, where both metal sites (M and M’) are occupied by iron. Furthermore, positron annihilation lifetime spectroscopy was explored as a potential method for determining the internal porosity in PBAs. A correlation between the average positron lifetime and varying PBA compositions was found and is compared to other standard characterization techniques. Secondly, the impact of the synthesis method of Na2Fe[Fe(CN)6]·zH2O was studied using X-ray and neutron diffraction. Independent of the synthesis method, it was found that a P21/n phase exists at room temperature, which transitions to an R-3 phase upon heating to 40 °C. The two structures were differentiated in terms of the octahedral tilting system and sodium-ion displacements. These results were compared to the structure of the dehydrated material. In addition, it was found that there is a significant difference in the rate and magnitude of thermal expansion of the hydrated relative to the dehydrated material.