Prussian blue analogues (PBAs) are a diverse family of microporous inorganic solids. Originally commercialized in the dye industry, they are known for their gas storage ability, ion conduction, and other magnetic, electronic and optical properties leading to applications in fields including energy storage and electro-catalysis.
Within the PBA formula, there is a large range of compositions that opens up possibilities for diverse functionality. For example, the introduction of a particular type of defect creates a network of voids inside the structure, which could make the materials suitable for gas storage or catalysis.
However, the relationship between the composition and physical properties is not well understood, despite the potential industrial applications. In this study, published in JACS, researchers used variable pressure X-ray and neutron diffraction at Diamond Light Source and the PEARL beamline at ISIS, to study a range of PBAs.
Study author, Hanna Boström, explains; “Neutron diffraction really is an excellent tool to study these materials, as the high neutron scattering length of carbon and nitrogen means that cyanide positions can be accurately determined. This is challenging for X-ray diffraction."
By studying multiple samples with different compositions, hydration levels and defect concentrations, the researchers were able to build a picture of how these influence the materials' physical properties such as mechanical stability, flexibility and phase transition behaviour.
The effect of defects was studied by comparing the behaviour of Mn[Co(CN)6]0.67 and MnPt(CN)6; the group found that these defects present in the Mn[Co(CN)6]0.67 structure enhanced its flexibility. The structure was also less prone to pressure-induced phase transitions.
Using the Paris-Edinburgh press on the Pearl beamline, the researchers looked at the combined role of hydration and defect concentration by comparing the structures of Mn[Co]0.67·nD2O and MnPt·nD2O at pressures up to 2.5 GPa. When defects were present, the addition of water caused a phase transition to occur whereas, in the defect-free MnPt·nD2O, the compressibility of the structure was not affected by adding water.
They expanded the study to investigate the effect of composition by comparing RbMnCo and CsMnCo, finding that the radius of the A-site alkali metal dictates the phase transition pressure. They also discovered that RbMnCo exhibited a polar phase under modest compression that could have ferroelectric properties. In general, polar distortions are very rare in PBAs.
Their results highlight the impact of compositional changes on the behaviour of PBAs, and how these behaviours could be tuned to design future materials.
“Our study improves the fundamental understanding of PBAs and we hope that it will ultimately aid the development of these materials into functional devices, whether catalysts or cathodes," adds Hanna.
The full paper can be found online at DOI: 10.1021/jacs.0c13181
A previous characterisation of a Prussian Blue Analogue using the ISIS beamline GEM is available as an ISIS science highlight.