Hidden diversity of vacancy networks in Prussian blue analogues.

Prussian blue analogues (PBAs) are a diverse family of microporous inorganic solids, known for their gas storage ability, metal-ion immobilization, proton conduction, and stimuli-dependent magnetic, electronic and optical properties. This family of materials includes the double-metal cyanide catalysts and the hexacyanoferrate/hexacyanomanganate battery materials. Central to the various physical properties of PBAs is their ability to reversibly transport mass, a process enabled by structural vacancies. Conventionally presumed to be random, vacancy arrangements are crucial because they control micropore-network characteristics, and hence the diffusivity and adsorption profiles. The long-standing obstacle to characterizing the vacancy networks of PBAs is the inaccessibility of single crystals. Here we report the growth of single crystals of various PBAs and the measurement and interpretation of their X-ray diffuse scattering patterns. We identify a diversity of non-random vacancy arrangements that is hidden from conventional crystallographic powder analysis. Moreover, we explain this unexpected phase complexity in terms of a simple microscopic model that is based on local rules of electroneutrality and centrosymmetry. The hidden phase boundaries that emerge demarcate vacancy-network polymorphs with very different micropore characteristics. Our results establish a foundation for correlated defect engineering in PBAs as a means of controlling storage capacity, anisotropy and transport efficiency.