Hidden diversity of vacancy networks in Prussian blue analogues

Abstract

Prussian blue analogues (PBAs) are a broad and important family of microporous inorganic solids, famous for their gas storage (1,2,3,4,5), metal-ion immobilisation (6,7), proton conduction (8,9), and stimuli-dependent magnetic (10, 11, 12), electronic (13) and optical (14) properties. The family also includes the widely-used double-metal cyanide (DMC) catalysts (15,16,17) and the topical hexacyanoferrate/hexacyanomanganate arXiv:1908.10596v1 [cond-mat.mtrl-sci] PBAs is the ability to transport mass reversibly, a process made possible by structural vacancies. Normally presumed random (21,22,23), vacancy arrangements are actually crucially important because they control the connectivity of the micropore network, and hence diffusivity and adsorption profiles (24,25). The long-standing obstacle to characterising PBA vacancy networks has always been the relative inaccessibility of single-crystal samples (26). Here we report the growth of single crystals of a range of PBAs. By measuring and interpreting their X-ray diffuse scattering patterns, we identify for the first time a striking diversity of non-random vacancy arrangements that is hidden from conventional crystallographic analysis of powder samples. Moreover, we show that this unexpected phase complexity can be understood in terms of a remarkably simple microscopic model based on local rules of electroneutrality and centrosymmetry. The hidden phase boundaries that emerge demarcate vacancy-network polymorphs with profoundly different micropore characteristics. Our results establish a clear foundation for correlated defect engineering in PBAs as a means of controlling storage capacity, anisotropy, and transport efficiency.The true crystal structures of PBAs-as of Prussian Blue itself-have long posed a difficult and important problem in solid-state chemistry because their ostensibly simple powder diffraction patterns [ Fig. 1(a)] belie a remarkable complexity at the atomic scale (27,28,29). The common parent structure is based on the cubic lattice and corresponds to the idealised composition M[M (CN) 6 ]. Atoms of type M and M (usually transition-metal cations) occupy alternate lattice vertices and are octahedrally coordinated by bridging cyanide ions (CN − ) at the lattice edges [ Fig. 1(b)]. There is a close conceptual parallel to the double perovskite structure (30); indeed the key considerations of covalency and octahedral coordination geometry that stabilise perovskites amongst oxide ceramics (31) also favour this same architecture for transition-metal cyanides, which accounts for the chemical diversity of PBAs (32). Charge balance

The structures of ordered defects in thiocyanate analogues of Prussian Blue

“…19 Even for single crystal and total scattering studies where information about the correlation of vacancies through the lattice can be obtained, the high symmetry of the parent structure means the local defect-structure determined through crystallography is superimposed on both the non-defective structure and the symmetry-related transformations of the defect, hindering detailed interpretation. 8,13,20,21 The presence of disorder also complicates the interpretation of spectroscopic data, as deconvolution of different defect sites can oen be challenging unless there are additional helpful features such as moderate paramagnetism. 22 These problems are by no means unique to PBAs 9 and thus, despite the utility of accurate representations of defect-structures, we are oen forced to resort to simplied models which omit the true complexity of defective molecular frameworks.…”

Ordered absences observed in porous framework materials

“…Second, the study adds to a growing body of evidence supporting the 'one polymorph, one disease' hypothesis [6][7][8] , which states that different structural forms (polymorphs) of the same aggregated protein can cause distinct pathologies and symptoms. What might lead a protein to adopt different structures?…”

“…However, studying the surprisingly complex atomic-scale structures of PBAs remains a long-standing challenge. On page 256, Simonov et al 7 report that they have successfully grown single crystals of PBAs, which have previously been notoriously elusive. By coupling X-ray measurements of the crystal lattices with a simple but effective theoretical model, the authors reveal an unexpected ordering of vacancies -absent nodes in the lattices that correspond to missing metal-anion units.…”

“…Networks of vacancies can form pathways that allow molecules or ions to be transported through PBAs, a potentially useful characteristic. Simonov et al7 have used X-ray measurements of single crystals of PBAs and numerical modelling to reveal the hidden order of vacancies in PBAs.…”

Filling Vacancies in a Prussian Blue Analogue Using Mechanochemical Post-Synthetic Modification

“…Our reasoning was that the success of chelation strategies 13,14 implies vacancies are largely a kinetic-rather than thermodynamic-phenomenon in PBAs, and so milling may provide an atom-efficient methodology for overcoming the kinetic barrier to ion incorporation. 6 ] were loaded within a ball-mill, and the mixture milled for up to three hours. The reaction we hoped to effect is given by the (idealised) scheme…”