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 synthesis of titanium-carboxylate metal-organic frameworks (MOFs) is hampered by the high reactivity of the commonly employed alkoxide precursors. Herein, we present an innovative approach to titanium-based MOFs by the use of titanocene dichloride to synthesize COK-69, the first breathing Ti MOF, which is built up from trans-1,4-cyclohexanedicarboxylate linkers and an unprecedented [Ti(IV)3(μ3-O)(O)2(COO)6] cluster. The photoactive properties of COK-69 were investigated in depth by proton-coupled electron-transfer experiments, which revealed that up to one Ti(IV) center per cluster can be photoreduced to Ti(III) while preserving the structural integrity of the framework. The electronic structure of COK-69 was determined by molecular modeling, and a band gap of 3.77 eV was found.

A new method has been developed for generating highly dispersed base sites on metal-organic framework (MOF) lattices. The base catalytic activity of two alkaline earth MOFs, M-2(BTC)(NO3)(DMF) (M = Ba or Sr, H3BTC = 1,3,5-benzenetricarboxylic acid, DMF = N,N-dimethylformamide) was studied as a function of their activation procedures. The catalytic activity in Knoevenagel condensation and Michael addition reactions was found to increase strongly with activation temperature. Physicochemical characterization using FTIR, C-13 CP MAS NMR, PXRD, XPS, TGA-MS, SEM, EPR, N-2 physisorption and nitrate content analysis shows that during activation, up to 85% of the nitrate anions are selectively removed from the structure and replaced with other charge compensating anions such as O-2(-). The defect sites generated via this activation act as new strong basic sites within the catalyst structure. A fluorescence microscopic visualization of the activity convincingly proves that it is exclusively associated with the hexagonal crystals, and that reaction proceeds inside the crystal's interior. Theoretical analysis of the Ba-material shows that the basicity of the proposed Ba2+-O2--Ba2+ motifs is close to that of the edge sites in BaO