Control of Multipolar and Orbital Order in Perovskite-like [C(NH2)(3)]CuxCd1-x(HCOO)(3) Metal-Organic Frameworks

We study the compositional dependence of molecular orientation (multipolar) and orbital (quadrupolar) order in the family of perovskite-like metal–organic frameworks [C(NH2)3]CuxCd1–x(HCOO)3. On increasing the fraction x of Jahn-Teller-active Cu2+, we observe first an orbital disorder/order transition and then a multipolar reorientation transition, each occurring at distinct critical compositions xo = 0.45(5) and xm = 0.55(5). We attribute these transitions to a combination of size, charge distribution, and percolation effects. The transitions we observe establish the accessibility in for-mate perovskites of novel structural degrees of freedom beyond the familiar dipolar terms responsible for (an-ti)ferroelectric order. We discuss the symmetry implica-tions of cooperative quadrupolar and multipolar states for the design of relaxor-like hybrid perovskites

Intrinsic flexibility of the EMT zeolite framework under pressure

The roles of organic additives in the assembly and crystallisation of zeolites are still not fully understood. This is important when attempting to prepare novel frameworks to produce new zeolites. We consider 18-crown-6 ether (18C6) as an additive, which has previously been shown to differentiate between the zeolite EMC-2 (EMT) and faujasite (FAU) frameworks. However, it is unclear whether this distinction is dictated by influences on the metastable free-energy landscape or geometric templating. Using high-pressure synchrotron X-ray diffraction, we have observed that the presence of 18C6 does not impact the EMT framework flexibility—agreeing with our previous geometric simulations and suggesting that 18C6 does not behave as a geometric template. This was further studied by computational modelling using solid-state density-functional theory and lattice dynamics calculations. It is shown that the lattice energy of FAU is lower than EMT, but is strongly impacted by the presence of solvent/guest molecules in the framework. Furthermore, the EMT topology possesses a greater vibrational entropy and is stabilised by free energy at a finite temperature. Overall, these findings demonstrate that the role of the 18C6 additive is to influence the free energy of crystallisation to assemble the EMT framework as opposed to FAU

Ellipsoidal analysis of coordination polyhedra

The idea of the coordination polyhedron is essential to understanding chemical structure. Simple polyhedra in crystalline compounds are often deformed due to structural complexity or electronic instabilities so distortion analysis methods are useful. Here we demonstrate that analysis of the minimum bounding ellipsoid of a coordination polyhedron provides a general method for studying distortion, yielding parameters that are sensitive to various orders in metal oxide examples. Ellipsoidal analysis leads to discovery of a general switching of polyhedral distortions at symmetry-disallowed transitions in perovskites that may evidence underlying coordination bistability, and reveals a weak off-centre ‘d(5) effect' for Fe(3+) ions that could be exploited in multiferroics. Separating electronic distortions from intrinsic deformations within the low temperature superstructure of magnetite provides new insights into the charge and trimeron orders. Ellipsoidal analysis can be useful for exploring local structure in many materials such as coordination complexes and frameworks, organometallics and organic molecules

Metal–organic frameworks under pressure

Metal–organic frameworks (MOFs) are a broad and interesting class of materials known for their mechanical flexibility. As such, their response to pressure is usually extreme and often counterintuitive. This tutorial review surveys the structural response of MOFs to pressure as observed experimentally. It describes the experimental tools exploited in high-pressure crystallographic measurements and highlights some of the experiment design choices that influence the actual physics probed in these measurements. The main focus of the review is a description of the key pressure-driven structural responses exhibited by MOFs: isosymmetric compression, including negative compressibility; symmetry-lowering transitions; changes in connectivity; amorphization; and inclusion of the pressure-transmitting medium within the MOF pores. The review concludes both by highlighting some functional implications of these responses and by flagging some future directions for the field

Geometric switching of linear to area negative thermal expansion in uniaxial metal-organic frameworks

Using variable-temperature neutron powder diffraction measurements, we show that the two quartz-like metal-organic frameworks (MOFs) deuterium indium(iii) terephthalate and zinc(ii) isonicotinate exhibit anisotropic positive and negative thermal expansion (PTE/NTE) behaviour. Whereas in the former the NTE response is uniaxial - occurring along the hexagonal crystal axis - this behaviour is inverted in the latter such that PTE occurs along the hexagonal axis and NTE is found in the entire plane of perpendicular directions. We show that this inversion of mechanical response can be explained on geometric grounds alone; specifically, we identify a critical framework geometry that demarcates a switch from linear to area NTE response. Extending this analysis to other common MOF topologies, we establish a generic predictive approach for establishing the dimensionality of NTE (or, by extension, negative compressibility) responses in a large range of different framework systems. Our analysis suggests that framework geometry plays a crucial role in determining the mechanical response of framework materials which show anisotropic responses via hinging. © 2014 the Partner Organisations

Static disorder and local structure in zinc(II) isonicotinate, a quartzlike metal-organic framework

Using a combination of Rietveld and reverse Monte Carlo (RMC) refinement of neutron total scattering data, we find that the 10 K structure of zinc(II) isonicotinate shows strong evidence of static disorder. This disorder takes the form of transverse displacements of the isonicotinate ligand and results in elongated atomic displacement parameters and dampening of the experimental G(r). We analyse the RMC configurations using an approach derived from geometric algebra. Complications regarding the inclusion of hydrogenous guest molecules within the pore structure are discussed. This study highlights the way in which structural flexibility can give rise to multiple lowenergy ground states in metal-organic framework materials. © by Oldenbourg Wissenschaftsverlag, München

Supramolecular mechanics in a metal-organic framework

A combination of variable-temperature and variable-pressure single-crystal and powder X-ray diffraction is used to study the thermo- and piezo-mechanical properties of the metal-organic framework (MOF) silver(i) 2-methylimidazolate, Ag(mim). We find the material to exhibit a number of anomalous mechanical properties: negative thermal expansion, colossal positive thermal expansion and the most extreme negative linear compressibility yet observed for a MOF. By considering the mechanical response of individual supramolecular motifs we are able to rationalise the varied and unconventional behaviour of the bulk material. A general inverse correspondence between strength of supramolecular interaction and magnitude of mechanical response is identified. We propose that the consideration of MOF structures in terms of their underlying mechanical building units provides a straightforward qualitative method of directing framework design in order to maximise anomalous mechanical response. © 2012 The Royal Society of Chemistry

Probing the influence of defects, hydration and composition on Prussian blue analogues with pressure

The vast compositional space of Prussian blue analogues (PBAs), formula AxM[M′(CN)6]y·nH2O, allows for a diverse range of functionality. Yet, the interplay between composition and physical properties—e.g., flexibility and propensity for phase transitions—is still largely unknown, despite its fundamental and industrial relevance. Here we use variable-pressure X-ray and neutron diffraction to explore how key structural features, i.e., defects, hydration, and composition, influence the compressibility and phase behavior of PBAs. Defects enhance the flexibility, manifesting as a remarkably low bulk modulus (B0 ≈ 6 GPa) for defective PBAs. Interstitial water increases B0 and enables a pressure-induced phase transition in defective systems. Conversely, hydration does not alter the compressibility of stoichiometric MnPt(CN)6, but changes the high-pressure phase transitions, suggesting an interplay between low-energy distortions. AMnCo(CN)6 (AI = Rb, Cs) transition from F4̅3m to P4̅n2 upon compression due to octahedral tilting, and the critical pressure can be tuned by the A-site cation. At 1 GPa, the symmetry of Rb0.87Mn[Co(CN)6]0.91 is further lowered to the polar space group Pn by an improper ferroelectric mechanism. These fundamental insights aim to facilitate the rational design of PBAs for applications within a wide range of fields