A new approach to the reverse Monte Carlo analysis of total scattering data from polycrystalline materials is presented. The essential new feature is the incorporation of an explicit analysis of the Bragg peaks using a profile refinement, taking account of the instrument resolution function. Other new features including fitting data from magnetic materials, modelling lattice site disorder and new restraint and constraint options. The new method is demonstrated by a brief review of studies carried out during its development. The new program RMCProfile represents a significant advance in the analysis of polycrystalline total scattering data, especially where the local structure is to be explored within the true constraints of the long-range average structure.

Metal-organic frameworks (MOFs) are a family of chemically diverse materials, with applications in a wide range of fields, covering engineering, physics, chemistry, biology and medicine. Until recently, research has focused almost entirely on crystalline structures, yet now a clear trend is emerging, shifting the emphasis onto disordered states, including 'defective by design' crystals, as well as amorphous phases such as glasses and gels. Here we introduce a strongly associated MOF liquid, obtained by melting a zeolitic imidazolate framework. We combine in situ variable temperature X-ray, ex situ neutron pair distribution function experiments, and first-principles molecular dynamics simulations to study the melting phenomenon and the nature of the liquid obtained. We demonstrate from structural, dynamical, and thermodynamical information that the chemical configuration, coordinative bonding, and porosity of the parent crystalline framework survive upon formation of the MOF liquid.

Crystalline solids dominate the field of metal-organic frameworks (MOFs), with access to the liquid and glass states of matter usually prohibited by relatively low temperatures of thermal decomposition. In this work, we give due consideration to framework chemistry and topology to expand the phenomenon of the melting of 3D MOFs, linking crystal chemistry to framework melting temperature and kinetic fragility of the glass-forming liquids. Here we show that melting temperatures can be lowered by altering the chemistry of the crystalline MOF state, which provides a route to facilitate the melting of other MOFs. The glasses formed upon vitrification are chemically and structurally distinct from the three other existing categories of melt-quenched glasses (inorganic nonmetallic, organic, and metallic), and retain the basic metal-ligand connectivity of crystalline MOFs, which connects their mechanical properties to their starting chemical composition. The transfer of functionality from crystal to glass points toward new routes to tunable, functional hybrid glasses.

Total scattering, an increasingly important crystallographic research area, is defined theoretically in terms of correlation functions. Different researchers use different definitions for these functions, frequently leading to confusion in the literature. Here, a consistent set of equations for total‐scattering correlation functions are developed and explicitly compared with other, often encountered, definitions. It is hoped that this will lead to increased transparency for newcomers to the field of total scattering.

We show that silver(I) hexacyanocobaltate(III), Ag3[Co(CN)6], exhibits positive and negative thermal expansion an order of magnitude greater than that seen in other crystalline materials. This framework material expands along one set of directions at a rate comparable to the most weakly bound solids known. By flexing like lattice fencing, the framework couples this to a contraction along a perpendicular direction. This gives negative thermal expansion that is 14 times larger than in ZrW2O8. Density functional theory calculations quantify both the low energy associated with this flexibility and the role of argentophilic (Ag+...Ag+) interactions. This study illustrates how the mechanical properties of a van der Waals solid might be engineered into a rigid, useable framework.

No abstract

ZIF-4, a metal-organic framework (MOF) with a zeolitic structure, undergoes a crystal-amorphous transition on heating to 300 degrees C. The amorphous form, which we term a-ZIF, is recoverable to ambient conditions or may be converted to a dense crystalline phase of the same composition by heating to 400 degrees C. Neutron and x-ray total scattering data collected during the amorphization process are used as a basis for reverse Monte Carlo refinement of an atomistic model of the structure of a-ZIF. The structure is best understood in terms of a continuous random network analogous to that of a-SiO2. Optical microscopy, electron diffraction and nanoindentation measurements reveal a-ZIF to be an isotropic glasslike phase capable of plastic flow on its formation. Our results suggest an avenue for designing broad new families of amorphous and glasslike materials that exploit the chemical and structural diversity of MOFs.

Silver(I) hexacyanocobaltate(III), Ag3[Co(CN)6], shows a large negative linear compressibility (NLC, linear expansion under hydrostatic pressure) at ambient temperature at all pressures up to our experimental limit of 7.65(2) GPa. This behavior is qualitatively unaffected by a transition at 0.19 GPa to a new phase Ag 3[Co(CN)6]-II, whose structure is reported here. The high-pressure phase also shows anisotropic thermal expansion with large uniaxial negative thermal expansion (NTE, expansion on cooling). In both phases, the NLC/NTE effect arises as the rapid compression/contraction of layers of silver atoms-weakly bound via argentophilic interactions-is translated via flexing of the covalent network lattice into an expansion along a perpendicular direction. It is proposed that framework materials that contract along a specific direction on heating while expanding macroscopically will, in general, also expand along the same direction under hydrostatic pressure while contracting macroscopically.negative linear compression ͉ negative thermal expansion ͉ high-pressure ͉ framework materials ͉ flexibility N egative linear compressibility (NLC), whereby a material expands along a specific direction on increasing hydrostatic pressure, is a very unusual effect. Indeed in a study of elastic constant data from 500 noncubic crystal phases, only 13 displayed NLC, and of those, 11 structures were of monoclinic or lower symmetry (1). Despite its rarity, NLC is a highly attractive mechanical property, with a key application being the development of effectively incompressible optical materials (1, 2). Such materials could be used in high-pressure environments, such as in optical telecommunications devices that must function at deep-sea pressures Ͼ1,000 atm. NLC also offers a means of producing ultrasensitive pressure detectors, such as interferometric optical sensors for sonar and aircraft altitude measurements. The effect is also often coupled to so-called ''auxetic'' behavior, which is itself being used to improve shock resistance in, e.g., body armor (3).Of the few known examples among inorganic materials, the most pronounced NLC effects have been reported for LaNbO 4 (4), elemental Se (5), the BXO 4 (X ϭ P, As) family (6), and the spin-Peierls compound ␣Ј-NaV 2 O 5 (7). In some other cases, transient NLC behavior may occur only at pressures just above a strain-induced phase transition to a structure of lower symmetry (e.g., refs. 8 and 9), or as a result of an uptake of additional interstitial molecules (10, 11).One fundamental barrier to the practical application of NLC is that its magnitude is generally much smaller than the ''normal'' (positive) compressibilities of standard materials.* By convention, linear compressibility is defined as the relative rate at which a given dimension ᐉ decreases with pressure (at constant temperature): K ᐉ ϭ Ϫ(Ѩlnᐉ/Ѩp) T . Typical values for crystalline materials lie in the range K ᐉ Ӎ 5-20 TPa Ϫ1 (12) (i.e., their linear dimensions decrease by Ϸ1% for each GPa increase in pressure). In cont...