Andrew L. Goodwin

Colossal Positive and Negative Thermal Expansion in the Framework Material Ag3[Co(CN)6]

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.

RMCProfile: reverse Monte Carlo for polycrystalline materials

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.

Correlated defect nanoregions in a metal–organic framework

Throughout much of condensed matter science, correlated disorder is key to material function. While structural and compositional defects are known to exist within a variety of metal–organic frameworks, the prevailing understanding is that these defects are only ever included in a random manner. Here we show—using a combination of diffuse scattering, electron microscopy, anomalous X-ray scattering, and pair distribution function measurements—that correlations between defects can in fact be introduced and controlled within a hafnium terephthalate metal–organic framework. The nanoscale defect structures that emerge are an analogue of correlated Schottky vacancies in rocksalt-structured transition metal monoxides and have implications for storage, transport, optical and mechanical responses. Our results suggest how the diffraction behaviour of some metal–organic frameworks might be reinterpreted, and establish a strategy of exploiting correlated nanoscale disorder as a targetable and desirable motif in metal–organic framework design.

PASCal: a principal axis strain calculator for thermal expansion and compressibility determination

This article describes a web-based tool (PASCal; principal axis strain calculator; http://pascal.chem.ox.ac.uk) designed to simplify the determination of principal coefficients of thermal expansion and compressibilities from variable-temperature and variable-pressure lattice parameter data. In a series of three case studies, PASCal is used to reanalyse previously published lattice parameter data and show that additional scientific insight is obtainable in each case. First, the two-dimensional metal–organic framework [Cu2(OH)(C8H3O7S)(H2O)]·2H2O is found to exhibit the strongest area negative thermal expansion (NTE) effect yet observed; second, the widely used explosive HMX exhibits much stronger mechanical anisotropy than had previously been anticipated, including uniaxial NTE driven by thermal changes in molecular conformation; and third, the high-pressure form of the mineral malayaite is shown to exhibit a strong negative linear compressibility effect that arises from correlated tilting of SnO6 and SiO4 coordination polyhedra.

Structure and properties of an amorphous metal-organic framework.

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.

Large negative linear compressibility of Ag3[Co(CN)6]

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 Ag3[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.

The crystallography of correlated disorder

Classical crystallography can determine structures as complicated as multi-component ribosomal assemblies with atomic resolution, but is inadequate for disordered systems—even those as simple as water ice—that occupy the complex middle ground between liquid-like randomness and crystalline periodic order. Correlated disorder nevertheless has clear crystallographic signatures that map to the type of disorder, irrespective of the underlying physical or chemical interactions and material involved. This mapping hints at a common language for disordered states that will help us to understand, control and exploit the disorder responsible for many interesting physical properties.

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.

Thermal Amorphization of Zeolitic Imidazolate Frameworks

Zeolitic imidazolate frameworks (ZIFs) are a family of metal–organic frameworks (MOFs) that display network topologies analogous to those seen in zeolites whereby the zeolitic building blocks of corner-sharing SiO4 tetrahedra are replicated by MN4 tetrahedra (M=metal) linked by imidazolate anions. Over 100 distinct ZIF phases adopting 40 network types currently exist. Interest has focused mainly on the tuneable gas sorption and separation properties of these porous materials, though their potential for catalytic activity is starting to be explored. The retention of thermal stability derived from their zeolitic structures makes them particularly attractive candidates for practical applications. Inorganic zeolites are known to undergo pressureor temperature-induced amorphization. Depending on the heating/pressurization rate, the amorphous materials thus formed can retain some aspects of crystalline topology, and consequently possess a lower configurational entropy than true glasses. Polyamorphism (the presence of structurally isomeric amorphous phases differing in density and entropy) has been identified both experimentally and theoretically in these materials. Given the comparison often drawn between between ZIFs and zeolites (ascribed to the common subtended angles of ca. 1458 at the metal-bridging species, see Figure 1a), it is not surprising that reports of pressure-induced ZIF phase transitions and amorphization exist, albeit at pressures far lower than those of their zeolitic counterparts. Recently, we reported an amorphous ZIF (a-ZIF) with a network topology comparable to that of silica glass, formed by thermal amorphization of the crystalline Zn-based ZIF-4 framework. Further heating of the a-ZIF yielded the dense ZIF-zni. The mechanical properties of the a-ZIF, studied using nanoindentation, were found to be isotropic and intermediate between ZIF-4 and ZIF-zni. Remarkably, the amorphization temperature is comparable to that of purely inorganic zeolites. Here, we show that a-ZIF and crystalline ZIF-zni can also be prepared by heating two structural isomers of ZIF-4 (see Figure 1c). Zn-based ZIF-1 and ZIF-3, possessing the BCT and DFT zeolitic topologies, undergo amorphization and recrystallization to ZIF-zni at similar temperatures to that of ZIF-4. The same process occurs in a cobalt analogue of ZIF-4 (Co-ZIF-4), yielding an amorphous MOF containing a spinactive transition-metal ion. We also show that five ZIFs incorporating substituted imidazolate bridging ions do not undergo thermal amorphization. These frameworks—ZIF-8, -9, -11, -14, and ZIF-bqtz—adopt four different network topologies and possess three different substituted imidazolate species. Solvothermal reaction of Zn(NO3)2 and imidazole (Im) under varying conditions yielded single-crystal samples of ZIF-1, ZIF-4, Co-ZIF-4, ZIF-8, ZIF-9, and ZIF-11 of typical size 0.2! 0.2! 0.1 mm [3,19] whilst a polycrystalline powder sample of ZIF-3 was prepared by a liquid-mixing method. Liquid-assisted grinding was used to synthesize polycrystalline samples of ZIF-14 and ZIF-bqtz. For ZIF-1, -3, and -4 (Co, Zn) thermogravimetric analysis shows that the structure-directing agent and solvent molecules trapped within the porous cavities of the frameworks are Figure 1. a) The similar Si-O-Si and Zn-Im-Zn linkages in zeolites and ZIFs, respectively. b) A snapshot of the continuous random network (CRN) topology of the a-ZIF gained from reverse Monte Carlo (RMC) modeling. c) Representative views of the expanded unit cells of ZIF1 (left), ZIF-3 (center), and ZIF-4 (right).

Zero thermal expansion in a flexible, stable framework : tetramethylammonium copper(I) zinc(II) cyanide.

Tetramethylammonium copper(I) zinc(II) cyanide, which consists of N(CH(3))(4)(+) ions trapped within a cristobalite-like metal cyanide framework, has been studied by variable-temperature powder and single-crystal X-ray diffraction. Its coefficient of thermal expansion is approximately zero over the temperature range 200-400 K and comparable with the best commercial zero thermal expansion materials. The atomic displacement parameters, apparent bond lengths, and structure of a low-temperature, low-symmetry phase reveal that the low-energy vibrational modes responsible for this behavior maintain approximately rigid Zn coordination tetrahedra but involve significant distortion of their Cu counterparts.