Publications
37 results found
Hitchings TJ, Scatena R, Allan DR, et al., 2024, Negative linear compressibility exhibited by the hybrid perovskite [(NH2)3C]Er(HCO2)2(C2O4)., Chem Commun (Camb), Vol: 60, Pages: 3271-3274
Extended framework materials with specific topologies can exhibit unusual mechanical behaviour, such as expanding in one direction under hydrostatic (uniform) pressure, known as negative linear compressibility (NLC). Here, two hybrid perovskite frameworks with winerack structures, a known NLC topology, are investigated under pressure. [C(NH2)3]Er(HCO2)2(C2O4) exhibits NLC from ambient pressure to 2.63(10) GPa and is the first reported NLC hybrid perovskite from ambient pressure. However, isostructural [(CH3)2NH2]Er(HCO2)2(C2O4) instead compresses relatively moderately along all axes before it undergoes a phase transition above 0.37(10) GPa. The differences in the mechanical properties can be interpreted from differences in host-guest interactions within these frameworks, primarily their hydrogen bond networks.
Boström HLB, Cairns AB, Chen M, et al., 2024, The pressure response of Jahn-Teller-distorted Prussian blue analogues, Chemical Science, Vol: 15, Pages: 3155-3164, ISSN: 2041-6520
Jahn-Teller (JT) distorted CuII-containing compounds often display interesting structural and functional behaviour upon compression. We use high-pressure X-ray and neutron diffraction to investigate four JT-distorted Prussian blue analogues: Cu[Co(CN)6]0.67, CuPt(CN)6, and ACuCo(CN)6 (A = Rb, Cs), where the first two were studied in both their hydrated and dehydrated forms. All compounds are less compressible than the JT-inactive MnII-based counterparts, indicating a coupling between the electronic and mechanical properties. The effect is particularly strong for Cu[Co(CN)6]0.67, where the local JT distortions are uncorrelated (so-called orbital disorder). This sample amorphises at 0.5 GPa when dehydrated. CuPt(CN)6 behaves similarly to the MnII-analogues, with phase transitions at around 1 GPa and low sensitivity to water. For ACuCo(CN)6, the JT distortions reduce the propensity for phase transitions, although RbCuCo(CN)6 transitions to a new phase (P2/m) around 3 GPa. Our results have a bearing on both the topical Prussian blue analogues and the wider field of flexible frameworks.
Geers M, Jarvis DM, Liu C, et al., 2023, High-pressure behavior of the magnetic van der Waals molecular framework Ni(NCS) 2, Physical Review B, Vol: 108, ISSN: 2469-9950
Two-dimensional materials offer a unique range of magnetic, electronic, and mechanical properties which can be controlled by external stimuli. Pressure is a particularly important stimulus, as it can be achieved readily and can produce large responses, especially in low-dimensional materials. In this paper, we explore the pressure dependence of the structural and magnetic properties of a two-dimensional van der Waals (vdW) molecular framework antiferromagnet with ferromagnetic layers, Ni(NCS)2, up to 8.4 kbar. Through a combination of x-ray and neutron diffraction analysis, we find that Ni(NCS)2 is significantly more compressible than comparable vdW metal halides, and its response is anisotropic not only out of the plane, but also within the layers. Using bulk magnetization and neutron diffraction data, we show that the ambient layered antiferromagnetic phase is maintained up to the largest investigated pressure, but with an enhanced Néel temperature, TN (ΔTN/TN=+19%), and a large pressure sensitivity (Q=1TNdTNdP=+2.3%kbar-1), one of the larger values of magnetic pressure responsiveness for a vdW material. Density functional theory calculations suggest that this is due to increasing three dimensionality. These results provide insights into the pressure response of molecular framework vdW magnets and suggest that the investigation of other molecular framework vdW magnets might uncover contenders for future pressure-switchable devices.
Fernando NK, Boström HLB, Murray CA, et al., 2022, Variability in X-ray induced effects in [Rh(COD)Cl]2 with changing experimental parameters., Phys Chem Chem Phys, Vol: 24, Pages: 28444-28456
X-ray characterisation methods have undoubtedly enabled cutting-edge advances in all aspects of materials research. Despite the enormous breadth of information that can be extracted from these techniques, the challenge of radiation-induced sample change and damage remains prevalent. This is largely due to the emergence of modern, high-intensity X-ray source technologies and the growing potential to carry out more complex, longer duration in situ or in operando studies. The tunability of synchrotron beamlines enables the routine application of photon energy-dependent experiments. This work explores the structural stability of [Rh(COD)Cl]2, a widely used catalyst and precursor in the chemical industry, across a range of beamline parameters that target X-ray energies of 8 keV, 15 keV, 18 keV and 25 keV, on a powder X-ray diffraction synchrotron beamline at room temperature. Structural changes are discussed with respect to absorbed X-ray dose at each experimental setting associated with the respective photon energy. In addition, the X-ray radiation hardness of the catalyst is discussed, by utilising the diffraction data collected at the different energies to determine a dose limit, which is often considered in protein crystallography and typically overlooked in small molecule crystallography. This work not only gives fundamental insight into how damage manifests in this organometallic catalyst, but will encourage careful consideration of experimental X-ray parameters before conducting diffraction on similar radiation-sensitive organometallic materials.
Boström HLB, Cairns AB, Chen M, et al., 2022, Radiation effects, zero thermal expansion, and pressure-induced phase transition in CsMnCo(CN)6., Phys Chem Chem Phys, Vol: 24, Pages: 25072-25076
The Prussian blue analogue CsMnCo(CN)6 is studied using powder X-ray and neutron diffraction under variable temperature, pressure, and X-ray exposure. It retains cubic F4̄3m symmetry in the range 85-500 K with minimal thermal expansion, whereas a phase transition to P4̄n2 occurs at ∼2 GPa, driven by octahedral tilting. A small lattice contraction occurs upon increased X-ray dose. Comparisons with related systems indicate that the CsI ions decrease the thermal expansion and suppress the likelihood of phase transformations. The results improve the understanding of the stimuli-responsive behaviour of coordination polymers.
Paddison JAM, Ehlers G, Cairns AB, et al., 2021, Suppressed-moment 2-k order in the canonical frustrated antiferromagnet Gd2Ti2O7, npj Quantum Materials, Vol: 6
<jats:title>Abstract</jats:title><jats:p>In partially ordered magnets, order and disorder coexist in the same magnetic phase, distinct from both spin liquids and spin solids. Here, we determine the nature of partial magnetic ordering in the canonical frustrated antiferromagnet Gd<jats:sub>2</jats:sub>Ti<jats:sub>2</jats:sub>O<jats:sub>7</jats:sub>, in which Gd<jats:sup>3+</jats:sup> spins occupy a pyrochlore lattice. Using single-crystal neutron-diffraction measurements in applied magnetic field, magnetic symmetry analysis, inelastic neutron-scattering measurements, and spin-wave modeling, we show that its low-temperature magnetic structure involves two propagation vectors (2-<jats:bold>k</jats:bold> structure) with suppressed ordered magnetic moments and enhanced spin-wave fluctuations. Our experimental results are consistent with theoretical predictions of thermal fluctuation-driven order in Gd<jats:sub>2</jats:sub>Ti<jats:sub>2</jats:sub>O<jats:sub>7</jats:sub>, and reveal that inelastic neutron-scattering measurements on powder samples can solve the longstanding problem of distinguishing single-<jats:bold>k</jats:bold> and multi-<jats:bold>k</jats:bold> magnetic structures.</jats:p>
Fernando NK, Cairns AB, Murray CA, et al., 2021, Structural and Electronic Effects of X-ray Irradiation on Prototypical [M(COD)Cl]2 Catalysts., J Phys Chem A, Vol: 125, Pages: 7473-7488
X-ray characterization techniques are invaluable for probing material characteristics and properties, and have been instrumental in discoveries across materials research. However, there is a current lack of understanding of how X-ray-induced effects manifest in small molecular crystals. This is of particular concern as new X-ray sources with ever-increasing brilliance are developed. In this paper, systematic studies of X-ray-matter interactions are reported on two industrially important catalysts, [Ir(COD)Cl]2 and [Rh(COD)Cl]2, exposed to radiation in X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) experiments. From these complementary techniques, changes to structure, chemical environments, and electronic structure are observed as a function of X-ray exposure, allowing comparisons of stability to be made between the two catalysts. Radiation dose is estimated using recent developments to the RADDOSE-3D software for small molecules and applied to powder XRD and XPS experiments. Further insights into the electronic structure of the catalysts and changes occurring as a result of the irradiation are drawn from density functional theory (DFT). The techniques combined here offer much needed insight into the X-ray-induced effects in transition-metal catalysts and, consequently, their intrinsic stabilities. There is enormous potential to extend the application of these methods to other small molecular systems of scientific or industrial relevance.
Boström HLB, Collings IE, Daisenberger D, et al., 2021, Probing the influence of defects, hydration and composition on Prussian blue analogues with pressure, Journal of the American Chemical Society, Vol: 143, Pages: 3544-3554, ISSN: 0002-7863
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.
Boström HLB, Cairns AB, Liu L, et al., 2020, Spin crossover in the Prussian blue analogue FePt(CN)6 induced by pressure or X-ray irradiation., Dalton Transactions: an international journal of inorganic chemistry, Vol: 49, Pages: 12940-12944, ISSN: 1477-9226
The spin state of the Prussian blue analogue FeIIPtIV(CN)6 is investigated in response to temperature, pressure, and X-ray irradiation. While cooling to 10 K maintains the high-spin state of FeII, compression at ambient temperature induces a first-order spin-crossover (SCO) transition with a small hysteresis loop (p↑ = 0.8 GPa, p↓ = 0.6 GPa). In addition, the high-spin to low-spin transition can be initiated at lower pressure through increased X-ray irradiation. Our study highlights a cooperative SCO with moderate pressure in a porous Prussian blue analogue.
Cairns AB, Catafesta J, Hermet P, et al., 2020, Effect of extra-framework cations on negative linear compressibility and high pressure phase transitions: a study of KCd[Ag(CN)2]3, The Journal of Physical Chemistry C, Vol: 124, Pages: 6896-6906, ISSN: 1932-7447
The negative thermal expansion material potassium cadmium dicyanoargentate, KCd[Ag(CN)2]3, is studied at high pressure using a combination of X-ray single crystal diffraction, X-ray powder diffraction, infrared and Raman spectroscopy, and density functional theory calculations. In common with the isostructural manganese analogue, KMn[Ag(CN)2]3, this material is shown to exhibit very strong negative linear compressibility (NLC) in the crystallographic c direction due to structure hinging. We find increased structural flexibility results in enhanced NLC and NTE properties, but this also leads to two pressure-induced phase transitions—to very large unit cells involving octahedral tilting and shearing of the structure—below 2 GPa. The presence of potassium cations has an important effect on the mechanical and thermodynamic properties of this family, whilst the chemical versatility demonstrated here is of considerable interest to tune unusual mechanical properties for application.
Nearchou A, Cornelius M-LU, Skelton JM, et al., 2019, Intrinsic flexibility of the EMT zeolite framework under pressure, Materials, Vol: 24, ISSN: 1420-3049
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.
Boström HLB, Collings IE, Cairns AB, et al., 2019, High-pressure behaviour of Prussian blue analogues: interplay of hydration, Jahn-Teller distortions and vacancies, Dalton Transactions, Vol: 48, Pages: 1647-1655, ISSN: 1477-9234
We report a high-pressure crystallographic study of four hydrated Prussian blue analogues: M[Pt(CN)6] and M[Co(CN)6]0.67 (M = Mn2+, Cu2+) in the range 0-3 GPa. Mn[Co(CN)6]0.67 was studied by single-crystal X-ray diffraction, whereas the other systems were only available in polycrystalline form. The Mn-containing compounds undergo pressure-induced phase transitions from Fm3[combining macron]m to R3[combining macron] at ∼1.0-1.5 GPa driven by cooperative tilting of the octahedral units. No phase transition was found for the orbitally disordered Cu[Co(CN)6]0.67 up to 3 GPa. Mn[Co(CN)6]0.67 is significantly softer than the other samples, with a bulk modulus of ∼14 GPa compared to ∼35 GPa of the powdered samples. The discrepant pressure responses are discussed in terms of the presence of structural defects, Jahn-Teller distortions, and hydration. The implications for the development of polar systems are reviewed based upon our high-pressure study.
Scelta D, Baldassarre A, Serrano-Ruiz M, et al., 2018, The p-sc structure in phosphorus: bringing order to the high pressure phases of group 15 elements, Chemical Communications, Vol: 54, Pages: 10554-10557, ISSN: 1359-7345
Black phosphorus was studied by state-of-the-art synchrotron X-ray diffraction in a Diamond Anvil Cell during room temperature compression in the presence of He, H2, N2 and Daphne Oil 7474. The data demonstrate that the existence of the pseudo simple-cubic (p-sc) structure above 10.5 GPa is an intrinsic feature of P independent from the pressure transmitting medium. In the case of He, the pressure evolution of the lattice parameters and unit cell volume of P across the A17, A7 and p-sc structures was obtained and the corresponding EOS derived, providing a deeper insight on the recently reported p-sc structure. The results presented in this letter highlight the key role of the s–p orbital mixing in the formation and stabilization of the p-sc structure up to ∼30 GPa, solving apparent contradictions emerging from previous literature and finally bringing order to the sequence of the high pressure A7 layered structure in group 15 elements.
Nobrega MM, Teixeira-Neto E, Cairns AB, et al., 2018, One-dimensional diamondoid polyaniline-like nanothreads from compressed crystal aniline, Chemical Science, Vol: 9, Pages: 254-260, ISSN: 2041-6520
Low-dimensional nanomaterials such as highly ordered polyaniline (PANI) have attracted considerable interest due to their expected extraordinary electronic and optoelectronic properties. In spite of several attempts, the attainment of atomically well-ordered PANI is a long-standing challenge. Pressure-induced polymerization of aromatic molecules in the crystal phase has been demonstrated as a practicable route for the synthesis of highly ordered polymers but this approach has never been tested to produce PANI. Here we show the synthesis of diamondoid polyaniline-like nanothreads at 33 GPa and 550 K by compressing aniline in crystal phase-II. Infrared spectroscopy, transmission electron microscopy, X-ray diffraction data, and DFT calculations support the formation of this totally new polyaniline-like nanothread. The NH2-enriched carbon nanothread combines the outstanding mechanical properties of carbon nanotubes with the versatility of NH2 groups decorating the exterior of the nanothreads representing potential active sites for doping and as linkers for molecules with biological interest and inorganic nanostructures. The synergy of all of these properties emphasizes the strong potential of this material to be applied in a broad range of areas, from chemistry to materials engineering.
Gupta MK, Singh B, Mittal R, et al., 2017, Anomalous thermal expansion, negative linear compressibility, and high-pressure phase transition in ZnAu2(CN)4: Neutron inelastic scattering and lattice dynamics studies, Physical Review B: Condensed Matter and Materials Physics, Vol: 96, ISSN: 1098-0121
We present temperature-dependent inelastic-neutron-scattering measurements, accompanied by ab initio calculations of the phonon spectra and elastic properties as a function of pressure to quantitatively explain an unusual combination of negative thermal expansion and negative linear compressibility behavior of ZnAu2(CN)4. The mechanism of the negative thermal expansion is identified in terms of specific anharmonic phonon modes that involve bending of the -Zn-NC-Au-CN-Zn- linkage. The soft phonon at the L point at the Brillouin zone boundary quantitatively relates to the high-pressure phase transition at about 2 GPa. The ambient pressure structure is also found to be close to an elastic instability that leads to a weakly first-order transition.
Scelta D, Baldassarre A, Serrano-Ruiz M, et al., 2017, Interlayer bond formation in black phosphorus at high pressure, Angewandte Chemie International Edition, Vol: 56, Pages: 14135-14140, ISSN: 1521-3757
Black phosphorus was compressed at room temperature across the A17, A7 and simple-cubic phases up to 30 GPa, using a diamond anvil cell and He as pressure transmitting medium. Synchrotron X-ray diffraction showed the persistence of two previously unreported peaks related to the A7 structure in the pressure range of the simple-cubic phase. The Rietveld refinement of the data demonstrates the occurrence of a two-step mechanism for the A7 to simple-cubic phase transition, indicating the existence of an intermediate pseudo simple-cubic structure. From a chemical point of view this study represents a deep insight on the mechanism of interlayer bond formation during the transformation from the layered A7 to the non-layered simple-cubic phase of phosphorus, opening new perspectives for the design, synthesis and stabilization of phosphorene-based systems. As superconductivity is concerned, a new experimental evidence to explain the anomalous pressure behavior of Tc in phosphorus below 30 GPa is provided.
Scelta D, Baldassarre A, Serrano-Ruiz M, et al., 2017, Interlayer bond formation in black phosphorus at high pressure, Angewardte chemie, Vol: 129, Pages: 14323-14328, ISSN: 0044-8249
Black phosphorus was compressed at room temperature across the A17, A7 and simple‐cubic phases up to 30 GPa, using a diamond anvil cell and He as pressure transmitting medium. Synchrotron X‐ray diffraction showed the persistence of two previously unreported peaks related to the A7 structure in the pressure range of the simple‐cubic phase. The Rietveld refinement of the data demonstrates the occurrence of a two‐step mechanism for the A7 to simple‐cubic phase transition, indicating the existence of an intermediate pseudo simple‐cubic structure. From a chemical point of view this study represents a deep insight on the mechanism of interlayer bond formation during the transformation from the layered A7 to the non‐layered simple‐cubic phase of phosphorus, opening new perspectives for the design, synthesis and stabilization of phosphorene‐based systems. As superconductivity is concerned, a new experimental evidence to explain the anomalous pressure behavior of Tc in phosphorus below 30 GPa is provided.
Dziubek K, Citroni M, Fanetti S, et al., 2017, Synthesis of high-quality crystalline carbon nitride oxide by selectively driving the high-temperature instability of urea with pressure, The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 121, Pages: 19872-19879, ISSN: 1932-7447
One-step synthesis using only physical tools is an appealing “green” method for the realization of technological materials. High-pressure conditions are particularly suitable in the attainment of extended supramolecular networks and in combination with high-temperature are effective in selecting specific reaction pathways to increase product quality. Here we show how pressures below 3 GPa and temperatures on the order of 420 K are effective for the synthesis, from urea crystal phase IV, of 2D graphitic carbon nitride oxide with enhanced crystalline quality. Angle-dispersive X-ray diffraction and Fourier transform IR spectroscopy show that the reaction becomes less selective at higher pressure with the formation of melamine and ammonium carbamate as byproducts. An anomalous positive slope is shown by the P–T instability boundary, likely arising from unfavorable hydrogen bonding with respect to the topochemical path, making urea an interesting case study for gaining insight into the role of hydrogen bonding in solid-state reactivity.
Dziubek K, Citroni M, Fanetti S, et al., 2017, High-Pressure High-Temperature Structural Properties of Urea, Journal of Physical Chemistry C, Vol: 121, Pages: 2380-2387, ISSN: 1932-7447
Angle-dispersive X-ray diffraction and Fourier transform infrared spectroscopy have been employed to study the phase diagram of urea crystal beyond 15 GPa and at temperatures in excess of 400 K. Previously reported Bridgman phase II was structurally characterized for the first time, and it is discovered that it coincides with room-temperature phase IV. Large metastability P–T regions were identified for all phases in the sequence I–III–IV–V, ascribed to the difficulty to disrupt the H-bonding network, a prerequisite to accomplish the molecular rearrangement necessary for the structural transformation. High-temperature studies and use of a hydrostatic compression medium allows the thermodynamic boundaries of phase III, and partly of phase IV, to be identified therefore making a considerable step forward in the knowledge of the phase diagram of urea.
Cairns AB, Cliffe MJ, Paddison JAM, et al., 2016, Encoding complexity within supramolecular analogues of frustrated magnets, Nature Chemistry, Vol: 8, Pages: 442-447, ISSN: 1755-4330
The solid phases of gold(I) and/or silver(I) cyanides are supramolecular assemblies of inorganic polymer chains in which the key structural degrees of freedom—namely, the relative vertical shifts of neighbouring chains—are mathematically equivalent to the phase angles of rotating planar (‘XY’) spins. Here, we show how the supramolecular interactions between chains can be tuned to mimic different magnetic interactions. In this way, the structures of gold(I) and/or silver(I) cyanides reflect the phase behaviour of triangular XY magnets. Complex magnetic states predicted for this family of magnets—including collective spin-vortices of relevance to data storage applications—are realized in the structural chemistry of these cyanide polymers. Our results demonstrate how chemically simple inorganic materials can behave as structural analogues of otherwise inaccessible ‘toy’ spin models and also how the theoretical understanding of those models allows control over collective (‘emergent’) phenomena in supramolecular systems.
Overy AR, Cairns AB, Cliffe MJ, et al., 2016, Design of crystal-like aperiodic solids with selective disorder–phonon coupling, Nature Communications, Vol: 7
<jats:title>Abstract</jats:title><jats:p>Functional materials design normally focuses on structurally ordered systems because disorder is considered detrimental to many functional properties. Here we challenge this paradigm by showing that particular types of strongly correlated disorder can give rise to useful characteristics that are inaccessible to ordered states. A judicious combination of low-symmetry building unit and high-symmetry topological template leads to aperiodic ‘procrystalline’ solids that harbour this type of disorder. We identify key classes of procrystalline states together with their characteristic diffraction behaviour, and establish mappings onto known and target materials. The strongly correlated disorder found in these systems is associated with specific sets of modulation periodicities distributed throughout the Brillouin zone. Lattice dynamical calculations reveal selective disorder-driven phonon broadening that resembles the poorly understood ‘waterfall’ effect observed in relaxor ferroelectrics. This property of procrystalline solids suggests a mechanism by which strongly correlated topological disorder might allow independently optimized thermal and electronic transport behaviour, such as required for high-performance thermoelectrics.</jats:p>
Adamson J, Lucas TC, Cairns AB, et al., 2015, Competing hydrostatic compression mechanisms in nickel cyanide, Physica B: Condensed Matter, Vol: 479, Pages: 35-40, ISSN: 0921-4526
We use variable-pressure neutron and X-ray diffraction measurements to determine the uniaxial and bulk compressibilities of nickel(II) cyanide, Ni(CN)2. Whereas other layered molecular framework materials are known to exhibit negative area compressibility, we find that Ni(CN)2 does not. We attribute this difference to the existence of low-energy in-plane tilt modes that provide a pressure-activated mechanism for layer contraction. The experimental bulk modulus we measure is about four times lower than that reported elsewhere on the basis of density functional theory methods [Phys. Rev. B 83 (2011) 024301].
Baxter EF, Bennett TD, Cairns AB, et al., 2015, A comparison of the amorphization of zeolitic imidazolate frameworks (ZIFs) and aluminosilicate zeolites by ball-milling, Dalton Transactions, Vol: 45, Pages: 4258-4268, ISSN: 1477-9234
X-ray diffraction has been used to investigate the kinetics of amorphization through ball-milling at 20 Hz, for five zeolitic imidazolate frameworks (ZIFs) – ZIF-8, ZIF-4, ZIF-zni, BIF-1-Li and CdIF-1. We find that the rates of amorphization for the zinc-containing ZIFs increase with increasing solvent accessible volume (SAV) in the sequence ZIF-8 > ZIF-4 > ZIF-zni. The Li–B analogue of the dense ZIF-zni amorphizes more slowly than the corresponding zinc phase, with the behaviour showing a correlation with their relative bulk moduli and SAVs. The cadmium analogue of ZIF-8 (CdIF-1) amorphizes more rapidly than the zinc counterpart, which we ascribe primarily to its relatively weak M–N bonds as well as the higher SAV. The results for the ZIFs are compared to three classical zeolites – Na-X, Na-Y and ZSM-5 – with these taking up to four times longer to amorphize. The presence of adsorbed solvent in the pores is found to render both ZIF and zeolite frameworks more resistant to amorphization. X-ray total scattering measurements show that amorphous ZIF-zni is structurally indistinguishable from amorphous ZIF-4 with both structures retaining the same short-range order that is present in their crystalline precursors. By contrast, both X-ray total scattering measurements and 113Cd NMR measurements point to changes in the local environment of amorphous CdIF-1 compared with its crystalline CdIF-1 precursor.
Collings IE, Hill JA, Cairns AB, et al., 2015, Compositional dependence of anomalous thermal expansion in perovskite-like ABX(3) formates, Dalton Transactions, Vol: 45, Pages: 4169-4178, ISSN: 1477-9234
The compositional dependence of thermal expansion behaviour in 19 different perovskite-like metal–organic frameworks (MOFs) of composition [AI][MII(HCOO)3] (A = alkylammonium cation; M = octahedrally-coordinated divalent metal) is studied using variable-temperature X-ray powder diffraction measurements. While all systems show essentially the same type of thermomechanical response—irrespective of their particular structural details—the magnitude of this response is shown to be a function of AI and MII cation radii, as well as the molecular anisotropy of AI. Flexibility is maximised for large MII and small AI, while the shape of AI has implications for the direction of framework hingeing.
Cairns AB, Goodwin AL, 2015, Negative linear compressibility, Physical Chemistry Chemical Physics, Vol: 17, Pages: 20449-20465, ISSN: 1463-9076
While all materials reduce their intrinsic volume under hydrostatic (uniform) compression, a select few actually expand along one or more directions during this process of densification. As rare as it is counterintuitive, such “negative compressibility” behaviour has application in the design of pressure sensors, artificial muscles and actuators. The recent discovery of surprisingly strong and persistent negative compressibility effects in a variety of new families of materials has ignited the field. Here we review the phenomenology of negative compressibility in this context of materials diversity, placing particular emphasis on the common structural motifs that recur amongst known examples. Our goal is to present a mechanistic understanding of negative compressibility that will help inform a clear strategy for future materials design.
Tominaka S, Hamoudi H, Suga T, et al., 2015, Topochemical conversion of a dense metal-organic framework from a crystalline insulator to an amorphous semiconductor, Chemical Science, Vol: 6, Pages: 1465-1473, ISSN: 2041-6520
The topochemical conversion of a dense, insulating metal–organic framework (MOF) into a semiconducting amorphous MOF is described. Treatment of single crystals of copper(I) chloride trithiocyanurate, CuICl(ttcH3) (ttcH3 = trithiocyanuric acid), 1, in aqueous ammonia solution yields monoliths of amorphous CuI1.8(ttc)0.6(ttcH3)0.4, 3. The treatment changes the transparent orange crystals of 1 into shiny black monoliths of 3 with retention of morphology, and moreover increases the electrical conductivity from insulating to semiconducting (conductivity of 3 ranges from 4.2 × 10−11 S cm−1 at 20 °C to 7.6 × 10−9 S cm−1 at 140 °C; activation energy = 0.59 eV; optical band gap = 0.6 eV). The structure and properties of the amorphous conductor are fully characterized by AC impedance spectroscopy, X-ray photoelectron spectroscopy, X-ray pair distribution function analysis, infrared spectroscopy, diffuse reflectance spectroscopy, electron spin resonance spectroscopy, elemental analysis, thermogravimetric analysis, and theoretical calculations.
Hill JA, Cairns AB, Lim JJK, et al., 2015, Zero-strain reductive intercalation in a molecular framework, CRYSTENGCOMM, Vol: 17, Pages: 2925-2928, ISSN: 1466-8033
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Hunt SJ, Cliffe MJ, Hill JA, et al., 2014, Flexibility transition and guest-driven reconstruction in a ferroelastic metal-organic framework, CRYSTENGCOMM, Vol: 17, Pages: 361-369
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Cairns A, Goodwin A, 2014, Designing next-generation negative compressibility materials., Acta Crystallographica Section A Foundations and Advances, Vol: 70, Pages: C262-C262
<jats:p>Negative compressibility is a rare but desirable property whereby a material's crystal structure actually expands in one (negative linear compressibility, NLC) or two (negative area compressibility, NAC) principal directions against application of increasing hydrostatic pressure. The performance of such materials–for use in, for e.g., sensitive interferometric or ferroelectric pressure sensing devices, advanced actuators, or prototype artificial muscle–critically depends on the magnitude of intrinsic negative response. NLC and NAC have been previously reported in a diverse range of materials: from the elemental forms of selenium and tellurium, to transition metal oxides, halides and chalcogenides [1], to more recent reports of NLC in molecular materials, and metal–organic and metal–cyanide frameworks. We explore, using examples from our work [2,3] as well as that of others, how understanding known NLC and NAC materials can inform material design, and how the versatile chemistry of molecular frameworks–the connecting of cationic metal nodes with anionic molecular linkers in one or more dimensions–allows for the targeting, enhancing and coupling of functionalities. By analysis of the negative response across all known NLC and NAC materials we develop new understanding into the underlying mechanisms of these unusual responses. Structural motifs identified point towards strategies for designing the next-generation of these materials, including the simple "wine-rack," "honeycomb" and "spring" mechanisms, where hinging about nodes requires volume reduction by simultaneous expansion and contraction in perpendicular directions (Figure 1). We discuss the first report of "giant" NLC in zinc(II) dicyanoaurate(I), Zn[Au(CN)<jats:sub>2</jats:sub>]<jats:sub>2</jats:sub>, where the crystal structure expands >10% over 1.8 GPa [2], the unprecedented prolonged NAC
Hodgson SA, Adamson J, Hunt SJ, et al., 2014, Negative area compressibility in silver(i) tricyanomethanide, Chemical Communications, Vol: 50, Pages: 5264-5266, ISSN: 1359-7345
The molecular framework Ag(tcm) (tcm− = tricyanomethanide) expands continuously in two orthogonal directions under hydrostatic compression. The first of its kind, this negative area compressibility behaviour arises from the flattening of honeycomb-like layers during rapid pressure-driven collapse of the interlayer separation.
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