I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[20934]
Open Access
Abstract: A single-component molecular crystal [Pd(dddt)2] has been shown to exhibit almost temperature-independent resistivity under high pressure, leading theoretical studies to propose it as a three-dimensional (3D) Dirac electron system. To obtain more experimental information about the high-pressure electronic states, detailed resistivity measurements were performed, which show temperature-independent behavior at 13 GPa and then an upturn in the low temperature region at higher pressures. High-pressure single-crystal structure analysis was also performed for the first time, revealing the presence of pressure-induced structural disorder, which is possibly related to the changes in resistivity in the higher-pressure region. Calculations based on the disordered structure reveal that the Dirac cone state and semiconducting state coexist, indicating that the electronic state at high pressure is not a simple Dirac electron system as previously believed. Finally, the first measurements of magnetoresistance on [Pd(dddt)2] under high pressure are reported, revealing unusual behavior that seems to originate from the Dirac electron state.
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May 2021
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I11-High Resolution Powder Diffraction
I12-JEEP: Joint Engineering, Environmental and Processing
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Kieran W. P.
Orr
,
Sean M.
Collins
,
Emily M.
Reynolds
,
Frank
Nightingale
,
Hanna L. B.
Bostroem
,
Simon J.
Cassidy
,
Daniel M.
Dawson
,
Sharon E.
Ashbrook
,
Oxana
Magdysyuk
,
Paul A.
Midgley
,
Andrew L.
Goodwin
,
Hamish H.-M.
Yeung
Diamond Proposal Number(s):
[20946, 18786]
Open Access
Abstract: Control over the spatial distribution of components in metal–organic frameworks has potential to unlock improved performance and new behaviour in separations, sensing and catalysis. We report an unprecedented single-step synthesis of multi-component metal–organic framework (MOF) nanoparticles based on the canonical ZIF-8 (Zn) system and its Cd analogue, which form with a core–shell structure whose internal interface can be systematically tuned. We use scanning transmission electron microscopy, X-ray energy dispersive spectroscopy and a new composition gradient model to fit high-resolution X-ray diffraction data to show how core–shell composition and interface characteristics are intricately controlled by synthesis temperature and reaction composition. Particle formation is investigated by in situ X-ray diffraction, which reveals that the spatial distribution of components evolves with time and is determined by the interplay of phase stability, crystallisation kinetics and diffusion. This work opens up new possibilities for the control and characterisation of functionality, component distribution and interfaces in MOF-based materials.
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Feb 2021
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[20934]
Abstract: We examined the high-pressure electronic structure of a single-component molecular conductor [Pd(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) at room temperature, on the basis of the crystal structure determined by single-crystal synchrotron X-ray diffraction measurements at 5.9 GPa. The monoclinic unit cell contains four molecules that form two crystallographically independent molecular layers. A tight-binding model of an 8 × 8 matrix Hamiltonian gives an electronic structure as a Dirac electron system. The Dirac point describes a loop within the first Brillouin zone, and a nodal line semimetal is obtained. The noticeable property of the Dirac cone with a linear dispersion is shown by calculating the density of states (DOS). The Dirac cone in this system is associated with the crossing of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) bands, which originates from the direct interaction between different molecular layers. This is a newly found mechanism in addition to the indirect interaction [J. Phys. Soc. Jpn. 86, 064705 (2017)]. The Dirac points emerge as a line when the HOMO and LUMO bands meet on the surface and the HOMO–LUMO couplings are absent. Such a mechanism is verified using a reduced model of a 4 × 4 matrix Hamiltonian. The deviation of the band energy (δE) at the Dirac point from the Fermi level is very small (δE < 0.4 meV). The nodal line is examined by calculating the parity of the occupied band eigenstates at time reversal invariant momentum (TRIM), which shows that the topological number is 1.
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Dec 2020
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I12-JEEP: Joint Engineering, Environmental and Processing
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Diamond Proposal Number(s):
[16450]
Open Access
Abstract: The formation processes of metal–organic frameworks are becoming more widely researched using in situ techniques, although there remains a scarcity of NMR studies in this field. In this work, the synthesis of framework MFM-500(Ni) has been investigated using an in situ NMR strategy that provides information on the time-evolution of the reaction and crystallization process. In our in situ NMR study of MFM-500(Ni) formation, liquid-phase 1H NMR data recorded as a function of time at fixed temperatures (between 60 and 100 °C) afford qualitative information on the solution-phase processes and quantitative information on the kinetics of crystallization, allowing the activation energies for nucleation (61.4 ± 9.7 kJ mol−1) and growth (72.9 ± 8.6 kJ mol−1) to be determined. Ex situ small-angle X-ray scattering studies (at 80 °C) provide complementary nanoscale information on the rapid self-assembly prior to MOF crystallization and in situ powder X-ray diffraction confirms that the only crystalline phase present during the reaction (at 90 °C) is phase-pure MFM-500(Ni). This work demonstrates that in situ NMR experiments can shed new light on MOF synthesis, opening up the technique to provide better understanding of how MOFs are formed.
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Nov 2020
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B22-Multimode InfraRed imaging And Microspectroscopy
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Open Access
Abstract: There is rising interest on low‐k dielectric materials based on porous metal–organic frameworks (MOFs) for improved electrical insulation in microelectronics. Herein, the concept of MOF dielectric sensor built from a single crystal of HKUST‐1 is demonstrated. Guest encapsulation effects of polar and non‐polar molecules are studied, by monitoring the transient dielectric response and AC conductivity of the crystal exposed to different vapors (water, iodine, methanol, and ethanol). The dielectric properties are measured along the <100> crystal direction in the frequency range of 100 Hz to 2 MHz. The dielectric data show the efficacy of MOF dielectric sensor for discriminating the guest analytes. The time‐dependent transient response reveals dynamics of the molecular inclusion and exclusion processes in the nanoscale pores. Since dielectric response is ubiquitous to all MOF materials (unlike DC conductivity and fluorescence), the results demonstrate the potential of dielectric MOF sensors compared to resistive sensors and luminescence‐based approaches.
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Jul 2020
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[15000]
Open Access
Abstract: Single-component molecular conductors form an important class of materials showing exotic quantum phenomena, owing to the range of behavior they exhibit under physical stimuli. We report the effect of high pressure on the electrical properties and crystal structure of the single-component crystal [Ni(dddt)2] (where dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate). The system is isoelectronic and isostructural with [Pd(dddt)2], which is the first example of a single-component molecular crystal that exhibits nodal line semimetallic behavior under high pressure. Systematic high pressure four-probe electrical resistivity measurements were performed up to 21.6 GPa, using a Diamond Anvil Cell (DAC), and high pressure single crystal synchrotron X-ray diffraction was performed up to 11.2 GPa. We found that [Ni(dddt)2] initially exhibits a decrease of resistivity upon increasing pressure but, unlike [Pd(dddt)2], it shows pressure-independent semiconductivity above 9.5 GPa. This correlates with decreasing changes in the unit cell parameters and intermolecular interactions, most notably the π-π stacking distance within chains of [Ni(dddt)2] molecules. Using first-principles density functional theory (DFT) calculations, based on the experimentally-determined crystal structures, we confirm that the band gap decreases with increasing pressure. Thus, we have been able to rationalize the electrical behavior of [Ni(dddt)2] in the pressure-dependent regime, and suggest possible explanations for its pressure-independent behavior at higher pressures.
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May 2019
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I12-JEEP: Joint Engineering, Environmental and Processing
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Hamish
Yeung
,
Adam F.
Sapnik
,
Felicity
Massingberd-mundy
,
Michael W.
Gaultois
,
Yue
Wu
,
Duncan X.
Fraser
,
Sebastian
Henke
,
Roman
Pallach
,
Niclas
Heidenreich
,
Oxana
Magdysyuk
,
Nghia T.
Vo
,
Andrew L.
Goodwin
Diamond Proposal Number(s):
[16354, 16450]
Abstract: There is an increasingly large amount of interest in metal‐organic frameworks (MOFs) for a variety of applications, from gas sensing and separations to electronics and catalysis. Their exciting properties arise from their modular architectures, which self‐assemble from different combinations of metal‐based and organic building units. However, the exact mechanisms by which they crystallize remain poorly understood, thus limiting any realisation of real “structure by design”. We report important new insight into MOF formation, gained using in situ X‐ray diffraction, pH and turbidity measurements to uncover for the first time the evolution of metastable intermediate species in the canonical zeolitic imidazolate framework system, ZIF‐8. We reveal that the intermediate species exist in a dynamic pre‐equilibrium prior to network assembly and, depending on the reactant concentrations and the progress of reaction, the pre‐equilibrium can be made to favour under‐ or over‐coordinated Zn‐imidazolate species, thus accelerating or inhibiting crystallization, respectively. We thereby find that concentration can be effectively used as a synthetic handle to directly control particle size, with great implications for industrial scale‐up and gas sorption applications. These finding enables us to rationalise the apparent contradictions between previous studies of ZIF‐8 and, importantly, opens up new opportunities for the control of crystallization in network solids more generally, from the design of local structure to assembly of particles with precise dimensions.
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Nov 2018
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[13284]
Abstract: We study the structural and thermomechanical effects of cation substitution in the compositional family of metal–organic frameworks Zn1−xCdx(mIm)2 (HmIm = 2-methylimidazole). We find complete miscibility for all compositions x, with evidence of inhomogeneous distributions of Cd and Zn that in turn affect framework aperture characteristics. Using variable-temperature X-ray powder diffraction measurements, we show that Cd substitution drives a threefold reduction in the magnitude of thermal expansion behaviour. We interpret this effect in terms of an increased density of negative thermal expansion modes in the more flexible Cd-rich frameworks.
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Aug 2018
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[9700]
Abstract: By decoupling the mechanical behaviour of building units for the first time in a wine-rack framework containing two different strut types, we show that lithium l-tartrate exhibits NLC with a maximum value, Kmax = -21 TPa-1, and an overall NLC capacity, ΧNLC = 5.1 %, that are comparable to the most exceptional materials to date. Furthermore, the contributions from molecular strut compression and angle opening interplay to give rise to so-called “hidden” negative linear compressibility, in which NLC is absent at ambient pressure, switched on at 2 GPa and sustained up to the limit of our experiment, 5.5 GPa. Analysis of the changes in crystal structure using variable-pressure synchrotron X-ray diffraction reveals new chemical and geometrical design rules to assist the discovery of other materials with exciting hidden anomalous mechanical properties.
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Jan 2017
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I12-JEEP: Joint Engineering, Environmental and Processing
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Diamond Proposal Number(s):
[9661]
Abstract: Understanding the driving forces controlling crys-tallization is essential for the efficient synthesis and design ofnew materials,particularly metal–organic frameworks(MOFs), where mild solvothermal synthesis often allowsaccess to various phases from the same reagents.Using high-energy in situ synchrotron X-ray powder diffraction, wemonitor the crystallization of lithium tartrate MOFs,observingthe successive crystallization and dissolution of three compet-ing phases in one reaction. By determining rate constants andactivation energies,wefully quantify the reaction energylandscape,gaining important predictive power for the choice ofreaction conditions.Different reaction rates are explained bythe structural relationships between the products and thereactants;larger changes in conformation result in higheractivation energies.The methods we demonstrate can easily beapplied to other materials,opening the door to agreaterunderstanding of crystallization in general.
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Feb 2016
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