I12-JEEP: Joint Engineering, Environmental and Processing
|
Open Access
Abstract: Despite being one of the most thoroughly characterised molecular crystals, hexamethylenetetramine (HMT) and its deuterated counterpart (DHMT), are still not fully understood, especially regarding anharmonic and nuclear quantum effects. In this work, an unprecedented combination of experimental techniques, including neutron and X-ray diffraction, inelastic neutron scattering, neutron transmission, and Compton scattering, all augmented ab initio by harmonic lattice dynamics calculations, was applied. The main question that motivated the presented work was the interplay between the phonon anharmonicity and isotope and nuclear quantum effects related to the zero-point energies of proton and deuteron. Signatures of the combined effects of isotopic substitution, temperature, anharmonicity and nuclear quantum effects were found in data from all experimental methods. In the case of neutron and X-ray diffraction, these signatures manifested as systematic discrepancies between the structural and atomic displacement parameters and thermal diffuse scattering obtained from harmonic lattice calculations and their experimental counterparts. To this end, an important effect was found that could not have been explained by the harmonic lattice modelling; the reverse Ubbelohde effect, i.e. the observation that deuteration decreases hydrogen bond length in HMT. In the case of neutron transmission, further discrepancies between theoretical predictions and experimental data were found at cryogenic temperatures. Finally, applying the diabatic theory of the local potential of the intermolecular hydrogen bond in HMT, it was possible to elucidate the degree of anharmonicity of the C–H···N bonds by relating it to the magnitude of the vibrational isotope effect for the C–H bond stretching observed in inelastic and neutron Compton scattering experiments. It was found that the combined nuclear quantum and anharmonic effects of the protons (deuterons) in hydrogen bonds in HMT (DHMT) manifest as systematic discrepancies between the ab initio predictions for the widths of nuclear momentum distributions and the experimental values.
|
Jan 2023
|
|
I19-Small Molecule Single Crystal Diffraction
|
Diamond Proposal Number(s):
[18193]
Open Access
Abstract: In this work we use high-resolution synchrotron X-ray diffraction for electron density mapping, in conjunction with ab initio modelling, to study short O—H⋯O and O+—H⋯O− hydrogen bonds whose behaviour is known to be tuneable by temperature. The short hydrogen bonds have donor–acceptor distances in the region of 2.45 Å and are formed in substituted urea and organic acid molecular complexes of N,N′-dimethylurea oxalic acid 2[thin space (1/6-em)]:[thin space (1/6-em)]1 (1), N,N-dimethylurea 2,4-dinitrobenzoate 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (2) and N,N-dimethylurea 3,5-dinitrobenzoic acid 2[thin space (1/6-em)]:[thin space (1/6-em)]2 (3). From the combined analyses, these complexes are found to fall within the salt-cocrystal continuum and exhibit short hydrogen bonds that can be characterised as both strong and electrostatic (1, 3) or very strong with a significant covalent contribution (2). An additional charge assisted component is found to be important in distinguishing the relatively uncommon O—H⋯O pseudo-covalent interaction from a typical strong hydrogen bond. The electron density is found to be sensitive to the extent of static proton transfer, presenting it as a useful parameter in the study of the salt–cocrystal continuum. From complementary calculated hydrogen atom potentials, we attribute changes in proton position to the molecular environment. Calculated potentials also show zero barrier to proton migration, forming an ‘energy slide’ between the donor and acceptor atoms. The better fundamental understanding of the short hydrogen bond in the ‘zone of fluctuation’ presented in a salt-cocrystal continuum, enabled by studies like this, provide greater insight into their related properties and can have implications in the regulation of pharmaceutical materials.
|
Aug 2021
|
|
I11-High Resolution Powder Diffraction
|
Diamond Proposal Number(s):
[16297]
Open Access
Abstract: There has been significant recent interest in exploiting the large dimension changes that can occur in molecular materials as a function of temperature, stress or under optical illumination. Here we report the remarkable thermal expansion properties of chloranilic acid pyrazine (CA-Pyz) co-crystals. We show that the compound shows uniaxial negative thermal expansion over a wide temperature range with a linear contraction coefficient as low as (−)1500×10−6 K−1 at 250 K. The corresponding 10% contraction between 200 and 300 K is an order of magnitude larger than in the so-called colossal contraction materials. We adopt a symmetry-inspired approach to describe both the structural changes that occur (using rotational-symmetry modes) and the thermal expansion (using strain modes). This allows an extremely compact description of the phase transition and detailed understanding of its atomic origins. We show how the coupling of primary and secondary strain modes in materials showing extreme expansion and contraction can lead to unusual reversals in the temperature dependence of cell parameters.
|
May 2019
|
|
I19-Small Molecule Single Crystal Diffraction
|
Diamond Proposal Number(s):
[18735]
Abstract: The compound Cp(IPr)Ru(H)2SiH(Ph)Cl (IPr = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene) (1) was subject to low temperature (30 K), high resolution X-ray structural analysis to obtain a high quality electron density map. This map was subject to a Bader analysis to ascertain the possibility of a Ru–H⋯Si interaction. For comparison, DFT calculations employing the well known B3LYP functional in conjunction with a triple zeta basis set was employed. Thus, we not only report the results of a possible Ru–H⋯Si interaction but also benchmark the use of the employed level of theory against experimental results.
|
Mar 2019
|
|
|
|
D. F.
Bowman
,
E.
Cemal
,
T.
Lehner
,
A. R.
Wildes
,
L.
Mangin-Thro
,
G. J.
Nilsen
,
M. J.
Gutmann
,
D. J.
Voneshen
,
D.
Prabhakaran
,
A. T.
Boothroyd
,
D. G.
Porter
,
C.
Castelnovo
,
K.
Refson
,
J. P.
Goff
Open Access
Abstract: Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. Classical spin ices provide the first context in which it is possible to control emergent magnetic monopoles, and anisotropic exchange leads to even richer behaviour associated with large quantum fluctuations. Whether the magnetic ground state of Yb2Ti2O7 is a quantum spin liquid or a ferromagnetic phase induced by a Higgs transition appears to be sample dependent. Here we have determined the role of structural defects on the magnetic ground state via the diffuse scattering of neutrons. We find that oxygen vacancies stabilise the spin liquid phase and the stuffing of Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state.
|
Feb 2019
|
|
I05-ARPES
|
S.
Ricco
,
M.
Kim
,
A.
Tamai
,
S.
Mckeown Walker
,
F. Y.
Bruno
,
I.
Cucchi
,
E.
Cappelli
,
C.
Besnard
,
T. K.
Kim
,
P.
Dudin
,
M.
Hoesch
,
M. J.
Gutmann
,
A.
Georges
,
R. S.
Perry
,
F.
Baumberger
Diamond Proposal Number(s):
[17381]
Open Access
Abstract: Pressure plays a key role in the study of quantum materials. Its application in angle resolved photoemission (ARPES) studies, however, has so far been limited. Here, we report the evolution of the k-space electronic structure of bulk Ca2RuO4, lightly doped with Pr, under uniaxial strain. Using ultrathin plate-like crystals, we achieve uniaxial strain levels up to −4.1%, sufficient to suppress the insulating Mott phase and access the previously unexplored electronic structure of the metallic state at low temperature. ARPES experiments performed while tuning the uniaxial strain reveal that metallicity emerges from a marked redistribution of charge within the Ru t2g shell, accompanied by a sudden collapse of the spectral weight in the lower Hubbard band and the emergence of a well-defined Fermi surface which is devoid of pseudogaps. Our results highlight the profound roles of lattice energetics and of the multiorbital nature of Ca2RuO4 in this archetypal Mott transition and open new perspectives for spectroscopic measurements.
|
Oct 2018
|
|
I16-Materials and Magnetism
|
D.
Pincini
,
S.
Boseggia
,
R.
Perry
,
Ma. J.
Gutmann
,
S.
Ricco
,
L. S. I.
Veiga
,
C. D.
Dashwood
,
S. P.
Collins
,
G.
Nisbet
,
A.
Bombardi
,
D. G.
Porter
,
F.
Baumberger
,
A. T.
Boothroyd
,
D. F.
Mcmorrow
Diamond Proposal Number(s):
[15323, 15952, 15867, 18934]
Abstract: The chemical and magnetic structures of the series of compounds Ca2−xLaxRuO4 [x=0, 0.05(1), 0.07(1), 0.12(1)] have been investigated using neutron diffraction and resonant elastic x-ray scattering. Upon La doping, the low-temperature S-Pbca space group of the parent compound is retained in all insulating samples [x≤0.07(1)], but with significant changes to the atomic positions within the unit cell. These changes can be characterized in terms of the local
RuO6 octahedral coordination: with increasing doping, the structure, crudely speaking, evolves from an orthorhombic unit cell with compressed octahedra to a quasitetragonal unit cell with elongated ones. The magnetic structure on the other hand, is found to be robust, with the basic k=(0,0,0), b-axis antiferromagnetic order of the parent compound preserved below the critical La doping concentration of x≈0.11. The only effects of La doping on the magnetic structure are to suppress the A-centred mode, favoring the B mode instead, and to reduce the Néel temperature somewhat. Our results are discussed with reference to previous experimental reports on the effects of cation substitution on the d4 Mott insulator Ca2RuO4, as well as with regard to theoretical studies on the evolution of its electronic and magnetic structure. In particular, our results rule out the presence of a proposed ferromagnetic phase, and suggest that the structural effects associated with La substitution play an important role in the physics of the system.
|
Jul 2018
|
|
|
|
Open Access
Abstract: High performance batteries based on the movement of Li ions in Li x CoO2 have made possible a revolution in mobile electronic technology, from laptops to mobile phones. However, the scarcity of Li and the demand for energy storage for renewables has led to intense interest in Na-ion batteries, including structurally-related Na x CoO2. Here we have determined the diffusion mechanism for Na0.8CoO2 using diffuse x-ray scattering, quasi-elastic neutron scattering and ab-initio molecular dynamics simulations, and we find that the sodium ordering provides diffusion pathways and governs the diffusion rate. Above T ~ 290 K the so-called partially disordered stripe superstructure provides channels for quasi-1D diffusion, and melting of the sodium ordering leads to 2D superionic diffusion above T ~ 370 K. We obtain quantitative agreement between our microscopic study of the hopping mechanism and bulk self-diffusion measurements. Our approach can be applied widely to other Na- or Li-ion battery materials.
|
Feb 2018
|
|
I16-Materials and Magnetism
|
Daniel
Porter
,
Eron
Cemal
,
David
Voneshen
,
Keith
Refson
,
Matthias
Gutmann
,
Alessandro
Bombardi
,
Andrew
Boothroyd
,
A.
Krzton-Maziopa
,
E.
Pomjakushina
,
K.
Conder
,
Jon
Goff
Diamond Proposal Number(s):
[8686, 10377]
Abstract: Single crystal neutron diffraction is combined with synchrotron x-ray scattering to identify the different superlattice phases present in
Cs
0.8
Fe
1.6
Se
2
. A combination of single crystal refinements and first principles modeling are used to provide structural solutions for the
√
5
×
√
5
and
√
2
×
√
2
superlattice phases. The
√
5
×
√
5
superlattice structure is predominantly composed of ordered Fe vacancies and Fe distortions, whereas the
√
2
×
√
2
superlattice is composed of ordered Cs vacancies. The Cs vacancies only order within the plane, causing Bragg rods in reciprocal space. By mapping x-ray diffraction measurements with narrow spatial resolution over the surface of the sample, the structural domain pattern was determined, consistent with the notion of a majority antiferromagnetic
√
5
×
√
5
phase and a superconducting
√
2
×
√
2
phase.
|
Apr 2015
|
|
I10-Beamline for Advanced Dichroism - scattering
|
Abstract: The first successful growth of neutron-size single-crystal LixCoO2 by the optical floating-zone technique is reported. Structural properties have been studied using the time-of-flight neutron Laue diffraction technique. Our experiment is the first report of the LixCoO2 single-crystal neutron study. The neutron diffraction profile yields sharp, strong Bragg reflections, indicating a single grain of high crystalline quality. The structural refinement from the single-crystal neutron diffraction data indicated a trigonal structure of space group R (3) over barm, and a Li concentration x=0.87. No superlattice reflections were detected. The surface morphology analysed by scanning electron microscopy revealed the absence of cracks. The magnetic susceptibility was measured in a field of 1 T with H parallel to c and H perpendicular to c, and an antiferromagnetic transition was observed at similar to 10 K, with no magnetic impurities. (C) 2013 Published by Elsevier B.V.
|
Sep 2014
|
|
