Publications by authors named "Keith Refson"

40 Publications

Pressure-induced Pb-Pb bonding and phase transition in PbSnO.

Acta Crystallogr B Struct Sci Cryst Eng Mater 2020 Dec 3;76(Pt 6):979-991. Epub 2020 Nov 3.

Goethe University, Institute of Geosciences, Crystallography, Frankfurt, Germany.

High-pressure single-crystal to 20 GPa and powder diffraction measurements to 50 GPa, show that the structure of PbSnO strongly distorts on compression with an elongation of one axis. A structural phase transition occurs between 10 GPa and 12 GPa, with a change of space group from Pbam to Pnam. The resistivity decreases by more than six orders of magnitude when pressure is increased from ambient conditions to 50 GPa. This insulator-to-semiconductor transition is accompanied by a reversible appearance change from transparent to opaque. Density functional theory-based calculations show that at ambient conditions the channels in the structure host the stereochemically-active Pb 6s lone electron pairs. On compression the lone electron pairs form bonds between Pb ions. Also provided is an assignment of irreducible representations to the experimentally observed Raman bands.
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http://dx.doi.org/10.1107/S205252062001238XDOI Listing
December 2020

Negative thermal expansion of cubic silicon dicarbodiimide, Si(NCN), studied by ab initio lattice dynamics.

J Phys Condens Matter 2020 Jul 23. Epub 2020 Jul 23.

Physics, Queen Mary University of London, London, UNITED KINGDOM OF GREAT BRITAIN AND NORTHERN IRELAND.

We report an ab initio calculation of crystal structure and lattice dynamics of cubic silicon dicarbodiimide, Si(NCN)2, using Density Functional Theory methods. The calculations reveal a low-energy spectrum of rigid unit modes that are shown to be associate with negative thermal expansion. Comparisons are drawn with the closely-related materials Zn(CN)2, and the cubic-cristobalite phase of SiO2. Instabilities in the spectrum of rigid unit modes point to the existence of disorder of the positions and orientations of the dicarbodiimide molecular anions.
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http://dx.doi.org/10.1088/1361-648X/aba8cbDOI Listing
July 2020

Structural phase transitions in malononitrile, CH(CN): crystal structure of the δ phase by neutron powder diffraction, and ab initio calculations of the structures and phonons of the α and δ phases.

J Phys Condens Matter 2019 Jun 20;31(25):255401. Epub 2019 Mar 20.

Department of Physics and Astronomy, Queen Mary University of London, Mile End Road, London, E1 4NS, United Kingdom.

The crystal structure of the low-temperature [Formula: see text] phase of crystalline malononitrile, CH(CN) (stable phase below 260 K), has been determined using Rietveld refinement on neutron powder diffraction data. The [Formula: see text] phase has a slightly lower density than the other three low-pressure phases, and unlike those phases it has a polar structure. The transition from the [Formula: see text] to [Formula: see text] phase involves a major reconstruction of the structure, including establishing a network of hydrogen bonds. DFT simulations of the structure and phonon dispersion curves of both [Formula: see text] and [Formula: see text] phases give free energy curves consistent with the phase transitions. It is noted that the transition from the [Formula: see text] to [Formula: see text] phase at 260 K is facilitated by the entropy arising from the low-frequency phonons associated with the soft mode for the sequence of [Formula: see text]-[Formula: see text]-[Formula: see text] phase transitions.
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http://dx.doi.org/10.1088/1361-648X/ab11a1DOI Listing
June 2019

Nanoscale momentum-resolved vibrational spectroscopy.

Sci Adv 2018 06 15;4(6):eaar7495. Epub 2018 Jun 15.

SuperSTEM Laboratory, SciTech Daresbury Campus, Keckwick Lane, Daresbury WA4 4AD, UK.

Vibrational modes affect fundamental physical properties such as the conduction of sound and heat and can be sensitive to nano- and atomic-scale structure. Probing the momentum transfer dependence of vibrational modes provides a wealth of information about a materials system; however, experimental work has been limited to essentially bulk and averaged surface approaches or to small wave vectors. We demonstrate a combined experimental and theoretical methodology for nanoscale mapping of optical and acoustic phonons across the first Brillouin zone, in the electron microscope, probing a volume ~10 to 10 times smaller than that of comparable bulk and surface techniques. In combination with more conventional electron microscopy techniques, the presented methodology should allow for direct correlation of nanoscale vibrational mode dispersions with atomic-scale structure and chemistry.
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http://dx.doi.org/10.1126/sciadv.aar7495DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6018998PMC
June 2018

Effect of Basicity on the Hydrolysis of the Bi(III) Aqua Ion in Solution: An Ab Initio Molecular Dynamics Study.

J Phys Chem A 2018 Feb 12;122(7):1905-1915. Epub 2018 Feb 12.

Royal Holloway University of London , Egham Hill, Egham, Surrey TW20 0EX, United Kingdom.

Hydrolysis of the Bi(III) aqua ion under a range of solution conditions has been studied by means of ab initio molecular dynamics simulations. While the Bi(III) aqua ion is stable in pure water, there is an increasing degree of hydrolysis with the number of hydroxide anions in the medium. This is accompanied by a monotonic decrease of the total coordination number to an asymptotic value of ∼6, reached under extreme basicity conditions. Comparison of the simulated Bi(III) hydrolyzed species with the experimental species distribution at different degrees of basicity suggests that, at the PBE/DFT level of theory here employed, liquid water shows an overly acidic character. Predictions of theoretical EXAFS and XANES spectra were generated from the AIMD trajectories for different Bi hydrolyzed species, [Bi(HO)(HO)], m = 0-3 and n = 7-2. Comparison with available experimental spectra is presented. Spectral features joined to the degree of hydrolysis and hydration are analyzed.
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http://dx.doi.org/10.1021/acs.jpca.7b12402DOI Listing
February 2018

Mechanism of enhancement of ferroelectricity of croconic acid with temperature.

Phys Chem Chem Phys 2017 Dec;19(48):32216-32225

ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK.

A detailed study of the thermal behaviour of atomic motions in the organic ferroelectric croconic acid is presented in the temperature range 5-300 K. Using high-resolution inelastic neutron scattering and first-principles electronic-structure calculations within the framework of density functional theory and a quasiharmonic phonon description of the material, we find that the frequencies of the well defined doublet in inelastic neutron scattering spectra associated with out-of-plane motions of hydrogen-bonded protons decrease monotonically with temperature indicating weakening of these bonding motifs and enhancement of proton motions. Theoretical mean-square displacements for these proton motions are within 5% of experimental values. A detailed analysis of this observable shows that it is unlikely that there is a facile proton transfer along the direction of ferroelectric polarization in the absence of an applied electric field. Calculations predict constrained thermal motion of proton along crystallographic lattice direction c retaining the hydrogen bond motif of the crystal at high temperature. Using the Berry-phase method, we have also calculated the spontaneous polarization of temperature dependent cell structures, and find that our computational model provides a satisfactory description of the anomalous and so far unexplained rise in bulk electric polarization with temperature. Correlating the thermal motion induced lattice strain with temperature dependent spontaneous polarizations, we conclude that increasing thermal strain with temperatures combined with constrained thermal motion along the hydrogen bond motif are responsible of this increase in ferroelectricity at high temperature.
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http://dx.doi.org/10.1039/c7cp06039dDOI Listing
December 2017

Visualization and processing of computed solid-state NMR parameters: MagresView and MagresPython.

Solid State Nucl Magn Reson 2016 09 4;78:64-70. Epub 2016 Jun 4.

Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom. Electronic address:

We introduce two open source tools to aid the processing and visualisation of ab-initio computed solid-state NMR parameters. The Magres file format for computed NMR parameters (as implemented in CASTEP v8.0 and QuantumEspresso v5.0.0) is implemented. MagresView is built upon the widely used Jmol crystal viewer, and provides an intuitive environment to display computed NMR parameters. It can provide simple pictorial representation of one- and two-dimensional NMR spectra as well as output a selected spin-system for exact simulations with dedicated spin-dynamics software. MagresPython provides a simple scripting environment to manipulate large numbers of computed NMR parameters to search for structural correlations.
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http://dx.doi.org/10.1016/j.ssnmr.2016.05.004DOI Listing
September 2016

Reproducibility in density functional theory calculations of solids.

Science 2016 Mar;351(6280):aad3000

Department of Physics, University of York, Heslington, York YO10 5DD, UK.

The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements.
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http://dx.doi.org/10.1126/science.aad3000DOI Listing
March 2016

Stabilization of 3d Transition Metal Hydrido Complexes in SrH2Mg2[Co(I)H5], BaH2Mg5[Co(-I)H4]2, and RbH2Mg5[Co(-I)H4 Ni(0)H4] via Easily Polarizable Hydride Ligands.

Inorg Chem 2016 Apr 18;55(7):3576-82. Epub 2016 Mar 18.

ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton , Didcot, Oxon OX11 0QX, United Kingdom.

A combined study using neutron diffraction, inelastic neutron scattering, and first-principles calculations describe cobalt with a very low formal oxidation state of (-I) in a slightly distorted tetrahedral Co(-I)H4-complex in BaH2Mg5[Co(-I)H4]2 and in the structurally related RbH2Mg5[Co(-I)H4 Ni(0)H4]. This indicates that the electron "back donating" effect via the polarizable hydride ions to the counterions in the solid state hydrides, can be compared to more conventional "back bonding" able to reduce the oxidation state down to -I. The hydrides were synthesized by hot sintering of transition metal powders with corresponding binary alkali- and alkaline earth hydrides. In the similarly synthesized SrH2Mg2[Co(I)H5], cobalt is formally + I-valent, showing a high sensitivity to differences in the counterion framework, which can also influence electrical properties.
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http://dx.doi.org/10.1021/acs.inorgchem.6b00074DOI Listing
April 2016

Dimer-mediated cation diffusion in the stoichiometric ionic conductor Li3N.

Phys Chem Chem Phys 2016 Feb;18(7):5605-13

ISIS Facility, Rutherford Appleton Laboratory, Chilton, Oxfordshire, OX11 0QX, UK. and Department of Physics, Royal Holloway, University of London, Egham, TW20 0EX, UK.

Non-equilibrium molecular dynamics has been used to model cation diffusion in stoichiometric Li3N over the temperature range 50 < T/K < 800. The resulting diffusion coefficients are in excellent agreement with the available experimental data. We present a detailed atomistic account of the diffusion process. Contrary to the conclusions drawn from previous studies, our calculations show that it is unnecessary to invoke the presence of a small concentration of intrinsic defects in order to initiate diffusion. The structure can be considered to consist of alternating layers of composition Li2N and Li. As the temperature increases an increasing number of cations leave the Li2N layers and migrate either to the interlayer space or to the Li layer. Those that move into the interlayer space form Li2 dimers with cations in the Li2N layers and those that move into the neighboring layer form dimers with cations therein. The two types of dimer are aligned parallel and perpendicular to [001], respectively and have lifetimes of ∼3 ps. The vacancies so created facilitate rapid diffusion in the Li2N layers and the interlayer cation motion results in slower diffusion perpendicular to the layers.
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http://dx.doi.org/10.1039/c5cp07625kDOI Listing
February 2016

Structure and spectroscopy of CuH prepared via borohydride reduction.

Acta Crystallogr B Struct Sci Cryst Eng Mater 2015 Dec 7;71(Pt 6):608-12. Epub 2015 Nov 7.

ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, England.

Copper(I) hydride (cuprous hydride, CuH) was the first binary metal hydride to be discovered (in 1844) and is singular in that it is synthesized in solution, at ambient temperature. There are several synthetic paths to CuH, one of which involves reduction of an aqueous solution of CuSO4·5H2O by borohydride ions. The product from this procedure has not been extensively characterized. Using a combination of diffraction methods (X-ray and neutron) and inelastic neutron scattering spectroscopy, we show that the CuH from the borohydride route has the same bulk structure as CuH produced by other routes. Our work shows that the product consists of a core of CuH with a shell of water and that this may be largely replaced by ethanol. This offers the possibility of modifying the properties of CuH produced by aqueous routes.
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http://dx.doi.org/10.1107/S2052520615015176DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4669994PMC
December 2015

Assignment of the Internal Vibrational Modes of C70 by Inelastic Neutron Scattering Spectroscopy and Periodic-DFT.

ChemistryOpen 2015 Oct 20;4(5):620-5. Epub 2015 May 20.

ISIS Facility, Science and Technology Facilities Council (STFC), Rutherford Appleton Laboratory Chilton, Didcot, OX11 0QX, UK.

The fullerene C70 may be considered as the shortest possible nanotube capped by a hemisphere of C60 at each end. Vibrational spectroscopy is a key tool in characterising fullerenes, and C70 has been studied several times and spectral assignments proposed. Unfortunately, many of the modes are either forbidden or have very low infrared or Raman intensity, even if allowed. Inelastic neutron scattering (INS) spectroscopy is not subject to selection rules, and all the modes are allowed. We have obtained a new INS spectrum from a large sample recorded at the highest resolution available. An advantage of INS spectroscopy is that it is straightforward to calculate the spectral intensity from a model. We demonstrate that all previous assignments are incorrect in at least some respects and propose a new assignment based on periodic density functional theory (DFT) that successfully reproduces the INS, infrared, and Raman spectra.
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http://dx.doi.org/10.1002/open.201500069DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4608530PMC
October 2015

A first-principles study of the vibrational properties of crystalline tetracene under pressure.

J Phys Condens Matter 2015 Sep 2;27(37):375402. Epub 2015 Sep 2.

Department of Physics, University of Cambridge, Cavendish Laboratory, 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UK.

We present a comprehensive study of the hydrostatic pressure dependence of the vibrational properties of tetracene using periodic density-functional theory (DFT) within the local density approximation (LDA). Despite the lack of van der Waals dispersion forces in LDA we find good agreement with experiment and are able to assess the suitability of this approach for simulating conjugated organic molecular crystals. Starting from the reported x-ray structure at ambient pressure and low temperature, optimized structures at ambient pressure and under 280 MPa hydrostatic pressure were obtained and the vibrational properties calculated by the linear response method. We report the complete phonon dispersion relation for tetracene crystal and the Raman and infrared spectra at the centre of the Brillouin zone. The intermolecular modes with low frequencies exhibit high sensitivity to pressure and we report mode-specific Grüneisen parameters as well as an overall Grüneisen parameter [Formula: see text]. Our results suggest that the experimentally reported improvement of the photocurrent under pressure may be ascribed to an increase in intermolecular interactions as also the dielectric tensor.
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http://dx.doi.org/10.1088/0953-8984/27/37/375402DOI Listing
September 2015

Computation of diffuse scattering arising from one-phonon excitations in a neutron time-of-flight single-crystal Laue diffraction experiment.

J Appl Crystallogr 2015 Aug 8;48(Pt 4):1122-1129. Epub 2015 Jul 8.

Deutsches Elektronen Synchrotron DESY , Notkestrasse 85, Hamburg 22603, Germany.

Direct phonon excitation in a neutron time-of-flight single-crystal Laue diffraction experiment has been observed in a single crystal of NaCl. At room temperature both phonon emission and excitation leave characteristic features in the diffuse scattering and these are well reproduced using phonons from density functional theory (DFT). A measurement at 20 K illustrates the effect of thermal population of the phonons, leaving the features corresponding to phonon excitation and strongly suppressing the phonon annihilation. A recipe is given to compute these effects combining DFT results with the geometry of the neutron experiment.
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http://dx.doi.org/10.1107/S1600576715010912DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4520289PMC
August 2015

Diffusion in Li₂O studied by non-equilibrium molecular dynamics for 873 < T/K < 1603.

Phys Chem Chem Phys 2015 Sep 29;17(33):21470-5. Epub 2015 Jul 29.

Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK.

The use of non-equilibrium molecular dynamics facilitates the calculation of the cation diffusion constant of Li2O at temperatures too low to be accessible by other methods. Excellent agreement with experimental diffusion coefficients has been obtained over the temperature range 873 < T/K < 1603. Diffusion below 1200 K was shown to be dominated by a concerted nearest-neighbour hopping process, whereas in the high-temperature superionic region an additional mechanism involving a six-coordinate interstitial cation site in the anti-fluorite structure becomes increasingly dominant. Our model thus accounts for the transition from the superionic regime to the non-superionic regime.
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http://dx.doi.org/10.1039/c5cp02628hDOI Listing
September 2015

Lattice dynamics of α-cristobalite and the Boson peak in silica glass.

J Phys Condens Matter 2015 Aug 14;27(30):305401. Epub 2015 Jul 14.

Department of Quantum Matter Physics, University of Geneva, 24, Quai Ernest Ansermet, CH-1211 Genève, Switzerland. Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland.

The lattice dynamics of the silica polymorph [Formula: see text]-cristobalite has been investigated by a combination of diffuse and inelastic x-ray scattering and ab initio lattice dynamics calculations. Phonon dispersion relations and vibrational density of states are reported and the phonon eigenvectors analyzed by a detailed comparison of scattering intensities. The experimentally validated calculation is used to identify the vibration contributing most to the first peak in the density of vibrational states. The comparison of its displacement pattern to the silica polymorphs [Formula: see text]-quartz and coesite and to vitreous silica reveals a distinct similarity and allows for decisive conclusions on the vibrations causing the so-called Boson peak in silica glass.
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http://dx.doi.org/10.1088/0953-8984/27/30/305401DOI Listing
August 2015

Chemical Descriptors of Yttria-Stabilized Zirconia at Low Defect Concentration: An ab Initio Study.

J Phys Chem A 2015 Jun 29;119(24):6412-20. Epub 2015 May 29.

‡Thomas Young Centre, Department of Chemistry, Imperial College London, London SW7 2AZ, U.K.

Yttria-stabilized zirconia (YSZ) is an important oxide ion conductor with applications in solid oxide fuel cells (SOFCs) and oxygen sensing devices. Doping the cubic phase of zirconia (c-ZrO2) with yttria (Y2O3) is isoelectronic, as two Zr(4+) ions are replaced by two Y(3+) ions, plus a charge compensating oxygen vacancy (Ovac). Typical doping concentrations include 3, 8, 10, and 12 mol %. For these concentrations, and all below 40 mol %, no phase with long-range order has been observed in either X-ray or neutron diffraction experiments. The prediction of local defect structure and the interaction between defects is therefore of great interest. This has not been possible to date as the number of possible defect topologies is very large and to perform reliable total energy calculations for all of them would be prohibitively expensive. Previous theoretical studies have only considered a selection of representative structures. In this study, a comprehensive search for low-energy defect structures using a combined classical modeling and density functional theory approach is used to identify the low-energy isolated defect structures at the dilute limit, 3.2 mol %. Through analysis of energetics computed using the best available Born-Mayer-Huggins empirical potential model, a point charge model, DFT, and a local strain energy estimated in the harmonic approximation, the main chemical and physical descriptors that correlate to the low-energy DFT structures are discussed. It is found that the empirical potential model reproduces a general trend of increasing DFT energetics across a series of locally strain relaxed structures but is unreliable both in predicting some incorrect low-energy structures and in finding some metastable structures to be unstable. A better predictor of low-energy defect structures is found to be the total electrostatic energy of a simple point charge model calculated at the unrelaxed geometries of the defects. In addition, the strain relaxation energy is estimated effectively in the harmonic approximation to the imaginary phonon modes of undoped c-ZrO2 but is found to be unimportant in determining the low-energy defect structures. These results allow us to propose a set of easily computed descriptors that can be used to identify the low-energy YSZ defect structures, negating the combinatorial complexity and number of defect structures that need to be considered.
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http://dx.doi.org/10.1021/acs.jpca.5b02031DOI Listing
June 2015

How the surface structure determines the properties of CuH.

Inorg Chem 2015 Mar 11;54(5):2213-20. Epub 2015 Feb 11.

School of Chemistry, Bangor University , Bangor LL57 2UW, U.K.

CuH is a material that appears in a wide diversity of circumstances ranging from catalysis to electrochemistry to organic synthesis. There are both aqueous and nonaqueous synthetic routes to CuH, each of which apparently leads to a different product. We developed synthetic methodologies that enable multigram quantities of CuH to be produced by both routes and characterized each product by a combination of spectroscopic, diffraction and computational methods. The results show that, while all methods for the synthesis of CuH result in the same bulk product, the synthetic path taken engenders differing surface properties. The different behaviors of CuH obtained by aqueous and nonaqueous routes can be ascribed to a combination of very different particle size and dissimilar surface termination, namely, bonded hydroxyls for the aqueous routes and a coordinated donor for the nonaqueous routes. This work provides a particularly clear example of how the nature of an adsorbed layer on a nanoparticle surface determines the properties.
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http://dx.doi.org/10.1021/ic5027009DOI Listing
March 2015

Inelastic incoherent neutron scattering study of the molecular properties of pure hydrogen peroxide and its water mixtures of different concentration.

J Chem Phys 2014 Apr;140(16):164504

ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom.

We have investigated the spectra of shock-frozen H2O2-H2O mixtures across the full composition range 99.1%-0.0% H2O2. In contrast to literature reports, we find that intermediate compositions (30%-70% H2O2) freeze to a solid solution rather than phase separating, which only occurs on annealing to just below the melting point. We have fully characterised the dihydrate H2O2·2H2O (48.6% H2O2) for the first time and shown that its spectrum can account for the features previously observed on the surface of a Au/TiO2 catalyst.
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http://dx.doi.org/10.1063/1.4871742DOI Listing
April 2014

Diffuse scattering in metallic tin polymorphs.

J Phys Condens Matter 2014 Mar 3;26(11):115401. Epub 2014 Mar 3.

European Synchrotron Radiation Facility, BP 220 F-38043 Grenoble Cedex 9, France.

The lattice dynamics of the metallic tin β and γ polymorphs has been studied by a combination of diffuse scattering, inelastic x-ray scattering and density functional perturbation theory. The non-symmorphic space group of the β-tin structure results in unusual asymmetry of thermal diffuse scattering. Strong resemblance of the diffuse scattering intensity distribution in β and γ-tin were observed, reflecting the structural relationship between the two phases and revealing the qualitative similarity of the underlying electronic potential. The strong influence of the electron subsystem on inter-ionic interactions creates anomalies in the phonon dispersion relations. All observed features are described in great detail by the density functional perturbation theory for both β- and γ-tin at arbitrary momentum transfers. The combined approach delivers thus a complete picture of the lattice dynamics in harmonic description.
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http://dx.doi.org/10.1088/0953-8984/26/11/115401DOI Listing
March 2014

Density functional theory in the solid state.

Philos Trans A Math Phys Eng Sci 2014 Mar 10;372(2011):20130270. Epub 2014 Feb 10.

Department of Physics, University of York, , York YO10 5DD, UK.

Density functional theory (DFT) has been used in many fields of the physical sciences, but none so successfully as in the solid state. From its origins in condensed matter physics, it has expanded into materials science, high-pressure physics and mineralogy, solid-state chemistry and more, powering entire computational subdisciplines. Modern DFT simulation codes can calculate a vast range of structural, chemical, optical, spectroscopic, elastic, vibrational and thermodynamic phenomena. The ability to predict structure-property relationships has revolutionized experimental fields, such as vibrational and solid-state NMR spectroscopy, where it is the primary method to analyse and interpret experimental spectra. In semiconductor physics, great progress has been made in the electronic structure of bulk and defect states despite the severe challenges presented by the description of excited states. Studies are no longer restricted to known crystallographic structures. DFT is increasingly used as an exploratory tool for materials discovery and computational experiments, culminating in ex nihilo crystal structure prediction, which addresses the long-standing difficult problem of how to predict crystal structure polymorphs from nothing but a specified chemical composition. We present an overview of the capabilities of solid-state DFT simulations in all of these topics, illustrated with recent examples using the CASTEP computer program.
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http://dx.doi.org/10.1098/rsta.2013.0270DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3928868PMC
March 2014

Molecular dynamics investigation of the disordered crystal structure of hexagonal LiBH4.

Phys Chem Chem Phys 2013 Jun 13;15(21):8081-7. Epub 2013 Feb 13.

ISIS Facility, Rutherford Appleton Laboratory, Chilton, Oxfordshire, OX11 0QX, UK.

The crystal structure of the hexagonal phase of solid lithium borohydride (LiBH4) is studied by ab initio molecular dynamics simulations of both the low and high-temperature phases. A temperature range of 200-535 K is simulated with the aim of characterising the disorder in the high-temperature structure in detail. The mechanism and kinetics of the reorientational motion of the borohydride units (BH4(-)) are determined and are consistent with published neutron scattering experiments; it is found that rotational diffusivity increases by an order of magnitude at the phase transition temperature. The average equilibrium orientation is characterised by a broad distribution of orientations, and reorientational jumps do not occur between sharply defined orientations. In addition, split positions with partial occupancy for the lithium and boron atoms are found (in agreement with previous theoretical studies), which, together with the disordered BH4(-) orientational distribution in equilibrium, lead to the conclusion that the correct crystallographic space group of the high-temperature phase is P63/mmc rather than P63mc.
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http://dx.doi.org/10.1039/c3cp44520hDOI Listing
June 2013

Assignment of metal-ligand modes in Pt(II) diimine complexes relevant to solar energy conversion.

Inorg Chem 2012 Sep 31;51(18):9748-56. Epub 2012 Aug 31.

ISIS Facility, STFC Rutherford Appleton Laboratory, STFC, Didcot, Chilton, Oxfordshire, OX11 0QX, UK.

This work describes a comprehensive assignment of the vibrational spectra of the platinum(II) diimine bisthiolate and chloride complexes as a prototype structure for a diversity of Pt(II) diimine chromophores. The dynamics and energy dissipation pathways in excited states of light harvesting molecules relies largely on the coupling between the high frequency and the low frequency modes. As such, the assignment of the vibrational spectrum of the chromophore is of utmost importance, especially in the low-frequency region, below 500 cm(-1), where the key metal-ligand framework modes occur. This region is experimentally difficult to access with infrared spectroscopy and hence frequently remains elusive. However, this region is easily accessible with Raman and inelastic neutron scattering (INS) spectroscopies. Accordingly, a combination of inelastic neutron scattering and Raman spectroscopy with the aid of computational results from periodic-DFT and the mode visualizations, as well as isotopic substitution, allowed for an identification of the modes that contain significant contributions from Pt-Cl, Pt-S, and Pt-N stretch modes. The results also demonstrate that it is not possible to assign transition energies to "pure", localized modes in the low frequency region, as a consequence of the anticipated severe coupling that occurs among the skeletal modes. The use of INS has proved invaluable in identifying and assigning the modes in the lowest frequency region, and overall the results will be of assistance in analyzing the structure of the electronic excited state in the families of chromophores containing a Pt(diimine) core.
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http://dx.doi.org/10.1021/ic3011184DOI Listing
September 2012

Ab initio nonequilibrium molecular dynamics in the solid superionic conductor LiBH4.

Phys Rev Lett 2012 Mar 29;108(9):095901. Epub 2012 Feb 29.

Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom.

The color-diffusion algorithm is applied to ab initio molecular dynamics simulation of hexagonal LiBH(4) to determine the lithium diffusion coefficient and diffusion mechanisms. Even in the best solid lithium ion conductors, the time scale of ion diffusion is too long to be readily accessible by ab initio molecular dynamics at a reasonable computational cost. In our nonequilibrium method, rare events are accelerated by the application of an artificial external field acting on the mobile species; the system response to this perturbation is accurately described in the framework of linear response theory and is directly related to the diffusion coefficient, thus resulting in a controllable approximation. The calculated lithium ionic conductivity of LiBH(4) closely matches published measurements, and the diffusion mechanism can be elucidated directly from the generated trajectory.
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http://dx.doi.org/10.1103/PhysRevLett.108.095901DOI Listing
March 2012

A combined experimental inelastic neutron scattering, Raman and ab initio lattice dynamics study of α-lithium amidoborane.

Phys Chem Chem Phys 2011 Jul 2;13(26):12249-53. Epub 2011 Jun 2.

Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, UK.

A combination of inelastic neutron scattering (INS) spectroscopy and Raman spectroscopy with periodic density functional theory calculations is used to provide a complete assignment of the vibrational spectra of α-lithium amidoborane (α-LiNH(2)BH(3)). The Born charge density and the atomic motion up to the decomposition temperature have been modelled. These models not only explain the nature of bonding in α-LiNH(2)BH(3) but also provide an insight into the atomic mechanisms of its decomposition. The (INS) measurements were performed in the range of 0-4000 cm(-1) on the high-resolution time-of-flight TOSCA INS spectrometer at the ISIS Spallation Neutron Source at the Rutherford Appleton Laboratory.
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http://dx.doi.org/10.1039/c1cp20587kDOI Listing
July 2011

Complete assignment of the vibrational modes of C60 by inelastic neutron scattering spectroscopy and periodic-DFT.

Phys Chem Chem Phys 2011 May 25;13(17):7789-804. Epub 2011 Mar 25.

ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK.

In this paper we exploit the complementarity of inelastic neutron scattering (INS), infrared and Raman spectroscopies with ab initio calculations to generate an updated assignment of the vibrational modes of C(60). We have carried out periodic-DFT calculations of the high temperature face centred cubic phase modelled as the standard structure and also of the low temperature simple cubic phase, the latter for the first time. Our assignment differs from all previous work, however, it is the only one that is able to successfully reproduce the INS spectrum in terms of both transition energies and intensities. In addition to the INS spectrum we are also able to quantitatively simulate the major features of the infrared and Raman spectra in the high temperature phase and the infrared spectrum in the low temperature phase.
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http://dx.doi.org/10.1039/c0cp02956dDOI Listing
May 2011

Structural compression and vibrational properties of Bi12SiO20 sillenite from experiment and theory.

J Phys Condens Matter 2010 Dec 26;22(50):505401. Epub 2010 Nov 26.

Institut für Geowissenschaften, Goethe Universität Frankfurt, Frankfurt aM, Germany.

The crystal structure of the bismuth silicon oxide Bi(12)SiO(20) was determined by single-crystal x-ray diffraction at ambient conditions and at high pressure. Single-crystal intensity data between 0.0001 and 16.8(3) GPa were collected in house with Mo Kα radiation and with synchrotron radiation (λ = 0.45 Å) at HASYLAB (D3), while lattice parameters were measured up to 23.0(3) GPa. The large cavities which exist in the crystal structure and host the lone electron pairs of the Bi(3 + ) ions are considerably compressed at high pressure. The crystal structure, however, remains stable and the lone electron pair is stereochemically active up to at least 16.8 GPa. A larger compression in the direction of the lone electron pairs by shear deformation was not observed. Raman spectra of Bi(12)SiO(20) were measured on powder samples during pressure decrease from 39.1(1) GPa down to ambient pressure and on single crystals during pressure increase up to 12.50(3) GPa. Density functional perturbation theory was used to compute Raman frequencies and intensities at ambient pressure and to investigate pressure-induced changes up to 50 GPa.
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http://dx.doi.org/10.1088/0953-8984/22/50/505401DOI Listing
December 2010

Assignment of the vibrational spectra of lithium hydroxide monohydrate, LiOH·H2O.

J Chem Phys 2011 Feb;134(8):084503

ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, United Kingdom.

The assignment of the vibrational spectra of lithium hydroxide monohydrate, LiOH·H(2)O, has been controversial for more than half-a-century. Here we show that only the combination of all three forms of vibrational spectroscopy: infrared, Raman and inelastic neutron scattering spectroscopies coupled with periodic-density functional theory calculations is able to satisfactorily assign the spectra. All previous work based on empirical criteria is, at least partially, incorrect. The librational modes of water do not follow the expected rock > wag > twist order and the calculations indicate that complete or partial deuterium substitution would not be useful in assigning the modes.
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http://dx.doi.org/10.1063/1.3553812DOI Listing
February 2011

Novel rhenium nitrides.

Phys Rev Lett 2010 Aug 20;105(8):085504. Epub 2010 Aug 20.

Geowissenschaften, Goethe-Universität, Altenhöferallee 1, D-60438 Frankfurt a M, Germany.

We report the synthesis, structure, and properties of novel bulk rhenium nitrides, hexagonal Re2N, and Re3N. Both phases have very high bulk moduli of >400 GPa, similar to the most incompressible binary transition-metal (TM) carbides and nitrides found to date. However, in contrast to other incompressible TM carbides and nitrides, Re3N is better placed for potential technological applications, as it can be formed at relatively moderate pressures (13-16 GPa) and temperatures (1600-2400 K).
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http://dx.doi.org/10.1103/PhysRevLett.105.085504DOI Listing
August 2010

Structure determination of adsorbed hydrogen on a real catalyst.

Chem Commun (Camb) 2010 May 5;46(17):2959-61. Epub 2010 Mar 5.

ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, UK OX11 0QX.

Here we show that neutron diffraction can experimentally measure distances involving hydrogen on real catalysts. This method is applicable to adsorbed hydrogen (and other hydrogenous species) on crystalline, nanocrystalline or amorphous materials, supported and unsupported metals as well as oxides and works at ambient temperature and pressure.
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http://dx.doi.org/10.1039/c001779eDOI Listing
May 2010
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