Chris J Pickard

University College London, Londinium, England, United Kingdom

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Publications (199)917.07 Total impact

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    ABSTRACT: The only known compound of sodium and hydrogen is archetypal ionic NaH. Application of high pressure is known to promote states with higher atomic coordination, but extensive searches for polyhydrides with unusual stoichiometry remain unsuccessful in spite of several theoretical predictions. Here we report the first observation of formation of polyhydrides of Na (NaH3 and NaH7) above 40 GPa and 2000 K. We combined synchrotron x-ray diffraction and Raman spectroscopy in a laser heated diamond anvil cell and theoretical Ab-Initio Random Structure search, which both agree in stable structures and compositions. Our results support the formation of multicenter bonding in a material with unusual stoichiometry. These results are applicable to the design of new energetic solids and high-temperature superconductors based on hydrogenrich materials.
    12/2014;
  • Jian Sun, Chris J. Pickard, Richard J. Needs
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    ABSTRACT: The cubic antifluorite structure comprises a face-centered cubic sublattice of anions with cations on the tetrahedral sites. The voids in the antifluorite structure that are crucial for superionicity in Li2O might also act as atomic traps. Trapping of guest atoms and small molecules within voids of a host structure leads to the formation of what are known as clathrate compounds. Here we investigate the possibility of trapping helium or larger neon guest atoms under pressure within alkali metal oxide and sulfide structures and find stable helium and neon bearing compounds even at pressures below 1 GPa. These structures are stabilized by a reduction in volume from incorporation of helium or neon atoms within the antifluorite structure. Our study suggests a novel class of alkali oxide and sulfide materials incorporating noble gas atoms that might potentially be useful for gas storage.
    09/2014;
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    ABSTRACT: Optical and synchrotron x-ray diffraction diamond anvil cell experiments have been combined with first principles theoretical structure predictions to investigate mixed N2 and H2 up to 55 GPa. We found the formation of oligomeric NxH (x>1) compounds using mechano- and photochemistry at pressures above 47 and 10 GPa, respectively, and room temperature. These compounds can be recovered to ambient pressure at T<130 K, whereas at room temperature, they can be metastable down to 3.5 GPa. Our results suggest new pathways for synthesis of environmentally benign high energy-density materials and alternative planetary ice.
    09/2014;
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    ABSTRACT: We study the effects of atomic vibrations on the solid-state chemical shielding tensor using first principles density functional theory calculations. At the harmonic level, we use a Monte Carlo method and a perturbative expansion. The Monte Carlo method is accurate but computationally expensive, while the perturbative method is computationally more efficient, but approximate. We find excellent agreement between the two methods for both the isotropic shift and the shielding anisotropy. The effects of zero-point quantum mechanical nuclear motion are important up to relatively high temperatures: at 500 K they still represent about half of the overall vibrational contribution. We also investigate the effects of anharmonic vibrations, finding that their contribution to the zero-point correction to the chemical shielding tensor is small. We exemplify these ideas using magnesium oxide and the molecular crystals L-alanine and beta-aspartyl-L-alanine. We therefore propose as the method of choice to incorporate the effects of temperature in solid state chemical shielding tensor calculations the perturbative expansion within the harmonic approximation. This approach is accurate and requires a computational effort that is about an order of magnitude smaller than that of dynamical or Monte Carlo approaches, so these effects might be routinely accounted for.
    The Journal of chemical physics. 08/2014; 141(13).
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    ABSTRACT: Solid-state (77)Se NMR measurements, first-principles molecular dynamics and DFT calculations of NMR parameters were performed to gain insight into the structure of selenium-rich GexSe(1-x) glasses. We recorded the fully-relaxed NMR spectra on natural abundance and 100% isotopically enriched GeSe4 samples, which led us to reconsider the level of structural heterogeneity in this material. In this paper, we propose an alternative procedure to initialise molecular dynamics runs for the chalcogenide glasses. The (77)Se NMR spectra calculated on the basis of the structural models deduced from these simulations are consistent with the experimental spectrum.
    Physical Chemistry Chemical Physics 07/2014; · 4.20 Impact Factor
  • Chris J Pickard, Richard J Needs
    Nature 07/2014; 511(7509):294-5. · 38.60 Impact Factor
  • Chris J. Pickard, Richard J. Needs
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    ABSTRACT: Ab initio random structure searching (AIRSS) and density functional theory methods are used to predict structures of calcium and magnesium carbonate (CaCO$_3$ and MgCO$_3$) at high pressures. We find a previously unknown CaCO$_3$ structure which is more stable than the aragonite and "post aragonite" phases in the range 32--48 GPa. At pressures from 67 GPa to well over 100 GPa the most stable phase is a previously unknown CaCO$_3$ structure of the pyroxene type with fourfold coordinated carbon atoms. We also predict a stable structure of MgCO$_3$ in the range 85--101 GPa. Our results lead to a revision of the phase diagram of CaCO$_3$ over more than half the pressure range encountered within the Earth's mantle, and smaller changes to the phase diagram of MgCO$_3$. We predict CaCO$_3$ to be more stable than MgCO$_3$ in the Earth's mantle above 100 GPa, and that CO$_2$ is not a thermodynamically stable compound under deep mantle conditions. Our results have significant implications for understanding the Earth's deep carbon cycle.
    07/2014;
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    Georg Schusteritsch, Chris J. Pickard
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    ABSTRACT: We present here a fully first-principles method for predicting the atomic structure of interfaces. Our method is based on the {\it ab initio} random structure searching (AIRSS) approach, applied here to treat two dimensional defects. The method relies on repeatedly generating random structures in the vicinity of the interface and relaxing them within the framework of density functional theory (DFT). The method is simple, requiring only a small set of parameters that can be easily connected to the chemistry of the system of interest, and efficient, ideally adapted to high-throughput first-principles calculations on modern parallel architectures. Being first-principles, our method is transferable, an important requirement for a generic computational method for the determination of the structure of interfaces. Results for two structurally and chemically very different interfaces are presented here, grain boundaries in graphene and grain boundaries in strontium titanate (SrTiO$_3$). We successfully find a previously unknown low energy grain boundary structure for the graphene system, as well as recover the previously known higher energy structures. For the SrTiO$_3$ system we study both stoichiometric and non-stoichiometric compositions near the grain boundary and find previously unknown low energy structures for all stoichiometries. We predict that these low energy structures have long-range distortions to the ground state crystal structure emanating into the bulk from the interface.
    Physical Review B 07/2014; 90(3). · 3.66 Impact Factor
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    ABSTRACT: Zirconium-based alloys are used in water-cooled nuclear reactors for both nuclear fuel cladding and structural components. Under this harsh environment, the main factor limiting the service life of zirconium cladding, and hence fuel burn-up efficiency, is water corrosion. This oxidation process has recently been linked to the presence of a sub-oxide phase with well-defined composition but unknown structure at the metal–oxide interface. In this paper, the combination of first-principles materials modeling and high-resolution electron microscopy is used to identify the structure of this sub-oxide phase, bringing us a step closer to developing strategies to mitigate aqueous oxidation in Zr alloys and prolong the operational lifetime of commercial fuel cladding alloys.
    Advanced Engineering Materials 06/2014; · 1.61 Impact Factor
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    ABSTRACT: We report experimental and theoretical evidence that solid molecular ammonia becomes unstable at room temperature and high pressures and transforms into an ionic crystalline form. This material has been characterized in both hydrogenated (NH 3) and deuterated (ND 3) ammonia samples up to about 180 and 200 GPa, respectively, by infrared absorption, Raman spectroscopy, and x-ray diffraction. The presence of a new strong infrared absorption band centered at 2500 cm −1 in NH 3 (1900 cm −1 in ND 3) is in line with previous theoretical predictions regarding the ionization of ammonia molecules into NH 2 − and NH 4 + ions. The experimental data suggest the coexistence of two crystalline ionic forms, which our ab initio structure searches predict to be the most stable at the relevant pressures. The ionic crystalline form of ammonia appears stable at low temperatures, which contrasts with the behavior of water in which no equivalent crystalline ionic phase has been found.
    Physical Review B 05/2014; 89:174103. · 3.66 Impact Factor
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    ABSTRACT: Two hexanuclear niobium halide cluster compounds with a [Nb6×12]2+ (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable Bo field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of 95Mo nuclei in structurally related compounds containing [Mo6Br8]4+ cluster cores. Experimentally determined δiso(93Nb) values are in the range from 2400 to 3000 ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (Vzz and δ33) coinciding with the molecular four-fold axis of the [Nb6×12]2+ unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (Vzz) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (Vyy) and the largest CS principal axis (δ11) are oriented in the radial direction with respect to the centre of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by 79/81Br NQR and 35Cl solid-state NMR measurements.
    Solid State Nuclear Magnetic Resonance 05/2014; · 2.10 Impact Factor
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    Zhu Li, Hanyu Liu, Chris J. Pickard, Guangtian Zou, Yanming Ma
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    ABSTRACT: Studies of the Earth's atmosphere have shown that more than 90% of the expected amount of Xe is depleted, a finding often referred to as the ‘missing Xe paradox’. Although several models for a Xe reservoir have been proposed, whether the missing Xe could be contained in the Earth's inner core has not yet been answered. The key to addressing this issue lies in the reactivity of Xe with Fe/Ni, the main constituents of the Earth's core. Here, we predict, through first-principles calculations and unbiased structure searching techniques, a chemical reaction of Xe with Fe/Ni at the temperatures and pressures found in the Earth's core. We find that, under these conditions, Xe and Fe/Ni can form intermetallic compounds, of which XeFe3 and XeNi3 are energetically the most stable. This shows that the Earth's inner core is a natural reservoir for Xe storage and provides a solution to the missing Xe paradox.
    Nature Chemistry 04/2014; · 21.76 Impact Factor
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    Andrew J. Morris, C. P. Grey, C. J Pickard
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    ABSTRACT: Density-functional-theory (DFT) calculations have been performed on the Li-Si and Li-Ge systems. Lithiated Si and Ge, including their metastable phases, play an important technological r\^ole as Li-ion battery (LIB) anodes. The calculations comprise structural optimisations on crystal structures obtained by swapping atomic species to Li-Si and Li-Ge from the X-Y structures in the International Crystal Structure Database, where X={Li,Na,K,Rb,Cs} and Y={Si,Ge,Sn,Pb}. To complement this at various Li-Si and Li-Ge stoichiometries, ab initio random structure searching (AIRSS) was also performed. Between the ground-state stoichiometries, including the recently found Li$_{17}$Si$_{4}$ phase, the average voltages were calculated, indicating that germanium may be a safer alternative to silicon anodes in LIB, due to its higher lithium insertion voltage. Calculations predict high-density Li$_1$Si$_1$ and Li$_1$Ge$_1$ $P4/mmm$ layered phases which become the ground state above 2.5 and 5 GPa respectively and reveal silicon and germanium's propensity to form dumbbells in the Li$_x$Si, $x=2.33-3.25$ stoichiometry range. DFT predicts the stability of the Li$_{11}$Ge$_6$ $Cmmm$, Li$_{12}$Ge$_7$ $Pnma$ and Li$_7$Ge$_3$ $P32_12$ phases and several new Li-Ge compounds, with stoichiometries Li$_5$Ge$_2$, Li$_{13}$Ge$_5$, Li$_8$Ge$_3$ and Li$_{13}$Ge$_4$.
    Physical Review B 02/2014; 90(5). · 3.66 Impact Factor
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    ABSTRACT: Solid He is studied in the pressure and temperature ranges 1-40 TPa and 0-10 000 K using first-principles methods. Anharmonic vibrational properties are calculated within a self-consistent field framework, including the internal and free energies, density-pressure relation, stress tensor, thermal expansion, and the electron-phonon coupling renormalization of the electronic band gap. We find that an accurate description of electron-phonon coupling requires us to use a nonperturbative approach. The metallization pressure of 32.9 TPa at 0 K is larger than found previously. The vibrational effects are large; for example, at P=30 TPa the band gap is increased by 2.8 eV by electron-phonon coupling and a further 0.1 eV by thermal expansion compared to the static value. The implications of the calculated metallization pressure for the cooling of white dwarfs are discussed.
    Physical Review Letters 02/2014; 112(5):055504. · 7.73 Impact Factor
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    ABSTRACT: We report experimental and theoretical evidence that solid molecular ammonia becomes unstable at room temperature and high pressures and transforms into an ionic crystalline form. This material has been characterised in both hydrogenated NH3 and deuterated ND3 ammonia samples up to about 180 and 200 GPa, respectively, by infrared absorption, Raman spectroscopy and x-ray diffraction. The presence of a new, strong IR absorption band centered at 2500 cm-1 in NH3 (1900 cm-1 in ND3) signals the ionization of ammonia molecules into NH2- and NH4+ ions, in line with previous theoretical predictions. We find experimental evidence for a coexistence of two crystalline ionic forms, which our ab-initio structure searches predict to be the most stable at the relevant pressures. The ionic crystalline form of ammonia is stable at low temperatures, which contrasts with the behaviour of water in which no equivalent crystalline ionic phase has been found.
    01/2014;
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    ABSTRACT: Periodic density functional theory (DFT) calculations have recently emerged as a popular tool for assigning solid-state nuclear magnetic resonance (NMR) spectra. However, in order for the calculations to yield accurate results, accurate structural models are also required. In many cases the structural model (often derived from crystallographic diffraction) must be optimised (i.e., to an energy minimum) using DFT prior to the calculation of NMR parameters. However, DFT does not reproduce weak long-range "dispersion" interactions well, and optimisation using some functionals can expand the crystallographic unit cell, particularly when dispersion interactions are important in defining the structure. Recently, dispersion-corrected DFT (DFT-D) has been extended to periodic calculations, to compensate for these missing interactions. Here, we investigate whether dispersion corrections are important for aluminophosphate zeolites (AlPOs) by comparing the structures optimised by DFT and DFT-D (using the PBE functional). For as-made AlPOs (containing cationic structure-directing agents (SDAs) and framework-bound anions) dispersion interactions appear to be important, with significant changes between the DFT and DFT-D unit cells. However, for calcined AlPOs, where the SDA-anion pairs are removed, dispersion interactions appear much less important, and the DFT and DFT-D unit cells are similar. We show that, while the different optimisation strategies yield similar calculated NMR parameters (providing that the atomic positions are optimised), the DFT-D optimisations provide structures in better agreement with the experimental diffraction measurements. Therefore, it appears that DFT-D calculations can, and should, be used for the optimisation of calcined and as-made AlPOs, in order to provide the closest agreement with all experimental measurements.
    Physical Chemistry Chemical Physics 01/2014; · 4.20 Impact Factor
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    ABSTRACT: 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.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 01/2014; 372(2011):20130270. · 2.89 Impact Factor
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    ABSTRACT: There is a great deal of fundamental and practical interest in the possibility of inducing superconductivity in a monolayer of graphene. But while bulk graphite can be made to superconduct when certain metal atoms are intercalated between its graphene sheets, the same has not been achieved in a single layer. Moreover, there is a considerable debate about the precise mechanism of superconductivity in intercalated graphite. Here we report angle-resolved photoelectron spectroscopy measurements of the superconducting graphite intercalation compound CaC6 that distinctly resolve both its intercalant-derived interlayer band and its graphene-derived π* band. Our results indicate the opening of a superconducting gap in the π* band and reveal a substantial contribution to the total electron-phonon-coupling strength from the π*-interlayer interband interaction. Combined with theoretical predictions, these results provide a complete account for the superconducting mechanism in graphite intercalation compounds and lend support to the idea of realizing superconducting graphene by creating an adatom superlattice.
    Nature Communications 01/2014; 5:3493. · 10.74 Impact Factor
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    ABSTRACT: We present OptaDOS, a program for calculating core-electron and low-loss electron energy loss spectra (EELS) and optical spectra along with total-, projected- and joint-density of electronic states (DOS) from single-particle eigenenergies and dipole transition coefficients. Energy-loss spectroscopy is an important tool for probing bonding within a material. Interpreting these spectra can be aided by first principles calculations. The spectra are generated from the eigenenergies through integration over the Brillouin zone. An important feature of this code is that this integration is performed using a choice of adaptive or linear extrapolation broadening methods which we show produces higher accuracy spectra than standard fixed-width Gaussian broadening. OptaDOS may be straightforwardly interfaced to any electronic structure code. OptaDOS is freely available under the GNU General Public licence from http://www.optados.org. Program summary Program title:OptaDOS Catalogue identifier: AESK_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AESK_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licencing provisions: GNU General Public License, version 3 No. of lines in distributed program, including test data, etc.: 110 299 No. of bytes in distributed program, including test data, etc.: 1 889 705 Distribution format: tar.gz Programming language: Fortran 95. Computer: Any architecture with a Fortran 95 compiler. Operating system: Linux, Mac OS X. Has the code been vectorised or parallelised?: Yes, using MPI RAM: 10 MB Word size: 32 or 64 Classification: 7.2, 7.3. External routines: MPI to run in parallel, CASTEP or any other electronic structure code capable of generating single-point eigenenergies and dipole transition coefficients. Nature of problem: Many properties of materials can be described using integration over the Brillouin zone such as core-level and low-loss EELS and optical spectra. This integration is performed computationally using a grid of kk-points. The discrete energy eigenvalues must be interpolated into a continuous spectra. The most common method broadens the eigenvalues using a Gaussian function. Gaussian broadening suffers from slow convergence with number of kk-points and a difficulty in resolving fine spectral features. Solution method: OptaDOS improves the underlying Brillouin zone integration beyond fixed-width Gaussian broadening by using band gradients to perform adaptive and linearly extrapolated broadening. This increases the resolution of the predicted spectra. Unusual features: Simple and user-friendly input system. Along with the usual band energies, band gradients are used to generate the linear extrapolation and adaptive broadening schemes producing a superior output able to represent both dispersive and localised bands concurrently. Additional comments: The input data to OptaDOS are single-particle eigenenergies and dipole transition coefficients. OptaDOS has an interface to obtain these from the CASTEP plane wave density-functional theory (DFT) code. The interfacing of OptaDOS with other electronic structure codes, which are also capable of generating such inputs, is currently being undertaken Running time: A few seconds to ∼10 min.
    Computer Physics Communications 01/2014; 185(5):1477–1485. · 2.41 Impact Factor
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    ABSTRACT: Carbon monoxide and nitrogen are among the potentially interesting high-energy density materials. However, in spite of the physical similarities of the molecules, they behave very differently at high pressures. Using density functional theory and structural prediction methods, we examine the ability of these systems to combine their respective properties and form novel mixed crystalline phases under pressures of up to 100 GPa. Interestingly, we find that CO catalyzes the molecular dissociation of N_{2}, which means mixed structures are favored at a relatively low pressure (below 18 GPa), and that a three-dimensional framework with Pbam symmetry becomes the most stable phase above 52 GPa, i.e., at much milder conditions than in pure solid nitrogen. This structure is dynamically stable at ambient pressure and has an energy density of approximately 2.2 kJ g^{-1}, making it a candidate for a high-energy density material, and one that could be achieved at less prohibitive experimental conditions.
    Physical Review Letters 12/2013; 111(23):235501. · 7.73 Impact Factor

Publication Stats

4k Citations
917.07 Total Impact Points

Institutions

  • 2009–2014
    • University College London
      • Department of Physics and Astronomy
      Londinium, England, United Kingdom
    • The University of Tokyo
      • Department of Systems Innovation
      Tokyo, Tokyo-to, Japan
    • University at Buffalo, The State University of New York
      • Department of Chemistry
      Buffalo, NY, United States
  • 2013
    • The University of York
      • Department of Physics
      York, England, United Kingdom
  • 2011–2013
    • Ruhr-Universität Bochum
      • Theoretical Chemistry
      Bochum, North Rhine-Westphalia, Germany
    • Lancaster University
      • Department of Physics
      Lancaster, ENG, United Kingdom
  • 2012
    • Linyi University
      Yichow, Shandong Sheng, China
  • 2009–2012
    • The University of Warwick
      • Department of Physics
      Warwick, ENG, United Kingdom
  • 2001–2012
    • Pierre and Marie Curie University - Paris 6
      • Laboratoire de Chimie de la Matière Condensée de Paris (LCMCP)
      Paris, Ile-de-France, France
    • Christian-Albrechts-Universität zu Kiel
      Kiel, Schleswig-Holstein, Germany
    • Fachhochschule Kiel
      Kiel, Schleswig-Holstein, Germany
  • 1998–2011
    • University of Cambridge
      • • Department of Chemistry
      • • Department of Physics: Cavendish Laboratory
      Cambridge, ENG, United Kingdom
  • 2010
    • Stony Brook University
      • Department of Chemistry
      Stony Brook, NY, United States
  • 2008–2010
    • University of St Andrews
      • • School of Chemistry
      • • School of Physics and Astronomy
      Saint Andrews, SCT, United Kingdom
  • 2007–2009
    • Scottish Universities Physics Alliance
      Glasgow, Scotland, United Kingdom
    • Ecole normale supérieure de Lyon
      • UMR 5182 - Laboratoire de Chimie
      Lyons, Rhône-Alpes, France
  • 2006–2009
    • Durham University
      • Department of Chemistry
      Durham, ENG, United Kingdom
  • 2007–2008
    • Tyndall National Institute
      Corcaigh, Munster, Ireland
  • 2006–2007
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, Germany
  • 2005
    • Université Paris-Est Marne-la-Vallée
      Champs, Île-de-France, France
  • 2004
    • University of Wuerzburg
      Würzburg, Bavaria, Germany