Browsing by Author "Kearley, GJ"

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    Ab initio determination of the structure of the ferroelectric phase of SrTi18O3
    (Australian Institute of Physics, 2011-02-02) Bartkowiak, M; Kearley, GJ; Yethiraj, M; Mulders, AM
    Strontium titanate (SrTi18O3) is known to display a quantum paraelectric behavior. Its dielectric constant saturates at low temperatures and does not increase with cooling due to quantum fluctuations present in the system. Only in 1999 Itoh et al [1] discovered that substituting regular 16O with the 18O isotope stabilizes the system and allows a transition into a ferroelectric phase below 23 K. The mechanism of the transition and the structure of the new phase have not been conclusively determined by experiment. The new phase displays ferroelectric properties and there are new peaks present in the Raman spectrum. However, diffraction experiments indicate that the structural distortion accompanying the transition is minimal, while Raman and NMR measurements provide evidence for both the order-disorder mechanism and the displacive mechanism to be an applicable explanation of the transition. We applied density functional theory calculations and lattice dynamics analysis to show that the paraelectric tetragonal phase of the regular SrTiO3 is inherently unstable. By distorting the structure along the direction of the soft mode present at the centre of the Brillouin zone we obtained an orthorhombic, ferroelectric structure of SrTiO3 which is energetically favourable over the paraelectric one. Lattice dynamics calculations show that our new structure is stable and the frequencies of the phonon modes present in it are in good agreement with the experimental values published so far.
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    Ab initio parametrized polarizable force field for rutile-type SnO2
    (Springer, 2012-05-01) Miiller, W; Kearley, GJ; Ling, CD
    We report a new, polarizable classical force field for the rutile-type phase of SnO2, casserite. This force field has been parametrized using results from ab initio (density functional theory) calculations as a basis for fitting. The force field was found to provide structural, dynamical and thermodynamic properties of tin oxide that compare well with both ab initio and experimental results at ambient and high pressures. © 2012, Springer.
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    Adsorption of CO2 and CD4 in UiO-66: a combination of neutron diffraction and modelling
    (Australian Institute of Physics, 2015-02-06) Chevreau, H; Laing, W; Kearley, GJ; Duyker, SG; D’Alessandro, DM; Peterson, VK
    Over the last twenty years, tremendous progress has been achieved in the field of Metal Organic Frameworks. Among these materials, the zirconium terephthalate UiO-66(Zr) [1] has attracted a growing attention because of its interesting thermal, chemical and water stability and has shown to be a promising material for the separation of CO2/CH4 gas mixtures. In order to get a better understanding of its sorption behavior towards CO2 and CH4, a Neutron Powder Diffraction (NPD) investigation of UiO-66 loaded with sequential doses of CO2 and CD4 has been carried out on the High Resolution Powder Diffractometer instrument “Echidna” at the OPAL reactor (ANSTO, Sydney). In total, three adsorption sites for CO2 and three adsorption sites for CD4 within the UiO- 66(Zr) have been located by neutron powder-diffraction then characterised by a combination of first-principles Density Functional Theory (DFT) calculations and Quantum Atoms In Molecules (QTAIM) theory. An example of the first CO2 adsorption site is given in figure 1.
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    Alkali metal dynamics in the β-pyrochlores A0s206 (A = K, Rb, Cs) and their prospects as thermoelectric materials
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2012-11-07) Shoko, E; Peterson, VK; Kearley, GJ
    The β-pyrochlores A0s206 (A = K, Rb, Cs)are extensively studied because of their superconducting properties that are believed to be linked to the rattling modes of the alkali metals. The rattling of small atoms encaged in large cavities has been mown to be important for the thermoelectric performance of both clathrates and skutterudites, and fuels our interest in the osmates. The vibrational dynamics of the K atom in KOs206, differs significantly from those of Rb and Cs in their respective pyrochlores and a complex low-energy signature was recently observed using inelastic neutron scattering (INS). To gain insight into the dynamics of the alkai metais in this system, we studied these materials using ab initio molecular dynamics (MD) simulations validated against experimental INS spectra. Combining the results of MD simulations with phonon dispersion curves, calculated from ab initio lattice dynamics (LD) enabled the prediction of thermoelectric properties for these materials using the Boltzmann transport equation. This talk will discuss the alkali metal dynamics in terms of their relevance in the potential development of this class of materials for thermoelectric applications.
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    Analysis of neutron-scattering data using atomistic modelling methods
    (Australian Nuclear Science and Technology Organisation, 2009) Kearley, GJ; Avdeev, M
    In many cases the scientific value of neutron-scattering data can be increased considerably by using atomistic or molecular modelling methods as an aid to understanding the experimental data. We give examples of how modelling helps to choose between ambiguous analyses, provide starting models for analytical methods and give understanding of how the structure and dynamics of a material relate to its function. © Australian Nuclear Science and Technology Organisation
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    Anomalous lattice parameter increase in alkali earth aluminium substituted tungsten defect pyrochlores
    (Elsevier, 2009-03) Thorogood, GJ; Kennedy, BJ; Peterson, VK; Elcombe, MM; Kearley, GJ; Hanna, JV; Luca, V
    The structures of the defect pyrochlores AAl(0.33)W(1.67)O(6) where A = K, Rb or Cs have been investigated using X-ray and neutron powder diffraction methods as well as the ab initio modelling program VASP The three cubic pyrochlores exhibit a non-linear increase in lattice parameter with respect to ionic radius of the A cation as a consequence of displacive disorder of the A-type cations. Solid state Al-27 MAS NMR studies of this pyrochlore system reveal shifts in the delta-21-22 ppm range that are indicative of pseudo-5 coordinate Al environments and emanate frorn distorted Al octahedral with one abnormally long Al. O bond. Solid state K-39, Rb-85, Rb-87 and Cs-133 MAS and static NMR Studies reflect the local cation disorder demonstrated in the structural Studies. © 2008, Elsevier Ltd.
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    Atomistic model of DNA: phonons and base-pair opening
    (American Physical Society, 2007-09) Merzel, F; Fontaine-Vive, F; Johnson, MR; Kearley, GJ
    A fully atomistic model of B-DNA using the CHARMM (chemistry at Harvard molecular mechanics) force field is presented. Molecular dynamics simulations were used to prepare an equilibrium structure. The Hessian of interatomic forces obtained from CHARMM for the equilibrium structure was used as input to a large scale phonon calculation. The calculated dispersion relations at low frequency are compared with recently published experimental data, which shows the model to have good accuracy for the low frequency, vibrational modes of DNA. These are discussed in the context of base-pair opening. In addition to the widely reported modes at, or below, ~12.5 meV, a continuous band of modes with strong base-pair opening character is found up to 40 meV, which coincides with the typical denaturation temperature of DNA. © 2007, American Physical Society
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    Cation order/disorder and local structures in alkaline earth pyrochlores
    (International Union of Crystallography (IUCr), 2008-08-23) Thorogood, GJ; Kennedy, BJ; Peterson, VK; Elcombe, MM; Kearley, GJ; Hanna, JV
    Materials that form the A2-yB2O7-x pyrochlore structure have various applications including use as catalysts, fuel cells, piezoelectrics, ferroelectric devices and ferro-magnets; have a wide range of electrical and ionic conductivities, including metallic, semi and super; can be used in nuclear-waste immobilisation due to radiation toughness, and have ion exchange properties. The degree of disorder of the A-cation is important and may play the major role in the effectiveness of these materials ion exchange properties; and contribute to the high relative permittivities displayed by (Bi,Zn)2(Zn,Nb)2O7. Previous Neutron and X-ray diffraction studies suggest that the disorder involves displacement of the A-cation along the six <112> or <110> directions. Our observation of diffuse scattering in electron diffraction patterns of CsTi0.5W1.5O6 pyrochlores suggests there are strong local correlations among the disordered ions. Movement of O at 48f away from 0.375 reduces the interaction between the two networks and so may increase the amount of disorder of the A-cation. The structures of the defect pyrochlores AAl0.33W1.67O6 where A= K, Rb or Cs have been investigated using an array of advanced structural probes; X-ray and neutron diffraction methods, NMR spectroscopy as well as the ab-initio modeling using VASP. The structures do not show a simple correlation between the radius of the A-type cation and the cubic lattice parameter. Our structural studies suggest that this may reflect the degree of local disorder of the A-cation. The results of these studies will be presented in this presentation. © 2008 International Union of Crystallography
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    Central-atom size effects on the methyl torsions of group XIV tetratolyls
    (Wiley-V C H Verlag GMBH, 2012-10-01) Ng, MCC; Harper, JB; Stampfl, APJ; Kearley, GJ; Rols, S; Stride, JA
    The Group XIV tetratolyl series X(C6H4-CH3)4 (X=C, Si, Ge, Sn, Pb) were studied by using inelastic neutron scattering to measure the low-energy phonon spectra to directly access the methyl-group torsional modes. The effect of increased molecular radius as a function of the size of the central atom was shown to have direct influence on the methyl dynamics, reinforced with the findings of molecular dynamics and contact surface calculations, based upon the solid-state structures. The torsional modes in the lightest analogue were found to be predominantly intramolecular: the Si and Ge analogues have a high degree of intermolecular methylmethyl group interactions, whilst the heaviest analogues (Sn and Pb) showed pronounced intermolecular methyl interactions with the whole phonon bath of the lattice modes. © 2012, Wiley-VCH Verlag GmbH & Co. KGaA
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    Collagen and component polypeptides: low frequency and amide vibrations.
    (Elsevier, 2009-01-27) Fontaine-Vive, F; Merzel, F; Johnson, MR; Kearley, GJ
    Collagen is a fibrous protein, which exists widely in the human body. The biomechanical properties of collagen depend on its triple helix structure and the corresponding low frequency vibrations. We use first-principles, density functional theory methods and analytical force fields to investigate the molecular vibrations of a model collagen compound, the results being validated by comparison with published, inelastic neutron scattering data. The results from these atomistic simulations are used at higher frequency to Study the Amide I and V vibrations and therefore the vibrational signature of secondary and tertiary structure formation. In addition to collagen, its component homopolymers, poly-glycine and poly-proline are also studied. The Amide V vibration of glycine is strongly modified in going from the single helix of poly-glycine II to the triple helix of collagen. The collagen models are hydrated and this work allows LIS to discuss the relative merits of density functional theory and force field methods when tackling complex, partially crystalline systems. © 2008, Elsevier Ltd.
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    Concentration-dependent binding of CO2 and CD4 in UiO-66 (Zr)
    (American Chemical Society, 2015-04-02) Chevreau, H; Liang, WB; Kearley, GJ; Duyker, SG; D'Alessandro, DM; Peterson, VK
    Porous metal–organic frameworks (MOFs) have emerged as promising materials for the capture of carbon dioxide (CO2) and its separation from methane (CH4) during the industrially important “sweetening” of sour natural-gas. The excellent thermal and chemical stability of the highly porous UiO-66(Zr) material, combined with good selectivity for CO2 over CH4, makes this material a prime candidate for such applications. Using a combination of neutron powder-diffraction and density-functional theory, we examine the details of the binding of CO2 and CH4 in UiO-66(Zr) over the industrially relevant 3.6–9.0 mmol/g concentration range, corresponding to the material that is half to fully saturated with CO2. This work builds on the previously reported preferred site for CO2 and CH4 in UiO-66(Zr), establishing further sites and determining the strength and nature of the guest–host interaction at these. We find the UiO-66(Zr)···CO2 interactions are significantly affected by the concentration of CO2 as the binding of CO2 is enhanced by interguest interactions. © 2015 American Chemical Society
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    Copper selenide: soft phonon modes and superionic phase transition
    (Australian Institute of Physics, 2010-02-03) Danilkin, SA; Yethiraj, M; Kearley, GJ
    This paper reports lattice dynamical measurements of Cu1.8Se superionic conductor having structure of the superionic α-phase at ambient temperature. Cu2δSe is a mixed ionic-electronic conductor with a superionic transition at 414K in stoichiometric compound Cu2Se. At room temperature the superionic α -phase exists in the composition range from δ = 0.15 to 0.25. The important features of the Cu1.8Se compound is the ordering of Cu atoms observed at ambient temperature [1] which is described as “disordered” α- phase in the literature and presence of low-energy transverse acoustic (TA) modes [2]. Measurements of phonon dispersion curves were performed with the new triple-axis spectrometer, TAIPAN, at the OPAL reactor [3]. We found that TA [100], TA [111] and TA1 [110] phonon branches demonstrate a decrease in frequency at wavevectors q/qm > 0.5 rather than the flattening observed previously. Results are compared with calculated density functional theoretical calculations showing the presence of unstable soft mode related to ordering of Cu atoms in Cu1.8Se at room temperature followed by α- β phase transition at a lower temperature. Superstructure arising from the ordering causes effects similar to the folding of the Brillouin zone, although phonon intensities at new Brillouin zone centres are weak. The coupling of low-energy phonon modes with displacement of mobile ions can explain the strong damping of phonons at q/qm > 0.5.
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    Coupled Li1+/Nb5+ and O2-/F- ordering on the Na and Cl sites of the average NaCl structure of Li4NbO4F
    (Elsevier, 2009-05) Norén, L; Withers, RL; Goossens, DJ; Elcombe, MM; Kearley, GJ
    The average, as well as the cation and anion 'disordered', crystal Structure of Li4NbO4F has been carefully investigated via coupled neutron and X-ray powder diffraction studies as well as via electron diffraction studies. The existence of a spectacular highly structured diffuse intensity distribution in the latter provides strong evidence for coupled Li1+/Nb5+ and O2-/F- ordering on the Na and Cl sites of the average NaCl structure of Li4NbO4F. Bond valence sum calculations have been used to investigate local crystal chemistry as well as to Suggest plausible local crystal chemical constraints while ab initio DFT based theoretical calculations of a 2 x 2 x 2 supercell have been carried Out in order to provide additional insight into the local crystal chemistry of this compound. © 2009, Elsevier Ltd.
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    Density functional calculations of potential energy surface and charge transfer integrals in molecular triphenylene derivative HAT(6)
    (Springer, 2010-03) Zbiri, M; Johnson, MR; Kearley, GJ; Mulder, FM
    We investigate the effect of structural fluctuations on charge transfer integrals, overlap integrals, and site energies in a system of two stacked molecular 2,3,6,7,10,11-hexakishexyloxytriphenylene (HAT(6)), which is a model system for conducting devices in organic photocell applications. A density functional based computational study is reported. Accurate potential energy surface calculations are carried out using an improved meta-hybrid density functional to determine the most stable configuration of the two weakly bound HAT(6) molecules. The equilibrium parameters in terms of the twist angle and co-facial separation are calculated. Adopting the fragment approach within the Kohn-Sham density functional framework, these parameters are combined to a lateral slide, to mimic structural/conformational fluctuations and variations in the columnar phase. The charge transfer and spatial overlap integrals, and site energies, which form the matrix element of the Kohn-Sham Hamiltonian are derived. It is found that these quantities are strongly affected by the conformational variations. The spatial overlap between stacked molecules is found to be of considerable importance since charge transfer integrals obtained using the fragment approach differ significantly from those using the dimer approach. © 2010, Springer.
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    Density functional modelling of silicate and aluminosilicate dimerisation solution chemistry
    (Royal Society of Chemistry, 2011-02-14) White, CE; Provis, JL; Kearley, GJ; Riley, DP; van Deventer, JSJ
    Common throughout sol–gel chemistry, including zeolite synthesis, aluminosilicate glass formation and geopolymerisation, is the process of inorganic oxide polymerisation and deprotonation. In this investigation, some of the fundamental reactions occurring during zeolite synthesis and geopolymerisation at high pH are investigated using density functional theory (DFT), and are compared with: (i) existing values reported in the literature, and (ii) new and previously published DFT-derived data for similar silicate reactions at near-neutral pH. From the results it is seen that the energetics of deprotonation and dimerisation reactions depend greatly on the pH value, and these results correlate well with existing experimental values and trends. Hence, this investigation exemplifies that an accurate replication of the solution environment is crucial for obtaining useful theoretical results for species dissolved in non-ideal environments. © 2011, Royal Society of Chemistry
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    Direct view on nanoionic proton mobility
    (Wiley-Blackwell, 2011-04-22) Chan, WK; Haverkate, LA; Borghols, WJH; Wagemaker, M; Picken, SJ; van Eck, ERH; Kentgens, APM; Johnson, MR; Kearley, GJ; Mulder, FM
    The field of nanoionics is of great importance for the development of superior materials for devices that rely on the transport of charged ions, like fuel cells, batteries, and sensors. Often nanostructuring leads to enhanced ionic mobilities due to the induced space-charge effects. Here these large space-charge effects occurring in composites of the proton-donating solid acid CsHSO4 and the proton-accepting TiO2 or SiO2 are studied. CsHSO4 is chosen for this study because it can operate effectively as a fuel-cell electrolyte at elevated temperature while its low-temperature conductivity is increased upon nanostructuring. The composites have a negative enthalpy of formation for defects involving the transfer of protons from the acid to the acceptor. Very high defect densities of up to 10% of the available sites are observed by neutron diffraction. The effect on the mobility of the protons is observed directly using quasielastic neutron scattering and nuclear magnetic resonance spectroscopy. Surprisingly large fractions of up to 25% of the hydrogen ions show orders-of-magnitude enhanced mobility in the nanostructured composites of TiO2 or SiO2, both in crystalline CsHSO4 and an amorphous fraction.© 2011, Wiley-Blackwell. The definitive version is available at www3.interscience.wiley.com
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    Dispersive kinetics in discotic liquid crystals
    (American Physical Society, 2010-11-22) Kruglova, O; Mulder, FM; Kearley, GJ; Picken, SJ; Stride, JA; Paraschiv, I; Zuilhof, H
    The dynamics of the discotic liquid-crystalline system, hexakis (n-hexyloxy) triphenylene (HAT6), is considered in the frame of the phenomenological model for rate processes proposed by Berlin. It describes the evolution of the system in the presence of the long-time scale correlations in the system, and we compare this with experimental quasielastic neutron scattering of the molecular assembly of HAT6 in the columnar phase. We interpret the parameters of this model in terms of nonextensive thermodynamics in which rare events in the local fast dynamics of some parts of the system control the slower dynamics of the larger molecular entity and lead to a fractional diffusion equation. The importance of these rare local events to the overall dynamics of the system is linked to the entropic index, this being obtained from the data within the model approach. Analysis of the waiting-time dependence from momentum transfer reveals a Lévy distribution of jump lengths, which allows us to construct the van Hove correlation function for discotic liquid-crystalline system. © 2010, American Physical Society
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    Driving forces for the phase transition of CuQ2-TCNQ molecular crystals
    (Royal Society of Chemistry, 2016-05-23) Yu, DH; Kearley, GJ; Liu, G; Mole, RA; McIntyre, GJ; Tao, X
    The driving forces for the phase transition and relative stability of the two forms of CuQ2-TCNQ molecular crystals have been studied using inelastic neutron scattering (INS), density functional theory (DFT), and Hirshfeld surface analysis. DFT molecular dynamics (MD) simulations show that form-II has a lower enthalpy, but with increasing temperature form-I becomes thermodynamically stable due to the greater entropy. INS and MD simulations both show that the entropy of the hydrogen-bond network that holds molecules together within layers is higher in form-I. The interlayer π–π interactions are also weaker in form-I, leading to an overall “loosening” of the structure. The phase transition is kinetically hindered by the requirement to re-optimize the orientation of the layers. The strong H-bond interactions keep the in-plane atomic arrangement stable, while the weak interlayer π–π interactions provide the coupling between layers during the phase-transition. This subtle interplay of the two interactions maintains the integrity of the crystal upon phase transition even with dramatic physical dimension changes. © The Royal Society of Chemistry 2016
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    Dynamical transition in a large globular protein: macroscopic properties and glass transition
    (Elsevier, 2010-01) Kealley, CS; Sokolova, AV; Kearley, GJ; Kemner, E; Russina, M; Faraone, A; Hamilton, WA; Gilbert, EP
    Hydrated soy-proteins display different macroscopic properties below and above approximately 25% moisture. This is relevant to the food industry in terms of processing and handling. Quasi-elastic neutron spectroscopy of a large globular soy-protein, glycinin, reveals that a similar moisture-content dependence exists for the microscopic dynamics as well. We find evidence of a transition analogous to those found in smaller proteins, when investigated as a function of temperature, at the so-called dynamical transition. In contrast, the glass transition seems to be unrelated. Small proteins are good model systems for the much larger proteins because the relaxation characteristics are rather similar despite the change in scale. For dry samples, which do not show the dynamical transition, the dynamics of the methyl group is probably the most important contribution to the QENS spectra, however a simple rotational model is not able to explain the data. Our results indicate that the dynamics that occurs above the transition temperature is unrelated to that at lower temperatures and that the transition is not simply related to the relaxation rate falling within the spectral window of the spectrometer. © 2010, Elsevier Ltd.
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    The effect of host relaxation and dynamics on guest molecule dynamics in H2/tetrahydrofuran-hydrate
    (Royal Society of Chemistry, 2011-01) Peterson, VK; Shoko, E; Kearley, GJ
    We use ab initio molecular dynamics simulations to obtain classically the effects of H2O cage motions on the potential-energy surface (PES) of encapsulated H2 in the H2/tetrahydrofuran-hydrate system. The significant differences between the PES for the H2 in rigid and flexible cages that we find will influence calculation of the quantum dynamics of the H2. Part of these differences arises from the relaxation of the H2O cage around the classical H2, with a second part arising from the coupling of both translational and rotational motions of H2 with the H2O cage. We find that isotopic substitution of 2H for 1H of the H2O cage affects the coupling, which has implications for experiments that require the use of 2H2O, including inelastic neutron scattering that uses 2H2O cages in order to focus on the H2 guest dynamics. Overall, this work emphasizes the importance of taking into account cage dynamics in any approach used to understand the dynamics of H2 guests in porous framework materials. © 2011, American Chemical Society