Browsing by Author "Shoko, E"

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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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    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
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    MD simulation and experimental INS: a marriage in atomic dynamics
    (Australian Institute of Physics, 2013-02-07) Shoko, E
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    Molecular dynamics evidence for alkali-metal rattling in the beta-pyrochlores, AOs(2)O(6) (A = K, Rb, Cs)
    (IOP Publishing, 2013-10-31) Shoko, E; Peterson, VK; Kearley, GJ
    We have used ab initio molecular dynamics simulations validated against inelastic neutron scattering data to study alkali-metal dynamics in the ?-pyrochlore osmates AOs2O6 (A=K, Rb, Cs) at 300?K to gain insight into the microscopic nature of rattling dynamics in these materials. Our results provide new evidence at the microscopic level for rattling dynamics: (1)?the elemental magnitude spectra calculated from the MD show a striking dominance by the alkali metals at low energies indicating weak coupling to the cage, (2) the atomic root-mean-square displacements for the alkali metals are significantly larger than for the other atoms, e.g., 25% and 150% larger than O and Os, respectively, in KOs2O6, and (3) motions of the alkali metals are weakly correlated to the dynamics in their immediate environment, e.g.?K in KOs2O6 is 6 times less sensitive to its local environment than Os, indicating weak bonding of the K. There is broadening of the elemental spectra of the alkali metals from Cs to K corresponding to a similar broadening of the local potential around these atoms as determined from potential of mean-force calculations. This feature of the spectra is partly explained by the well-known increase in the relative cage volume with decreasing atomic size of the alkali metal. We find that for the smallest rattler in this series (K) the larger relative cage volume allows this atom freedom to explore a large space inside the cage leading to vibration at a broader range of frequencies, hence a broader spectrum. Thus, since K is considered the best rattler in this series, these findings suggest that a significant feature of a good rattler is the ability to vibrate at several different but closely spaced frequencies. © 2013, IOP Publishing Ltd.
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    Novel K rattling: a new route to thermoelectric materials?
    (AIP Publishing, 2014-01-15) Shoko, E; Okamoto, Y; Kearley, GJ; Peterson, VK; Thorogood, GJ
    We have performed ab initio molecular dynamics simulations to study the alkali-metal dynamics in the Al-doped (KAl0.33W1.67O6 and RbAl0.33W1.67O6) and undoped (KW2O6 and RbW2O6) defect pyrochlore tungstates. The K atoms exhibit novel rattling dynamics in both the doped and undoped tungstates while the Rb atoms do not. The KAl0.33W1.67O6 experimental thermal conductivity curve shows an unusual depression between ∼50 K and ∼250 K, coinciding with two crossovers in the K dynamics: the first at ∼50 K, from oscillatory to diffusive, and the second at ∼250 K, from diffusive back to oscillatory. We found that the low-temperature crossover is a result of the system transitioning below the activation energy of the diffusive dynamics, whereas the high-temperature crossover is driven by a complex reconstruction of the local potential around the K atoms due to the cage dynamics. This leads to a hardening of the K potential with increasing temperature. This unusual reconstruction of the potential may have important implications for the interpretation of finite-temperature dynamics based on zero-temperature potentials in similar materials. The key result is that the novel K rattling, involving local diffusion, leads to a significant reduction in the thermal conductivity. We suggest that this may open a new route in the phonon engineering of cage compounds for thermoelectric materials, where the rattlers are specifically selected to reduce the lattice thermal conductivity by the mechanism of local diffusion. © 2014 AIP Publishing LLC.
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    Novel rattling of K atoms in aluminium-doped defect pyrochlore tungstate
    (IOP Science, 2014-07-04) Shoko, E; Kearley, GJ; Peterson, VK; Mutka, H; Koza, MM; Yamaura, J; Hiroi, Z; Thorogood, GJ
    Rattling dynamics have been identified as fundamental to superconductivity in defect pyrochlore osmates and aluminium vanadium intermetallics, as well as low thermal conductivity in clathrates and filled skutterudites. Combining inelastic neutron scattering (INS) measurements and ab initio molecular dynamics (MD) simulations, we use a new approach to investigate rattling in the Al-doped defect pyrochlore tungstates: AAl0.33W1.67O6 (A = K, Rb, Cs). We find that although all the alkali metals rattle, the rattling of the K atoms is unique, not only among the tungstates but also among the analogous defect osmates, KOs2O6 and RbOs2O6. Detailed analysis of the MD trajectories reveals that two unique features set the K dynamics apart from the rest, namely, (1) quasi one-dimensional local diffusion within a cage, and (2) vibration at a range of frequencies. The local diffusion is driven by strongly anharmonic local potentials around the K atoms exhibiting a double-well structure in the direction of maximum displacement, which is also the direction of local diffusion. On the other hand, vibration at a range of frequencies is a consequence of the strong anisotropy in the local potentials around the K atoms as revealed by directional magnitude spectra. We present evidence to show that it is the smaller size rather than the smaller mass of the K rattler which leads to the unusual dynamics. Finally, we suggest that the occurrence of local diffusion and vibration at a range of frequencies in the dynamics of a single rattler, as found here for the K atoms, may open new possibilities for phonon engineering in thermoelectric materials. © 2014, IOP Publishing Ltd.
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    Temperature dependence of alkali-metal rattling dynamics in the β-pyrochlores, AOs2O6 (A = K, Rb, Cs), from MD simulation
    (IOP Science, 2014-05-16) Shoko, E; Peterson, VK; Kearley, GJ
    We investigate the temperature response of the alkali-metal rattling modes in β-pyrochlores, AOs2O6 (A = K, Rb, Cs), from the results of ab initio molecular dynamics (MD) simulations performed at 20 K, 100 K and 300 K. Our results show that the temperature response of the T1u mode is clearly different from that of the T2g mode for all three pyrochlores. In this regard, two features are of particular note for both K and Rb; (1) the T1u mode exhibits a distinctly stronger softening response with decreasing temperature compared to the T2g mode, and (2) the T1u mode becomes stronger and sharper with decreasing temperature. These two findings suggest that the T1u mode is significantly more anharmonic and sensitive to the cage dynamics than the T2g mode. Examination of the local potentials around the alkali-metal atoms reveals that K has the flattest and most anharmonic potential at all temperatures while Cs exhibits the narrowest potential. The temperature dependence of the local potentials reveals that, for K, the potential at a higher temperature is not a simple extrapolation to higher energy of that at a lower temperature. Instead, we find significant reconstruction of the potential at different temperatures. Finally, we explore the temperature response of the coupling between the alkali metals and find a complex temperature dependence which suggests that the origin of the coupling may be more complex than a pure Coulomb interaction. We also find an unexpected increase in the static disorder of the system at low temperatures for the K and Rb pyrochlores. © 2014 IOP Publishing Ltd
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    Toward a mechanism of rattler coupling in the β-pyrochlores AOs2O6 (A = K, Rb, Cs)
    (Springer Link, 2014-03-03) Shoko, E; Peterson, VK; Kearley, GJ
    We have applied ab initio molecular dynamics simulations to study metal–metal coupling on the alkali-metal sublattice in the β-pyrochlore osmates, AOs2O6 (A = K, Rb, Cs) at 300 K. We find that the dynamics of the alkali-metal atoms (rattlers) exhibit stronger rattler–rattler correlations than rattler–cage correlations, and that, at 300 K, this correlation is strongest for Cs. We show that the rattler–rattler correlations control the dominant dynamics in the rattling of these atoms. We provide preliminary evidence that the rattler correlated motion occurs primarily through two somewhat distinct vibrational modes: a high-energy mode (peak A) couples the rattlers to each other and a low-energy mode (peak B) couples the rattlers to the cage modes. Rattler–rattler correlated motion through the high-energy mode provides insight into the trend in spectral broadening from Cs to K. The spectral broadening is inversely proportional to the strength of the dynamical correlations on the alkali-metal sublattice which in turn depend on the atomic size of the rattler, decreasing from Cs to K. Thus, the broadest spectrum exhibited by the K is partly a consequence of the small size of this rattler which permits a greater range of motions involving combinations of both correlated and anti-correlated dynamics. We emphasize that the identification of the somewhat distinct roles of the high-energy (peak A) and low-energy (peak B) modes in rattler coupling reported in this work is a significant step toward a complete fundamental mechanism of rattler dynamical coupling in these osmates. We believe that such a mechanism will have profound implications for a broad class of cage compounds, including clathrates and skutterudites.© 2014, Springer Science+Business Media New York.