Browsing by Author "Mutka, H"

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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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    Propagating vibrational modes in the Zr-Be metallic glasses
    (Institute of Physics, 2008-03-12) Syrykh, GF; Ivanov, AS; Klimenko, NA; Lisichkin, YV; Mutka, H; Stride, JA
    The dynamic structure factors, S(Q, E), for the Zr70Be30, Zr60Be40 and Zr50Be50 metallic glasses have been measured using neutron inelastic scattering on the IN4 spectrometer (ILL, Grenoble, France), over the range of momentum transfers 0.7 < h Q (angstrom(-1)) < 7 and energy transfers 2 < E (meV) < 60. The constant- Q cuts through the dynamic structure factor and longitudinal current correlation function, J(l) ( Q, E), were obtained. The dispersion relations for two representations, S( Q, E) and J(l) (Q, E), were derived by fitting a Gaussian to the peaks. The specific low-energy modes (similar to 5 meV) as well as 'acoustic-phonon like' (similar to 16 meV) and 'optic-phonon like' excitations (similar to 47 meV) were observed in these metallic glasses with disparate mass and ionic radius components. © 2008, Institute of Physics
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    Studies of finite molecular chains: synthesis, structural, magnetic and inelastic neutron scattering studies of hexa- and heptanuclear chromium horseshoes
    (Wiley, 2008-02-05) Ochsenbein, ST; Tuna, F; Rancan, M; Davies, RSG; Muryn, CA; Waldmann, O; Bircher, R; Sieber, A; Carver, G; Mutka, H; Fernandez-Alonso, F; Podlesnyak, A; Engelhardt, LP; Timco, GA; Gudel, HU; Winpenny, REP
    We report the synthesis and structural characterisation of a family of finite molecular chains, specifically [{[R2NH2](3) [Cr6F11 (O2CCMe3)(10)]}(2)] (in which R=nPr 1, Et 2, nBu 3), [{Et2NH}(2) {[Et2NH2](3)[Cr7F12(O2C-CMe3)(12)][HO2CCMe3](2)}(2)] (4), [{[Me2NH2](3)[Cr6F11(O2CCMe3)(10)]center dot 2.5 H2O}(4)] (5) and [I{iPr(2)NH(2)](3)[Cr7F12(O2CCMe3)(12)]}(2)] (6). The structures all contain horseshoes of chromium centres, with each Cr center dot center dot center dot Cr contact within the horseshoe bridged by a fluoride and two pivalates. The horseshoes are linked through hydrogen bonds to the secondary ammonium cations in the structure, leading to di- and tetrahorseshoe structures. Through magnetic measurements and inelastic neutron scattering studies we have determined the exchange coupling constants in 1 and 6. In 1 it is possible to distinguish two exchange interactions, J(A) = -1.1 meV and J(B) = -1.4 meV; J(A) is the exchange interactions at the tips of the horseshoe and JB is the exchange within the body of the horseshoe (1 meV = 8.066 cm(-1)). For 6 only one interaction was needed to model the data: J=-1.18meV The single-ion anisotropy parameters for Cr-III were also derived for the two compounds as: for 1, D-Cr = -0.028 meV and vertical bar ECr vertical bar = 0.005meV; for 6, D-Cr=-0.031meV. Magnetic-field-dependent inelastic neutron scattering experiments on I allowed the Zeeman splitting of the first two excited states and level crossings to be observed. For the tetramer of horseshoes (5), quantum Monte Carlo calculations were used to fit the magnetic susceptibility behaviour, giving two exchange interactions within the horseshoe (-1.32 and -1.65 meV) and a weak inter-horseshoe coupling of +0.12meV. Multi-frequency variable-temperature EPR studies on 1, 2 and 6 have also been performed, allowing further characterisation of the spin Hamiltonian parameters of these chains. © 1999-2020 John Wiley & Sons, Inc.