Browsing by Author "Capelli, SC"

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    “Glass-like” thermal conductivity gradually induced in thermoelectric Sr8Ga16Ge30 clathrate by off-centered guest atoms
    (AIP Publishing, 2016-05-10) Christensen, S; Schmøkel, MS; Borup, KA; Madsen, GKH; McIntyre, GJ; Capelli, SC; Christensen, M; Iversen, BB
    The origin of the “glass-like” plateau in thermal conductivity of inorganic type I clathrates has been debated for more than a decade. Here, it is demonstrated that the low temperature thermal conductivity of Sr8Ga16Ge30 can be controlled by the synthesis method: A flux-grown sample has a “glass-like” plateau in thermal conductivity at low temperature, while a zone-melted sample instead has a crystalline peak. A combination of flux-growth and zone-melting produces an intermediate thermal conductivity. In a comprehensive study of three single crystal samples, it is shown by neutron diffraction that the transition from crystalline peak to “glass-like” plateau is related to an increase in Sr guest atom off-centering distance from 0.24 Å to 0.43 Å. By modifying ab initio calculated force constants for the guest atom to an isotropic model, we reproduce both measured heat capacity and inelastic neutron scattering data. The transition from peak to plateau in the thermal conductivity can be modeled by a combined increase of Rayleigh and disorder scattering. Measurement of heat capacity refutes simple models for tunneling of Sr between off-center sites. Furthermore, the electronic properties of the same samples are characterized by Hall carrier density, Seebeck coefficient, and resistivity. The present comprehensive analysis excludes tunneling and charge carrier scattering as dominant contributors to the “glass-like” plateau. The increased guest atom off-centering distance controlled by synthesis provides a possible microscopic mechanism for reducing the low temperature thermal conductivity of clathrates. © 2021 AIP Publishing LLC.
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    Neutron diffraction investigations of magnetism in BiFeO(3) epitaxial films
    (Wiley-Blackwell, 2011-05-10) Ratcliff, W; Kan, D; Chen, WC; Watson, S; Chi, SX; Erwin, R; McIntyre, GJ; Capelli, SC; Takeuchi, I
    The recovery of a modulated magnetic structure in epitaxial BiFeO(3) thin films as revealed by neutron diffraction is reported. The magnetic structure in thin films is found to strongly depend on substrate orientation. The substrate orientation causes different strain-relaxation processes resulting in different thin-film crystal structures. The (110) oriented film with a monoclinic structural phase has a single-domain modulated magnetic structure where the magnetic moment lies in the HHL plane. For the (111) oriented film that has a rhombohedral structure, a modulated structure superimposed on the G-type antiferromagnetic order is found. These results indicate that slight structural modifications in the BiFeO(3) thin film cause drastic changes in the magnetic structure. © 2011, Wiley-Blackwell.
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    Proton cascade in a molecular solid: H/D exchange on mobile and immobile water
    (Wiley, 2013-12-09) Capelli, SC; Falvello, LR; Forcén-Vázquez, E; McIntyre, GJ; Palacio, F; Sanz, S; Tomás, M
    Working without pores: In non-porous, channel-free crystals of a manganese citrate coordination polymer, neutron diffraction reveals that the water molecules, both metal-bound and co-crystallized, undergo full hydrogen/deuterium exchange. Neutron diffraction analyses show a pattern of hydrogen disorder that can be interpreted in terms of the Grotthuss proton-cascade mechanism. © 1999-2020 John Wiley & Sons, Inc.