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See detailLattice dynamics in Bi2Te3 and Sb2Te3: Te and Sb density of phonon states
Bessas, D.; Sergueev, I.; Wille, H.-C. et al

in PHYSICAL REVIEW B (2012), 86(22), 224301-9

The lattice dynamics in Bi2Te3 and Sb2Te3 were investigated both microscopically and macroscopically using Sb-121 and Te-125 nuclear inelastic scattering, x-ray diffraction, and heat capacity measurements ... [more ▼]

The lattice dynamics in Bi2Te3 and Sb2Te3 were investigated both microscopically and macroscopically using Sb-121 and Te-125 nuclear inelastic scattering, x-ray diffraction, and heat capacity measurements. In combination with earlier inelastic neutron scattering data, the element-specific density of phonon states was obtained for both compounds and phonon polarization analysis was carried out for Bi2Te3. A prominent peak in the Te specific density of phonon states at 13 meV, that involves mainly in-plane vibrations, is mostly unaffected upon substitution of Sb with Bi revealing vibrations with essentially Te character. A significant softening is observed for the density of vibrational states of Bi with respect to Sb, consistently with the mass homology relation in the long-wavelength limit. In order to explain the energy mismatch in the optical phonon region, a similar to 20% force constant softening of the Sb-Te bond with respect to the Bi-Te bond is required. The reduced average speed of sound at 20 K in Bi2Te3, 1.75(1) km/s, compared to Sb2Te3, 1.85(4) km/s, is not only related to the larger mass density but also to a larger Debye level. The observed low lattice thermal conductivity at 295 K, 2.4 Wm(-1)K(-1) for Sb2Te3 and 1.6 Wm(-1)K(-1) for Bi2Te3, cannot be explained by anharmonicity alone given the rather modest Gruneisen parameters, 1.7(1) for Sb2Te3 and 1.5(1) for Bi2Te3, without accounting for the reduced speed of sound and more importantly the low acoustic cutoff energy. [less ▲]

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See detailLattice dynamics in intermetallic Mg2Ge and Mg2Si
Bessas, D.; Simon, R. E.; Friese, K. et al

in Journal of Physics : Condensed Matter (2014), 26(48),

The lattice dynamics of polycrystalline Mg2Ge and Mg2Si are compared using both microscopic and macroscopic measurements as well as theoretical calculations. The volume thermal expansion coefficient ... [more ▼]

The lattice dynamics of polycrystalline Mg2Ge and Mg2Si are compared using both microscopic and macroscopic measurements as well as theoretical calculations. The volume thermal expansion coefficient between 200 and 300K was found to be 4.37(5)•10-5K-1 in Mg2Ge, compared to 3.69(5)•10-5K-1 in Mg2Si. Inelastic neutron scattering measurements yield densities of phonon states which are in line with theoretical calculations. The microscopic data were corroborated with macroscopic calorimetry measurements and provide quantified values for anharmonicity. The estimated macroscopic Grneisen parameter is, γMg 2Si=1.17(5) and γMg 2Ge=1.46(5) at 295K, in excellent agreement with Raman scattering data. Although the element specific mean force constants are practically the same, in Mg2Ge and Mg2Si, a mass homology relation alone cannot reproduce the difference in the partial densities of vibrational states in these compounds and differences in elemental bonding should be taken into account. © 2014 IOP Publishing Ltd. [less ▲]

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See detailLattice dynamics in the FeSb3 skutterudite
Moechel, A.; Sergueev, I.; Long, Gary J. et al

in Physical Review (2011), 84(6),

Thin films of FeSb3 were characterized by electronic transport magnetometry, x-ray diffraction, Fe-57 and Sb-121 nuclear inelastic scattering, and Fe-57 Mossbauer spectroscopy. Resistivity and ... [more ▼]

Thin films of FeSb3 were characterized by electronic transport magnetometry, x-ray diffraction, Fe-57 and Sb-121 nuclear inelastic scattering, and Fe-57 Mossbauer spectroscopy. Resistivity and magnetometry measurements reveal semiconducting behavior with a 16.3(4) meV band gap and an effective paramagnetic moment of 0.57(6) mu(B) respectively. A systematic comparison of the lattice dynamics with CoSb3 and EuFe4Sb12 reveals that the Fe4Sb12 framework is softer than the Co4Sb12 framework, and that the observed softening and the associated lowering of the lattice thermal conductivity in the RFe4Sb12 filled skutterudites are not only related to the filler but also to the Fe4Sb12 framework. [less ▲]

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See detailLattice dynamics in the thermoelectric Zintl compound Yb14MnSb11
Möchel, A.; Sergueev, I.; Wille, H.-C. et al

in Physical Review (2011), 84(18),

The density of phonon states in the thermoelectric material Yb14MnSb11 has been studied first by inelastic neutron scattering and second in an element-specific way by nuclear inelastic x-ray scattering ... [more ▼]

The density of phonon states in the thermoelectric material Yb14MnSb11 has been studied first by inelastic neutron scattering and second in an element-specific way by nuclear inelastic x-ray scattering. The low sound velocity of 1880(50) m/s as obtained from the density of phonon states can be identified as an important reason for the low heat transport in this system. The high melting temperature of Yb14MnSb11 contrasts with the low energy of all phonons (<25 meV) and relates to an unusual lack of softening of phonon modes with temperature, when comparing the phonon density of states observed at ambient temperatures and at 1200 K. We have also measured the density of phonon states of the related Eu14MnSb11 compound and of the thermoelectric Zintl phase Zn4Sb3 in order to compare with related thermodynamic properties of Yb14MnSb11 and to elucidate the different mechanisms of the heat conductivity reduction in Zintl phases. [less ▲]

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See detailLattice dynamics in thermoelectric Zintl phases
Möchel, Anne ULg

Doctoral thesis (2011)

A brief introduction to the thermoelectric effects, the studied materials, and the experimen- tal methods is given in the first chapter. The introduction is not intended to be exhaustive, but only to ... [more ▼]

A brief introduction to the thermoelectric effects, the studied materials, and the experimen- tal methods is given in the first chapter. The introduction is not intended to be exhaustive, but only to summarize important basic information for the reader. The introduction is followed by four chapters dedicated to detailed experimental studies of the lattice dynamics in selected thermoelectric Zintl phases. First, the lattice dynamics in the unfilled and filled skutterudites FeSb3 , CoSb3 , and YbFe4 Sb12 were studied by nuclear inelastic scattering, inelastic neutron scattering, and several macroscopic methods. These studies reveal that FeSb3 exhibits softer Sb bonds than CoSb3 , that the density of phonon states is shifted towards lower energies and the velocity of sound is lower in FeSb3 as compared to CoSb3 . It appears thus that the soft [Fe4 Sb12 ] framework dynamics might play an important role in the thermoelectric properties of filled skutterudites. The observed anomalous temperature dependence of the elastic constants and the rearrangement of the spectral weight of the Yb phonon states in YbFe4 Sb12 can be explained by a change of the Yb valence state with the temperature. Second, the lattice dynamics in the Sr8 Ga16 Ge30 clathrate was investigated by inelastic neutron scattering measurements on a single crystal. We found that several mechanisms contribute to the low thermal conductivity in this system and that the reduction of the heat capacity contribution to the thermal conductivity plays a significant role to the low thermal conductivity, besides the reduction in the phonon lifetime and the phonon group velocity that is related to the guest atom. Third, the lattice dynamics of the Zintl phase Yb14 MnSb11 was studied by inelastic neu- tron scattering and nuclear inelastic scattering measurements. All phonon modes of these complex crystal systems are in a narrow energy range below ∼25 meV and the Debye tem- perature, the velocity of sound and the mean force constants are small compared to those of other thermoelectric materials such as Zn4 Sb3 . By comparing the lattice dynamics in Yb14 MnSb11 and Zn4 Sb3 different mechanisms which lead to the low thermal conductivity in Zintl phases have been identified. Between 300 and 1200 K no softening of the velocity of sound in Yb14 MnSb11 was observed by temperature dependent inelastic neutron scattering measurements, which is in line with its large melting temperature. [less ▲]

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See detailLattice dynamics of BaTiO3, PbTiO3, and PbZrO3: A comparative first-principles study
Ghosez, Philippe ULg; Cockayne, E.; Waghmare, U. V. et al

in Physical Review. B : Condensed Matter (1999), 60(2), 836-843

The full phonon dispersion relations of lead titanate and lead zirconate in the cubic perovskite structure are computed using first-principles variational density-functional perturbation theory, with nb ... [more ▼]

The full phonon dispersion relations of lead titanate and lead zirconate in the cubic perovskite structure are computed using first-principles variational density-functional perturbation theory, with nb initio pseudopotentials and a plane-wave basis set. Comparison with the results previously obtained for barium titanate shows that the change of a single constituent (Ba to Pb, Ti to Zr) has profound effects on the character and dispersion of unstable modes, with significant implications for the nature of the phase transitions and the dielectric and piezoelectric responses of the compounds. Examination of the interatomic force constants in real space, obtained by a transformation which correctly treats the long-range dipolar contribution, shows that most are strikingly similar, while it is the differences in a few key interactions which produce the observed changes in the phonon dispersions. These trends suggest the possibility of the transferability of force constants to predict the lattice dynamics of perovskite solid solutions. [less ▲]

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See detailLattice dynamics of multiferroic BiFeO3 studied by inelastic x-ray scattering
Borissenko, Elena; Goffinet, Marco ULg; Bosak, Alexei et al

in Journal of Physics : Condensed Matter (2013), 25

We report an experimental study of the phonon dispersion in BiFeO3 single crystals at ambient conditions by inelastic x-ray scattering (IXS). The phonon dispersions were recorded along several symmetry ... [more ▼]

We report an experimental study of the phonon dispersion in BiFeO3 single crystals at ambient conditions by inelastic x-ray scattering (IXS). The phonon dispersions were recorded along several symmetry directions up to 35 meV. Our results compare favorably with first-principles calculations performed using density functional theory (DFT) within the local-density approximation (LDA). We resolve a discrepancy concerning the symmetry of the optical phonon branches observed by Raman spectroscopy, determine the energy of the lowest Raman and infrared silent mode, and derive a subset of the elastic moduli of BiFeO3. [less ▲]

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See detailLattice Dynamics of Nanostructured Thermoelectric Materials
Claudio Weber, Tania ULg

Doctoral thesis (2013)

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See detailLattice gas model of gradual evolution
Ausloos, Marcel ULg; Mroz, I.; Pekalski, A. et al

in Physica A: Statistical Mechanics and its Applications (1998), 248(1-2), 155-164

A simple dynamical model is presented for describing the gradual evolution of a variable number of species. The system is studied through Monte Carlo simulations using a lattice gas formalism. Each ... [more ▼]

A simple dynamical model is presented for describing the gradual evolution of a variable number of species. The system is studied through Monte Carlo simulations using a lattice gas formalism. Each species is characterized by a single, scalar parameter ("adaptation") which is changed, within limits depending on the adaptation itself, at each time step. There are two independent mechanisms for removing a species from the system and one for creating a new species. We find that, regardless of the initial concentration of species, the system always reaches the same final state, characterized by the same concentration and the same average adaptation. The system is not homogeneous and contains species with different values of adaptation. The better adapted ones are found to form more symmetrical spatial patterns. Behavior similar to the one determined in the present model has been found in the evolving ecological and biological systems. [less ▲]

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See detailLattice instabilities in bulk EuTiO3
Bessas, D.; Rushchanskii, K. Z.; Kachlik, M. et al

in PHYSICAL REVIEW B (2013), 88(14),

The phase purity and the lattice dynamics in bulk EuTiO3 were investigated both microscopically, using x-ray and neutron diffraction, Eu-151-Mossbauer spectroscopy, and Eu-151 nuclear inelastic scattering ... [more ▼]

The phase purity and the lattice dynamics in bulk EuTiO3 were investigated both microscopically, using x-ray and neutron diffraction, Eu-151-Mossbauer spectroscopy, and Eu-151 nuclear inelastic scattering, and macroscopically using calorimetry, resonant ultrasound spectroscopy, and magnetometry. Furthermore, our investigations were corroborated by ab initio theoretical studies. The perovskite symmetry, Pm (3) over barm, is unstable at the M- and R-points of the Brillouin zone. The lattice instabilities are lifted when the structure relaxes in one of the symmetries: I4/mcm, Imma, R (3) over barc with relative relaxation energy around -25 meV. Intimate phase analysis confirmed phase purity of our ceramics. A prominent peak in the Eu specific density of phonon states at 11.5 meV can be modeled in all candidate symmetries. A stiffening on heating around room temperature is indicative of a phase transition similar to the one observed in SrTiO3, however, although previous studies reported the structural phase transition to the tetragonal I4/mcm phase our detailed sample purity analysis and thorough structural studies using complementary techniques did not confirm a direct phase transition. Instead, in the same temperature range, Eu delocalization is observed which might explain the lattice dynamical instabilities. [less ▲]

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See detailLattice involvement in low temperature phase of U(Ru,Rh)(2)Si-2
Jaime, M.; Silhanek, Alejandro ULg; Kim, K. H. et al

in Physica B: Condensed Matter (2006), 378-80

Specific heat, magnetocaloric effect and magnetization experiments were performed on the 5f-electron system U(Ru0.96Rh0.04)(2)Si-2. This compound exhibits a non-magnetic ground state with a single ... [more ▼]

Specific heat, magnetocaloric effect and magnetization experiments were performed on the 5f-electron system U(Ru0.96Rh0.04)(2)Si-2. This compound exhibits a non-magnetic ground state with a single magnetic field-induced phase between H = 26 and 38 T. We show that the transition into this phase is first-order as revealed by a large irreversible specific heat anomaly and asymmetric magnetocaloric effect. A comparative analysis with the valence transition compound YbInCu4, where similar irreversibility and asymmetry exists, is presented. (c) 2006 Published by Elsevier B.V. [less ▲]

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See detailA lattice model for BNE and RBNE
Ludewig, François ULg; Vandewalle, Nicolas ULg

in Powders and Grains 2005 (2005)

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See detailLattice of subalgebras in the finitely generated varieties of MV-algebras
Teheux, Bruno ULg

in Discrete Mathematics (2007), 307(17-18), 2261-2275

In this paper, we use the theory of natural duality to study subalgebra lattices in the finitely generated varieties of MV-algebras, With this tool, we obtain the dual atomicity of these lattices, and ... [more ▼]

In this paper, we use the theory of natural duality to study subalgebra lattices in the finitely generated varieties of MV-algebras, With this tool, we obtain the dual atomicity of these lattices, and characterize the members of these varieties in which every subalgebra is an intersection of maximal subalgebras. Then, we determine the algebras that have a modular or distributive lattice of subalgebras. [less ▲]

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See detailLATTICE-DYNAMICS AND DIELECTRIC-PROPERTIES OF INCIPIENT FERROELECTRIC TIO2 RUTILE
LEE, C.; Ghosez, Philippe ULg; GONZE, X.

in Physical Review. B : Condensed Matter (1994), 50(18), 13379-13387

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See detailLaudanum. Notes pour un rêve de Tracy Moffat
Havelange, Carl ULg

in Strivay, Lucienne; LE ROUX, Géraldine (Eds.) La revanche des genres. Art contemporain australien/The revenge of Genres. Australian Contemporary Art (2007)

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See detailLaudatio Alberto Manguel
Durand, Pascal ULg

Speech/Talk (2007)

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See detailLaudatio Hubert Nyssen
Durand, Pascal ULg

Speech/Talk (2003)

Présentation de la double carrière d'Hubert Nyssen écrivain et éditeur à l'occasion de son doctorat honoris causa à l'Université de Liège

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