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See detailFirst-principles study of ferroelectricity in oxide superlattices
Zhao, Jinzhu ULg

Doctoral thesis (2013)

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See detailFirst-principles study of filled and unfilled antimony skutterudites
Ghosez, Philippe ULg; Veithen, Marek

in Journal of Physics : Condensed Matter (2007), 19

Using a first-principles approach based on density-functional theory, the electronic, dielectric and dynamical properties of the skutterudites CoSb3 and TlFeCo3Sb12 are studied. In particular, the ... [more ▼]

Using a first-principles approach based on density-functional theory, the electronic, dielectric and dynamical properties of the skutterudites CoSb3 and TlFeCo3Sb12 are studied. In particular, the electron localization tensor, static and dynamic effective charges, static and optical dielectric constants and phonon dispersion curves are computed. The Born effective charges are found to be significantly larger than the static charges of the ions. Moreover, the static dielectric constant of TlFeCo3Sb12 is found to be significantly larger than that of CoSb3. The analysis of the phonon dispersion curves reveals a low-energy mode due to coupled vibrations of Tl and Sb. This mode is at the origin of a well-defined peak in the phonon density of states of TlFeCo3Sb12 and its mode effective charge is related to the increase of the dielectric constant in TlFeCo3Sb12. Our results are compared to recent experiments performed on CoSb3 and TlFeCo3Sb12, and differences between the lattice dynamics of TlFeCo3Sb12 and other filled skutterudites are highlighted. [less ▲]

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See detailFirst-Principles Study of High-Pressure Phase Transitions and Thermal Properties of Al2O3
Xu, Bin ULg; Dong, Jianjun

Poster (2007, June)

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See detailFirst-Principles Study of High-Pressure Phase Transitions in Si3N4
Xu, Bin ULg; Dong, Jianjun

Poster (2005, June)

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See detailFirst-Principles Study of High-Pressure Phase Transitions in Si3N4
Xu, Bin ULg; Dong, Jianjun

Conference (2006, March)

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See detailFirst-Principles Study of Lattice Vibration in Ga2O3 nanowires
Xu, Bin ULg; Dong, Jianjun

Poster (2005, October)

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See detailFirst-principles study of multiferroic materials : BiFeO3 and Bi2FeCrO6
Goffinet, Marco ULg

Doctoral thesis (2011)

The present work is devoted to an ab-initio characterization, using planewave and localized basis sets, of multiferroic and magnetoelectric materials. After a general description of the underlying ... [more ▼]

The present work is devoted to an ab-initio characterization, using planewave and localized basis sets, of multiferroic and magnetoelectric materials. After a general description of the underlying concepts and computational techniques, we focus on BiFeO3, which is probably the most intensively studied multiferroic. We start our investigation by comparing the ability of different exchange-correlation functionals (LSDA(+U), GGA and hybrid) to accurately describe the most fundamental properties of BiFeO3 (crystallographic structure, electronic band structure, magnetism, phonons at Gamma, Born effective charges). Next we present infrared reflectivity and Raman spectra obtained from first-principles and compare them to experimental data. We then extend the study of the dynamical properties of BiFeO3 to the whole Brillouin zone by analyzing the phonon band structure, density of states and interatomic force constants. We also use combined Einstein and Debye models to fit the specific heat as obtained from first-principles and compare it to experimental data. The final part of this thesis is devoted to the investigation of pressure driven spin phase transitions in Bi2FeCrO6 . [less ▲]

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See detailFirst-Principles Study of Oxidation Mechanism for SiC-based Materials
Xu, Bin ULg; Kroll, Peter

Poster (2010, August)

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See detailFirst-Principles Study of Phase Transitions in Silicon Nitride at High Pressure
Xu, Bin ULg; Dong, Jianjun; Shebanova, Olga et al

Poster (2009, June)

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See detailFirst-principles study of structural and vibrational properties of SrZrO3
Amisi, Safari ULg; Bousquet, Eric ULg; Katcho, Karume et al

in Physical Review. B, Condensed Matter and Materials Physics (2012), 85

Using first-principles calculations, we investigate the electronic, structural, and vibrational properties of SrZrO3. We start from the high-symmetry cubic perovskite phase, for which the phonon ... [more ▼]

Using first-principles calculations, we investigate the electronic, structural, and vibrational properties of SrZrO3. We start from the high-symmetry cubic perovskite phase, for which the phonon dispersion curves are reported.We point out the coexistence of structural antiferrodistortive instabilities at the R and M zone-boundary points and a ferroelectric instability at the zone center.We showthat the strong antiferrodistortive motions suppress ferroelectricity and are responsible for the orthorhombic ground state as in CaTiO3. The structural properties <br />of possible intermediate phases and of the orthorhombic Pnma ground state are reported. For the latter, an assignment of IR and Raman zone-center phonon modes is proposed. The main features of the ferroelectric instability are also discussed, and we show that a ferroelectric ground state can even be induced in SrZrO3 by strain engineering. [less ▲]

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See detailFirst-principles study of the dielectric and dynamical properties of lithium niobate
Veithen, M.; Ghosez, Philippe ULg

in Physical Review. B, Condensed Matter and Materials Physics (2002), 65

Using a first-principles approach based on density-functional theory, the electronic, dielectric, and dynamical properties of the two phases of lithium niobate are studied. In particular, the spontaneous ... [more ▼]

Using a first-principles approach based on density-functional theory, the electronic, dielectric, and dynamical properties of the two phases of lithium niobate are studied. In particular, the spontaneous polarization, the optical dielectric tensors, the Born effective charges, and the zone-center phonons are computed. The Born effective charges are found to be significantly larger than the nominal ionic charges of the ions, a feature similar to what is observed in related ABO(3) compounds and attributed to the hybridization between the O 2p and Nb 4d states. The analysis of the zone-center phonons in the paraelectric phase reveals an unstable A(2u) mode to be responsible for the phase transition. The origin of the structural instability is attributed to destabilizing long-range dipolar interactions, which are not fully compensated by stabilizing short-range forces. Finally, the identification of the E modes in the ferroelectric phase, which is still the cause for debates in spite of the numerous experimental and theoretical studies, is discussed and an assignation based on the analysis of the mode-oscillator strengths and the angular dispersion relation of extraordinary phonons is proposed. [less ▲]

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See detailFirst-principles study of the dielectric and dynamical properties of orthorhombic CaMnO3
Bhattacharjee, Satadeep; Bousquet, Eric ULg; Ghosez, Philippe ULg

in Journal of Physics : Condensed Matter (2008), 20

The structural, dielectric and dynamical properties of the low temperature antiferromagnetic orthorhombic phase of CaMnO3 have been computed from first principles, using a density functional theory ... [more ▼]

The structural, dielectric and dynamical properties of the low temperature antiferromagnetic orthorhombic phase of CaMnO3 have been computed from first principles, using a density functional theory approach within the local spin density approximation. The theoretical structural parameters are in good agreement with experiment. The full set of zone-center phonons is reported, allowing new assignment of experimental Raman data and providing reference values for the interpretation of future infrared phonon measurements. It is shown that the static dielectric constant is very large and comparable in amplitude to that of isostructural CaTiO3. In contrast to the pseudocubic structure, it is also highly anisotropic. These features are discussed in relationship to the anomalous Born effective charges and the presence of low frequency polar modes. [less ▲]

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See detailFirst-principles study of the dynamical and nonlinear optical properties of urea single crystals
Hermet, Patrick ULg; Ghosez, Philippe ULg

in Physical Chemistry Chemical Physics [=PCCP] (2010), 12

Raman and infrared spectra of urea single crystals have been calculated using the density functional theory. This allowed us to assign the remaining experimental low-frequency phonon modes. Then, we have ... [more ▼]

Raman and infrared spectra of urea single crystals have been calculated using the density functional theory. This allowed us to assign the remaining experimental low-frequency phonon modes. Then, we have determined the sign of the second-harmonic nonlinear optical susceptibility coefficient in urea to be negative, clarifying a long debate in the literature. Finally, we computed for the first time the electro-optic coefficients of urea. We found that the electronic and ionic contributions have a similar order of magnitude and an opposite sign, yielding a smaller value than that expected, and necessitating further experimental clarifications. [less ▲]

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See detailFirst-principles study of the electro-optic effect in ferroelectric oxides
Veithen, M.; Gonze, X.; Ghosez, Philippe ULg

in Physical Review Letters (2004), 93

We first present a method to compute the electro-optic tensor from first principles, explicitly taking into account the electronic, ionic and piezoelectric contributions. We then study the nonlinear optic ... [more ▼]

We first present a method to compute the electro-optic tensor from first principles, explicitly taking into account the electronic, ionic and piezoelectric contributions. We then study the nonlinear optic behavior of three paradigmatic ferroelectric oxides. Our calculations reveal the dominant contribution of the soft mode to the electro-optic coefficients in LiNbO3 and BaTiO3 and its minor role in PbTiO3. We identify the coupling between the electric field and the polar atomic displacements along the B-O chains as the origin of the large electro-optic response in perovskite ABO(3) compounds. [less ▲]

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See detailFirst-principles study of the ferroelectric Aurivillius phase Bi2WO6
Djani-Ait, Hania ULg; Bousquet, Eric ULg; Kellou, Abdelhafid et al

in Physical Review. B, Condensed Matter and Materials Physics (2012), 86

In order to better understand the reconstructive ferroelectric-paraelectric transition of Bi2WO6, which is unusual within the Aurivillius family of compounds, we performed first-principles calculations of ... [more ▼]

In order to better understand the reconstructive ferroelectric-paraelectric transition of Bi2WO6, which is unusual within the Aurivillius family of compounds, we performed first-principles calculations of the dielectric and dynamical properties of two possible high-temperature paraelectric structures—the monoclinic phase of A2/m symmetry observed experimentally and the tetragonal phase of I4/mmm symmetry—common to most Aurivillius-phase components. Both paraelectric structures exhibit various unstable modes, which, after their condensation, bring the system toward more stable structures of lower symmetry. The calculations confirm that, starting from the paraelectric A2/m phase at high temperatures, the system must undergo a reconstructive transition to reach the P21ab ferroelectric ground state. [less ▲]

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See detailFirst-principles study of the lattice dynamical properties of strontium ruthenate
Miao, Naihua ULg; Bristowe, Nicholas ULg; Xu, Bin et al

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

By means of first-principles calculations, various properties of SrRuO3 are investigated, focusing on its lattice dynamical properties. Despite having a Goldschmidt tolerance factor very close to 1, the ... [more ▼]

By means of first-principles calculations, various properties of SrRuO3 are investigated, focusing on its lattice dynamical properties. Despite having a Goldschmidt tolerance factor very close to 1, the phonon dispersion curves of the high-temperature cubic phase of SrRuO3 show strong antiferrodistortive instabilities. The energetics of metastable phases with different tilt patterns are discussed, concluding that the coupling of oxygen rotation modes with anti-polar Sr motion plays a key role in stabilizing the Pnma phase with respect to alternative rotation patterns. Our systematic analysis confirms previous expectations and contributes to rationalizing better why many ABO3 perovskites, including metallic compounds, exhibit an orthorhombic ground state. The zone-center phonon modes of the Pnma phase have been computed, from which we propose partial reassignment of available experimental data. The full dispersion curves have also been obtained, constituting benchmark results for the interpretation of future measurements and providing access to thermodynamical properties. [less ▲]

Detailed reference viewed: 42 (15 ULg)
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See detailFirst-Principles Study of the Mechanism for B4-to-B1 Phase Transformation in AlN
Xu, Bin ULg; Dong, Jianjun; Stokes, Harold et al

Poster (2009, June)

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See detailFirst-Principles Study of Thermal and Elastic Properties of Al2O3
Xu, Bin ULg; Dong, Jianjun

Poster (2007, February)

Detailed reference viewed: 20 (3 ULg)