References of "Ghosez, Philippe"
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See detailLong-range Coulomb interaction in ZrO2
Detraux, F.; Ghosez, Philippe ULg; Gonze, X.

in Physical Review Letters (1998), 81(15), 3297-3297

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See detailAnomalously large Born effective charges in cubic WO3
Detraux, F.; Ghosez, Philippe ULg; Gonze, X.

in Physical Review. B : Condensed Matter (1997), 56(3), 983-985

Within density-functional theory, we compute the Born effective charges of tungsten trioxyde in its reference cubic phase (defect-perovskite structure). For the tungsten atom, the effective charge tensor ... [more ▼]

Within density-functional theory, we compute the Born effective charges of tungsten trioxyde in its reference cubic phase (defect-perovskite structure). For the tungsten atom, the effective charge tensor is isotropic, with Z(W)*=+12.51. For the oxygen atoms, the two independent components of the tensor, corresponding, respectively, to a displacement of the atom parallel or perpendicular to the W-O bond, have the values Z(O parallel to)* = -9.13 and Z(O perpendicular to)* = -1.68. Z(W)* and Z(O parallel to)* are anomalously large with respect to the nominal ionic charges (+6 on W and -2 on O), but compatible with the Born effective charges found in related ABO(3)-perovskite compounds. [less ▲]

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See detailLong-wavelength behavior of the exchange-correlation kernel in the Kohn-Sham theory of periodic systems
Ghosez, Philippe ULg; Gonze, X.; Godby, R. W.

in Physical Review. B : Condensed Matter (1997), 56(20), 12811-12817

The polarization dependence of the exchange-correlation (re) energy functional of periodic insulators within Kohn-Sham (KS) density-functional theory requires a O(1/q(2)) divergence in the re kernel for ... [more ▼]

The polarization dependence of the exchange-correlation (re) energy functional of periodic insulators within Kohn-Sham (KS) density-functional theory requires a O(1/q(2)) divergence in the re kernel for small vectors q. This behavior, exemplified for a one-dimensional model semiconductor, is also observed when an insulator happens to be described asa KS metal, or vice versa. Although it can occur in the exchange-only kernel, it is not found in the usual local, semilocal, or even nonlocal approximations to KS theory. We also show that the test-charge and. electronic definitions of the macroscopic dielectric constant differ from one another in exact KS theory, but are equivalent in the above-mentioned approximations. [less ▲]

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See detailPolarization dependence of the exchange energy
Gonze, X.; Ghosez, Philippe ULg; Godby, R. W.

in Physical Review Letters (1997), 78(10), 2029-2029

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See detailLattice dynamics and ferroelectric instability of barium titanate
Ghosez, Philippe ULg; Gonze, X.; Michenaud, J. P.

in Ferroelectrics (1997), 194

Using first-principles calculations, we investigate the lattice dynamics of cubic and rhombohedral BaTiO3 and rye discuss the origin of the structural instability of this ferroelectric material. First, we ... [more ▼]

Using first-principles calculations, we investigate the lattice dynamics of cubic and rhombohedral BaTiO3 and rye discuss the origin of the structural instability of this ferroelectric material. First, we report results on the Born effective charges and the dielectric tensor and we emphasize the important reduction of these quantities in the ferroelectric phase, Then, the phonon frequencies at the Gamma point are calculated. We point out the similarity of theoretical eigenvectors in the cubic and rhombohedral phases. We examine the interaction of the vibration modes with the electric field and in particular the giant LO-TO splitting of the ferroelectric mode. Finally, separating the dipole-dipole interaction from the remaining short-range forces, we quantify the balance of forces leading to an unstable phonon in the cubic phase and we demonstrate its sensitivity to tiny effective charge changes. Within our decomposition, the stabilization of the unstable mode in the rhombohedral phase is produced by a reduction of the Born effective charges, while its stabilization under isotropic pressure is associated with a modification of the short-range forces. [less ▲]

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See detailDensity-functional theory of polar insulators
Gonze, X.; Ghosez, Philippe ULg; Godby, R. W.

in Physical Review Letters (1997), 78(2), 294-297

We examine the density-functional theory of macroscopic insulators, obtained in the large-cluster limit or under periodic boundary conditions. For polar crystals, we find that the two procedures are not ... [more ▼]

We examine the density-functional theory of macroscopic insulators, obtained in the large-cluster limit or under periodic boundary conditions. For polar crystals, we find that the two procedures are not equivalent. In a large-cluster case, the exact exchange-correlation potential acquires a homogeneous ''electric field'' which is absent from the usual local approximations, and the Kohn-Sham electronic system becomes metallic. With periodic boundary conditions, such a field ist forbidden, and the polarization deduced from Kohn-Sham wave functions is incorrect even if the exact functional is used. [less ▲]

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See detailRole of the Coulomb interaction on the ferroelectric instability of Barium Titanate
Ghosez, Philippe ULg; Gonze, X.; Michenaud, J. P.

in Ferroelectrics (1996), 186

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See detailCoulomb interaction and ferroelectric instability of BaTiO3
Ghosez, Philippe ULg; Gonze, X.; Michenaud, J. P.

in Europhysics Letters [=EPL] (1996), 33(9), 713-718

Using first-principles calculations, the phonon frequencies at the Gamma-point and the dielectric tensor are determined and analysed for the cubic and rhombohedral phases of BaTiO3. The dipole-dipole ... [more ▼]

Using first-principles calculations, the phonon frequencies at the Gamma-point and the dielectric tensor are determined and analysed for the cubic and rhombohedral phases of BaTiO3. The dipole-dipole interaction is then separated a la Cochran from the remaining short-range forces, in order to investigate their respective influence on lattice dynamics. This analysis highlights the delicate balance of forces leading to an unstable phonon in the cubic phase and demonstrates its extreme sensitivity to effective charge changes. Within our decomposition, the stabilization of the unstable mode in the rhombohedral phase or under isotropic pressure has a different origin. [less ▲]

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See detailA microscopic study of barium titanate
Ghosez, Philippe ULg; Gonze, X.; Michenaud, J. P.

in Ferroelectrics (1995), 164

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See detailSCALING HYPOTHESIS FOR CORRECTIONS TO TOTAL-ENERGY AND STRESS IN PLANE-WAVE-BASED AB-INITIO CALCULATIONS
RIGNANESE, G. M.; Ghosez, Philippe ULg; CHARLIER, J. C. et al

in Physical Review. B : Condensed Matter (1995), 52(11), 8160-8178

We present a technique aimed at preventing plane-wave-based total energy and stress calculations from the effect of abrupt changes in basis set size. This scheme relies on the interpolation of energy as a ... [more ▼]

We present a technique aimed at preventing plane-wave-based total energy and stress calculations from the effect of abrupt changes in basis set size. This scheme relies on the interpolation of energy as a function of the number of plane waves and on a scaling hypothesis that allows us to perform the interpolation for a unique reference volume. From a theoretical point of view, the method is compared to those already proposed in the literature, and its more rigorous derivation is emphasized. From a practical point of view, we illustrate the importance of the correction on different materials (Si, BaTiO3, and He) corresponding to different types of bonding, and to different k-point samplings and cutoff energies. Then, we compare the different approaches for the calculation of a(0), B-0, and B-0' in bulk silicon. [less ▲]

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See detailBORN EFFECTIVE CHARGES OF BARIUM-TITANATE - BAND-BY-BAND DECOMPOSITION AND SENSITIVITY TO STRUCTURAL FEATURES
Ghosez, Philippe ULg; GONZE, X.; LAMBIN, P. et al

in Physical Review. B : Condensed Matter (1995), 51(10), 6765-6768

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See detailDENSITY-POLARIZATION FUNCTIONAL THEORY OF THE RESPONSE OF A PERIODIC INSULATING SOLID TO AN ELECTRIC-FIELD
GONZE, X.; Ghosez, Philippe ULg; GODBY, R. W.

in Physical Review Letters (1995), 74(20), 4035-4038

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See detailFirst-principles calculations of dielectric and effective charge tensors in Barium Titanate
Ghosez, Philippe ULg; Gonze, X.; Michenaud, J. P.

in Ferroelectrics (1994), 153

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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 detailCHARACTERIZATION OF A HIGHLY PHOTOACTIVE MOLECULAR SEMICONDUCTOR - OXOTITANIUM PHTHALOCYANINE
Ghosez, Philippe ULg; COTE, R.; GASTONGUAY, L. et al

in Chemistry of Materials (1993), 5(10), 1581-1590

Oxotitanium phthalocyanine (OTiPc) thin films have been sublimed at various thicknesses on SnO2 and glass substrates. The morphology, crystallinity, and photoelectrochemical activity of the films have ... [more ▼]

Oxotitanium phthalocyanine (OTiPc) thin films have been sublimed at various thicknesses on SnO2 and glass substrates. The morphology, crystallinity, and photoelectrochemical activity of the films have been studied. It was found that the physical and photoelectrochemical properties of the films are greatly influenced by the temperature reached by the substrate during the sublimation. Below 80-degrees-C, amorphous films are obtained while the films are partially crystalline when the substrate is allowed to reach about 140-degrees-C. Amorphous films are made of tightly packed aggregates of circular section while partially crystalline films consists of platelets. All films are porous and permeable to the I3-/I- redox system. The dominant polymorph in partially crystalline films is not the same for all film thicknesses. It is phase IV OTiPc (as deduced by electron diffraction) for films thinner than about 2000 angstrom. On the other hand, for films thicker than about 8000 angstrom, phase I OTiPc becomes the dominant polymorph (as deduced by X-ray diffraction). It is replaced by phase II for 20 000-angstrom-thick films of OTiPc. Partially crystalline films are the only ones to absorb in the near-infrared (NIR) region. This typical absorption is going along with an improvement of the photoactivity of the films. In partially crystalline films, energy is transferred from the amorphous to the crystalline regions where most of the charges are generated. Quantum yields for electron collection per incident photon may reach over 25% in short circuit conditions, at 850 nm, the Q-band absorption maximum for 8000-angstrom-thick films. Those films are phase I OTiPc. Under 35 MW CM-2 polychromatic illumination, the same films are characterized by short circuit photocurrents of 1.5 mA cm-2. A NIR absorbance is an important factor required to obtain a high photoactivity, but it is not the only one. Interaction of OTiPc with oxygen at the purification level of the crude material is very important as well. [less ▲]

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