Showing papers by "Francesco Mauri published in 2004"

Kohn anomalies and electron-phonon interactions in graphite.

Abstract: We demonstrate that graphite phonon dispersions have two Kohn anomalies at the Gamma-E(2g) and K-A'1 modes. The anomalies are revealed by two sharp kinks. By an exact analytic derivation, we show that the slope of these kinks is proportional to the square of the electron-phonon coupling (EPC). Thus, we can directly measure the EPC from the experimental dispersions. The Gamma-E(2g) and K-A'1 EPCs are particularly large, while they are negligible for all the other modes at Gamma and K.

First-Principles Calculation of 17O, 29Si, and 23Na NMR Spectra of Sodium Silicate Crystals and Glasses

Abstract: This paper presents results of first-principles calculations of nuclear magnetic resonance (NMR) parameters: the chemical shielding tensor and the electric field gradient tensor, of some crystalline and amorphous sodium silicate systems. The calculations have been performed using the recently introduced gauge including projector augmented wave (GIPAW) method, which was especially devised for periodic systems. It provides an attractive alternative to the cluster approximation, used in the previous NMR theoretical studies of silicates systems. Moreover, within the GIPAW formalism, amorphous systems can be efficiently described via a supercell approach as demonstrated in this work. Five reference crystalline compounds of known structure (α-quartz, α-cristobalite SiO2, and the sodium silicates Na2SiO3, α-Na2Si2O5, and β-Na2Si2O5) and two molecular dynamics models of the sodium tetrasilicate glass Na2Si4O9 (NS4) have been studied. The NS4 glass models were generated by a combination of classical and Car-Parrin...

First-principles calculation of the 17O NMR parameters in Ca oxide and Ca aluminosilicates: the partially covalent nature of the Ca-O bond, a challenge for density functional theory.

Abstract: We apply density functional theory (DFT) to the calculation of the 17O NMR parameters in Ca and Mg oxides and aluminosilicates. We study the accuracy of the Perdew, Burke, and Ernzerhof (PBE) generalized-gradient approximation to DFT in the description of these systems and the origin of the experimentally observed large dependence of the 17O chemical shift on the alkaline earth ion. We find that (i) the partially covalent nature of the Ca−O bond has a huge impact on the O chemical shifts. The Ca−O covalence alone explains why in Ca oxides and aluminosilicates the 17O chemical shifts are much more deshielded than those of the corresponding Mg compounds. (ii) The Ca−O covalence is overestimated by the PBE functional. Thus PBE-DFT is not able to reproduce the measured 17O NMR parameters in Ca oxide and Ca aluminosilicates. (iii) It is possible to correct for the PBE-DFT deficiency in a simple and transferable way and to predict very accurate 17O NMR parameters. Such accuracy allows us to assign the 17O NMR s...

Combined ab initio computational and experimental multinuclear solid-state magnetic resonance study of phenylphosphonic acid.

Abstract: 1H, 13C, 17O and 31P NMR parameters, including chemical shift tensors and quadrupolar parameters for 17O, were calculated for phenylphosphonic acid, C6H5PO(OH)2, under periodic boundary conditions. The results are in very good agreement with experimental data and permit the unambiguous assignment of all the sites present in the structure. In particular, the 17O NMR parameters of the P=O and P-OH environments were precisely determined, which should help in the characterization of the bonding mode of phosphonate molecules in hybrid solids. Moreover, the effect of intermolecular interactions on the NMR parameters were investigated by comparing the results of the calculations in the crystal and in an isolated molecule of phenylphosphonic acid.

Theoretical Investigation of Oxygen-17 NMR Shielding and Electric Field Gradients in Glutamic Acid Polymorphs

Abstract: We present an assignment of the experimental 17O NMR shielding parameters for l-glutamic acid·HCl (Lemaitre, V.; Pike, K. J.; Watts, A.; Anupold, T.; Samoson, A.; Smith, M. E.; Dupree, R. Chem. Phys. Lett. 2003, 371, 91) based on first-principles quantum mechanical calculations. We use density functional theory and the gauge-including projector-augmented wave method (Pickard, C. J.; Mauri, F. Phys. Rev. B 2001, 63, 245101), which treats the true periodic nature of the crystal structure. We perform further theoretical calculations on a range of glutamic acid polymorphs and draw general conclusions about the influence of hydrogen bonding on 17O NMR shielding parameters.

Superconductivity from doping boron icosahedra

Abstract: We propose an alternative route to achieve the superconducting state in boron-rich solids, the hole doping of ${\mathrm{B}}_{12}$ icosahedra. For this purpose we consider a prototype metallic phase of ${\mathrm{B}}_{13}{\mathrm{C}}_{2}$. We show that in this compound the boron icosahedral units are mainly responsible for the large phonon frequencies logarithmic average, $65.8\phantom{\rule{0.3em}{0ex}}\text{meV}$, and the moderate electron-phonon coupling $\ensuremath{\lambda}=0.81$. We suggest that this high ${T}_{c}$ could be a general feature of hole-doped boron icosahedral solids. Moreover our calculated moderate value of $\ensuremath{\lambda}$ excludes the formation of bipolarons localized on the icosahedral length scale as suggested by previous authors.

Ab Initio Calculations of NMR Parameters of Highly Coordinated Oxygen Sites in Aluminosilicates

Abstract: Ab initio calculations have been carried out for the three polymorphs of SiAl2O5 in order to study the 17O NMR characteristics of tricoordinated O atoms, O[3], and tetracoordinated O atoms, O[4], that are possibly present in the structure of aluminosilicate glasses. We present δiso, CQ, and η calculations using the density functional theory-based GIPAW method for all silicon, aluminum, and oxygen sites and in particular for the 14 O[3] atoms, which are bonded to three Al or Si atoms in the andalusite, sillimanite, and kyanite polymorphs of SiAl2O5. The O[4] parameters calculated in kyanite are also compared with the corresponding site in α-Al2O3. The calculated values for the 29Si and 27Al isotropic chemical shift values as well as 27Al quadrupolar coupling constants (CQ) and asymmetry parameters (η) are in good agreement with experimental data. Apart from in alumina and in grossite CaAl4O7, 17O NMR parameters have not been measured experimentally yet but the CQ and η values obtained for all sites are con...

Resonant diffraction in FeS 2 : Determination of the x-ray polarization anisotropy of iron atoms

Abstract: For pyrite, FeS2, both the imaginary and real parts of the anisotropy of the iron atomic scattering factor are experimentally determined as functions of the x-ray energy near the iron K-edge and compared with ab initio calculations. The anisotropy appears due to the deformations of the electronic states induced by the asymmetric atomic environment and thus provides a quantitative measure of these deformations. As a consequence, reflections expected to be forbidden by screw-axis or glide-plane symmetry operations can be excited, with structure factors being proportional to the anisotropy. The azimuthal angle dependencies and energy spectra of such anisotropy-induced "forbidden" reflections are studied and the phase of the anisotropy is determined from interferences of the forbidden reflections with different multiple-wave reflections. The energy dependencies of the real and imaginary parts of the anisotropy are shown to be in good agreement with theoretical results obtained from two different approaches, i.e., the full multiple-scattering method employing a cluster muffin-tin potential and pseudopotential ab initio calculations. It is found that the anisotropy in pyrite is much more sensitive to the Fe environment than the average absorption coefficient.

Equations of state of Rb x C 60 ( x = 3 , 4, and 6)

Abstract: The equations of state (EOS) of ${\mathrm{Rb}}_{x}{\mathrm{C}}_{60}$ ($x=3$, 4, and 6) were determined by high pressure x-ray diffraction measurements. We focus on ${\mathrm{Rb}}_{4}{\mathrm{C}}_{60}$ since a Mott insulator-metal transition can be induced at around $0.8\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. We observed an abrupt jump in the compressibility of ${\mathrm{Rb}}_{4}{\mathrm{C}}_{60}$ between $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $0.8\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ which is attributed to a structural phase transition preserving the tetragonal symmetry. By ab initio calculations we are able to reproduce the experimental equations of state of ${\mathrm{Rb}}_{x}{\mathrm{C}}_{60}$ ($x=3$,4). These calculations permit us to study the pressure dependence of the internal coordinates.

Kohn Anomalies in Graphite and Nanotubes

Abstract: Atomic vibrations are partially screened by electrons. In a metal this screening can change rapidly for vibrations associated to certain points of the Brillouin zone, entirely determined by the shape of the Fermi surface. The consequent anomalous behaviour of the phonon dispersion is called Kohn anomaly. Graphite is a semimetal. Nanotubes can be metals or semiconductors. We demonstrate that two Kohn anomalies are present in the phonon dispersion of graphite and that their slope is proportional to the square of the electron-phonon coupling. Metallic nanotubes have much stronger anomalies than graphite, due to their reduced dimensionality. Semiconducting nanotubes have no Kohn anomalies.

X-ray absorption near-edge and pre-edge structure: the contributions of ab initio plane-wave pseudopotential calculations and of multiplet theory.

Abstract: Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Ab initio Calculations of NMR Parameters of Highly Coordinated Oxygen Sites in Aluminosilicates.

Abstract: Ab initio calculations have been carried out for the three polymorphs of SiAl2O5 in order to study the 17O NMR characteristics of tricoordinated O atoms, O[3], and tetracoordinated O atoms, O[4], that are possibly present in the structure of aluminosilicate glasses. We present δiso, CQ, and η calculations using the density functional theory-based GIPAW method for all silicon, aluminum, and oxygen sites and in particular for the 14 O[3] atoms, which are bonded to three Al or Si atoms in the andalusite, sillimanite, and kyanite polymorphs of SiAl2O5. The O[4] parameters calculated in kyanite are also compared with the corresponding site in α-Al2O3. The calculated values for the 29Si and 27Al isotropic chemical shift values as well as 27Al quadrupolar coupling constants (CQ) and asymmetry parameters (η) are in good agreement with experimental data. Apart from in alumina and in grossite CaAl4O7, 17O NMR parameters have not been measured experimentally yet but the CQ and η values obtained for all sites are con...