Jacques Soullard

Pressure and size effects in endohedrally confined hydrogen clusters

Density functional theory is used to carry out a systematic study of zero-temperature structural and energy properties of endohedrally confined hydrogen clusters as a function of pressure and the cluster size. At low pressures, the most stable structural forms of (H(2))(n) possess rotational symmetry that changes from C(4) through C(5) to C(6) as the cluster grows in size from n=8 through n=12 to n=15. The equilibrium configurational energy of the clusters increases with an increase of the pressure. The rate of this increase, however, as gauged on the per atom basis is different for different clusters sizes. As a consequence, the size dependencies of the configurational energies per atom at different fixed values of pressure are nonmonotonic functions. At high pressures, the molecular (H(2))(n) clusters gradually become atomic or dominantly atomic. The pressure-induced changes in the HOMO-LUMO gap of the clusters indicate a finite-size analog of the pressure-driven metallization of the bulk hydrogen. The ionization potentials of the clusters decrease with the increase of pressure on them.

Bond stopping cross sections for protons incident on molecular targets within the OLPA/FSGO implementation of the kinetic theory☆

Abstract We report the predictions of the kinetic theory for proton stopping in molecular targets within the orbital local plasma approximation (OLPA) introduced by Meltzer, Sabin and Trickey [Phys. Rev. A 41 (1990) 220]. In order to present a self-contained treatment, all the quantities entering in the calculations are obtained through use of the floating spherical Gaussian orbital (FSGO) representation of localized molecular orbitals. The calculations are performed for representative molecular targets, generating S e values for cores, single, double, triple-bonds and lone pairs as a function of projectile velocity. Fair agreement is observed with available experimental data and with other theoretical estimates. The adequacy of the FSGO representation in the study of molecular stopping for protons within the spirit of the kinetic theory and the orbital implementation of the local plasma approximation is confirmed in this work.

Electronic structure of ceramics at the MP2 electron correlation level

The electronic structure of the superconducting ground state of ceramics is calculated at the MP2 electron correlation level using the self-consistent embedded-cluster method. The quantum cluster is embedded in a finite array of point charges which produce the correct value of the Madelung potential at each cluster site. The results obtained reveal the strong influence of electron correlation effects on the charge distribution. The coupling between chains and planes is found to be essentially ionic while covalent bonding is predominant within both chains and planes. The symmetry of the holes on oxygens in the planes is in accordance with nuclear magnetic resonance and x-ray absorption experiments. A partially unoccupied state associated with the orbitals and is found to be present on the copper ions. The unpaired spin is located on the Cu1 atom in the chains; in the planes, the hypothetical Zhang-Rice singlet is indicated.

Advances in the Core-and-Bond Formalism for Proton Stopping in Molecular Targets

Abstract An account is presented of the main efforts leading to the development of the Core-and-Bond (CAB) treatment of chemical binding and phase state effects in the stopping of ions in molecular target materials. The relevance of molecular structure and properties in the stopping process is highlighted through revision of theoretical schemes developed to distinguish between the different contributions from inner-shell (core) and valence molecular orbitals. New results are presented for a CAB model of molecular confinement advanced to study the stopping of protons in dense matter.

Thermal behavior of a 13-molecule hydrogen cluster under pressure

The thermal behavior of a 13-molecule hydrogen cluster is studied as a function of pressure and temperature using a combination of trajectory and density functional theory simulations. The analysis is performed in terms of characteristic descriptors such as caloric curve, root-mean-square bond length fluctuation, pair correlation function, velocity autocorrelation function, volume thermal expansion, and diffusion coefficients. The discussion addresses on the peculiarities of the transition from the ordered-to-disordered state as exhibited by the cluster under different pressures and temperatures.

Vacancy clustering in manganese oxide

Abstract The binding energy of polydefects has been calculated by means of a static simulation of an ionic lattice for the four transition metal oxides MnO, FeO, CoO and NiO. In the case of MnO a new defect model is proposed which includes monovacancies, divacancies and 4:1 clusters. The results of the calculation of defect concentrations show that point defects are the dominant defects in MnO.

Thermodynamic states of nanoclusters at low pressure and low temperature: the case of 13 H(2).

A confinement model of finite-size systems that embodies an equation of state is presented. The temperature and pressure of the system are obtained from the positions and velocities of the enclosed particles after a number of molecular dynamics simulations. The pressure has static and dynamic (thermal) contributions, extending the Mie-Grüneisen equation of state to include weakly interacting anharmonic oscillators. The model is applied to a system of 13 H(2) molecules under low-pressure and low-temperature conditions in the classical regime. The confining cage in this case is a spherical hydrogen cavity. The Born-Oppenheimer molecular dynamics in conjunction with density functional theory are used for the time evolution of the particle system. The hydrogen molecules form a noncrystalline cluster structure with icosahedral symmetry that remains so in the whole temperature range investigated. The fluctuations of the interatomic distances increase with the temperature, while the orientational order of the enclosed system of molecules fades out, suggesting a gradual order-disorder transition.

Pressure-Induced Metallization of Li + ‑Doped Hydrogen Clusters

Endohedrally encapsulated hydrogen clusters doped with inert helium (H24He) and ionic lithium (H24Li(+)) are investigated. The confinement model is a nanoscopic analogue of the experimental compression of solid hydrogen. The structural and electronic properties of the doped hydrogen clusters are determined under the effects of pressure. The results are compared with these of the isoelectronic (pure) hydrogen counterpart H26 under similar physical conditions. Pressure increase rates with respect to H26 of approximately 1.1 are observed with the insertion of helium or lithium. The changes of geometrical structures and HOMO-LUMO gap energies with the pressure point out the pressure-induced metallization of the Li(+)-doped cluster. The computations are done using density functional theory in the form implemented for molecules; they include zero-point energy effects and, to our best knowledge, are the first of their kind.

Study of the electronic structure of Zn-doped Y123 ceramics

Abstract The electronic structure of undoped and Zn-doped Y123 ceramics is studied by means of the embedded cluster method at the MP2 electron correlation level. The Zn impurity is found to be a source of nonhomogeneity in the hole density distribution in the CuO2 planes which acts as an extended and strong scattering center. In Zn-doped ceramics the orbital symmetry of holes on nearest-neighbor oxygens is changed: in addition to the 2pσ symmetry which characterizes the pure material, a 30% contribution of the 2pπ (in plane) symmetry is revealed. The latter can hinder the hole-pair formation on oxygen ions.

Evolution of the vibrational spectra of doped hydrogen clusters with pressure

The evolution of the vibrational spectra of the isoelectronic hydrogen clusters H26, H24He, and H24Li(+) is determined with pressure. We establish the vibrational modes with collective character common to the clusters, identify their individual vibrational fingerprints and discuss frequency shifts in the giga-Pascal pressure region. The results are of interest for the identification of doping elements such as inert He and ionic Li(+) in hydrogen under confinement or, conversely, establish the pressure of doped hydrogen when the vibrational spectrum is known. At high pressure, the spectra of the nanoclusters resemble the spectrum of a solid, and the nanoclusters may be considered crystals of nanometer scale. The computations are performed at the gradient-corrected level of density functional theory. The investigation is the first of its kind.