J.R. Soto

Nonadiabatic structure instability of planar hexagonal gold cluster cation Au7 and its spectral signature

Determining the structure for gold nanoclusters at which the 2D–3D transition occurs has been a challenging problem for both theoretical and experimental studies. For gold cation clusters, quantum chemistry calculations have produced controversial results concerning the last most stable planar structure, some indicating that corresponds to n = 7, whereas others conclude that at this value, the structure is already 3D. Experimentally, ion mobility spectroscopy measurements, assisted by DFT calculations, have been interpreted as a planar structure for Au7+. Here, we show that the usually assumed 2D non-degenerate ground state cluster structure for Au7+ with D6h symmetry is unstable against a puckering distortion, ending in a 3D stable structure with D3d symmetry. The analysis is based on the pseudo Jahn–Teller effect through vibronic coupling between the ground state and two excited states. This structure presents, in average, the same surface area than the 2D, being therefore compatible with experimental results. We discuss the effects the vibronic coupling would have on Raman, IR and UV-vis spectra. The study is based on a scalar and spin orbit relativistic DFT calculation in the Zero Order Regular Approximation (ZORA).

Theoretical analysis of the Jahn-Teller effect in the LaBaCuO superconductor

Abstract We just commemorated the 10 years anniversary of the discovery of the high Tc superconductivity of Bednorz and Muller (BM). In their celebrated paper BM mentioned the possible role of the Jahn-Teller (JT) effect in the enhancement of the transition temperature to the superconducting state. In the present work we analize, under the quasi-molecular approximation, the conditions for the existence of the JT effect in the CuO6 cluster belongin to the LaBaCuO superconductor of BM. The electronic part is obtained from self consistent ab initio calculations and the force constants are calculated taking the second derivatives of the energy using the Hartree-Fock approximation for the electronic part. Both, the electronic and vibrational parts are classified accordingly to the irreducible representations of the D4h group in order to establish the JT theorem. The physical consecuences of the presence of the JT effect in CuO6 cluster are discussed.

Catalytic Reduction of Nitrous Oxide by the Low-Symmetry Pt8 Cluster

The search for a catalyst for the reduction of nitrous oxide (N2O) is now imperative, as this molecule is a very dangerous pollutant. We found that the low-symmetry Pt8 cluster presents multiple reaction pathways for N2O rupture, which are regioselective. This result was revealed by means of density functional theory calculations within the zero-order-regular approximation, ZORA, explicitly including relativistic effects. It is further proved that Pt8 is a competitive N2O catalyst compared to sub-nanometric rhodium clusters, obtaining similar reaction barriers. The hot adsorption site, a tip atom of Pt8, and the rotation of the N2O molecule over the metallic cluster promote the formation of a frustrated bridge activated transition state, Pt8-N2O. This transition structure yields to spontaneous dissociation of N2O without bridge formation. Along this catalytic process, rearrangements within the metal cluster take place, preserving its stability. Moreover, in addition to being important attributes of the Pt8 particle in the N2O reduction, fluxionality and multiple reaction pathways may also prevent poisoning effects. Overall, this differs from reported results for more symmetric metal particles also used as catalysts.

Surface properties for α-cluster nuclear matter

We introduce a new microscopic model for α-cluster matter, which simulates the properties of ordinary nuclear matter and α-clustering in a curved surface of a large but finite nucleus. The model is based on a nested icosahedral fullerene-like multiple-shell structure, where each vertex is occupied by a microscopic α-particle. The novel aspect of this model is that it allows a consistent description of nuclear surface properties from microscopic parameters to be made without using the leptodermous expansion. In particular, we show that the calculated surface energy is in excellent agreement with the corresponding coefficient of the Bethe-Weizacker semi-empirical mass formula. We discuss the properties of the surface α-cluster state, which resembles an ultra cold bosonic quantum gas trapped in an optical lattice. By comparing the surface and interior states we are able to estimate the α preformation probability. Possible extensions of this model to study nuclear dynamics through surface vibrations and departures from approximate sphericity are mentioned.

Structural Properties of the Bimetallic Cluster Au 12 Ag 6 on the NO x (x=1,2,3) Adsorption Processes

It has been shown that Au and Ag nanoparticles present catalytic properties of pollutant gases, product of the hydrocarbons combustion processes, whose efficiency is strongly dependant on their size, morphology and supporting material. The activity of Ag subnanometric particles supported on alumina in the NO x reduction mechanism, for the reaction HC-SCR (Hydrocarbon Selective Catalytic Reaction) is a good example of these catalytic properties. It is also well established that both Au and Ag nanoparticles are oxygen poisoning resistant and that in certain reactions the bimetallic combination may be more efficient than their isolated parts. In this work we present a DFT calculation of the NO x (x =1, 2, 3) adsorption energies on the Au 12 Ag 6 cluster surface. It is also discussed the possible synergetic effect of the different configurations, forming small Ag islands composed of one to five atoms, on the surface of the bimetallic cluster. The analysis was done using DFT in the ZORA approximation.

Density functional electronic spectrum of the CuO6-10 cluster and possible local Jahn-Teller distortions in the La-Ba-Cu-O superconductor

We present a density functional theory (DFT) calculation in the generalized gradient approximation to study the possibility for the existence of Jahn–Teller (JT) or pseudo Jahn–Teller (PJT) type local distortions in the La–Ba–Cu–O superconducting system. We performed the calculation and correspondingly group theory classification of the electronic ground state of the CuO6-10 elongated octahedra cluster, immersed in a background simulating the superconductor. Part of the motivation to do this study is that the origin of the apical deformation of the CuO6-10 cluster is not due to a pure JT effect, having therefore a non a priori condition to remove the degeneracy of the electronic ground state of the parent regular octahedron. We present a comparative analysis of the symmetry classified electron spectrum with previously reported results using unrestricted Hartree–Fock calculations (UHF). Both the DFT and UHF calculations produced a non-degenerate electronic ground state, not having therefore the necessary condition for a pure JT effect. However, the appearance of a degenerate Eg state near to the highest occupied molecular orbital in the DFT calculation, suggests the possibility for a PJT effect responsible for a local distortion of the oxidized cluster.

Theoretical analysis for the specific heat and thermal parameters of solid C60

Abstract We present the results of a theoretical analysis for the thermal parameters and phonon contribution to the specific heat in solid C 60 . The phonon contribution to the specific heat is calculated through the solution of the corresponding dynamical matrix, for different points in the Brillouin zone, and the construccion of the partial and generalized phonon density of states. The force constants are obtained from a first principle calculation, using a SCF Hartree-Fock wave function from the Gaussian 92 program. The thermal parameters reported are the effective temperatures and vibrational amplitudes as a function of temperature. Using this model we present a parametization scheme in order to reproduce the general behaviour of the experimental specific heat for these materials.

Orientational order phase transition in solid C-60

The phonon spectrum and thermal parameters such as specific heat, effective temperature and amplitude of vibration of solid C{sub 60} are studied from ab-initio calculations. The relative orientations between the C{sub 60} balls are taken into account in a sc phase below 261.4K. The same properties are studied above the transition temperature (261.4K) where the balls are randomly oriented in a fcc structure. A difference is found in some of the thermal parameters due to the relative inter-molecular orientation. In particular, the low energy vibrational spectrum (which is separated by a {approximately}20 meV gap from the intra-molecular spectrum) is changed.

APICAL OXYGEN MOVEMENT IN YBA2CU3O6+X AS A FUNCTION OF THE OXYGEN CONCENTRATION

Abstract We report the results of an analysis of the vibrational properties of the apical oxygen O(4) in YBa2Cu3O6+δ as a function of the the oxygen concentration O(1) in the chains. The analysis is based on the calculation of the behaviour of the partial phonon density of states for O(4) for different x values. Following the movement of the peaks corresponding to different modes we pay special attention to the discussion of possible anomalous displacement of the apical oxygen in the c direction.

Phonon contibution for th elow temperature specific heat of solid C60 and K3C60

Abstract We present the result of a theoretical analysis of the phonon contribution to the specific heat for both pure C 60 and alkaline doped K 3 C 60 fullerenes. This is based on the construction of the phonon density of states through the solution of the dynamical matrix, using a Born-Mayer type potential, for the different points in the Brillouin zone. Our results show the contribution to the specific heat for the different modes and in particular we study the low-lying modes coming from intramolecular motions appearing at low temperatures.