J.F. Rivas-Silva

The compressed helium atom variationally treated via a correlated Hylleraas wave function

Abstract We calculate ground state energy, critical cage and ionization radius for the confined helium atom centered in a spherical impenetrable box. Total and ionization energies, pressure on the confining boundary and the expectation value 〈 r 12 〉 are obtained as a function of the box radii. Three nonlinear parameters and N (=1,5 and 10) linear coefficients are variationally optimized within wave functions expressed in a generalized Hylleraas basis set that explicitly incorporates the interelectronic distance r 12 , both, via a Slater type exponent and through polynomial terms entering the expansion. The wave function includes a cut-off factor to ensure correct fulfillment of the boundary condition (vanishing wave function at the box edge). With the 10-term basis set we obtain energy values very close (within millihartrees) to the nearly exact results of Joslin and Goldman [J. Phys. B 25 (1992) 1965] obtained by means of a Quantum Monte Carlo method which, computationally, is a great deal more demanding than our variational approach.

Theoretical explanation of the quenching of luminescence in cerium‐doped ytterbium oxyorthosilicate

A remarkable result in applied solid-state physics is that whereas Ce-doped yttrium oxyorthosilicate, Y2(SiO4)O:Ce, is an excellent scintillator, the related Ce-doped ytterbium oxyorthosilicate, Yb2(SiO4)O:Ce, does not scintillate at all at room temperature. These compounds, Y and Yb, besides possessing the same crystal structure, both are trivalent and yield almost identical ionic radii. In order to understand the difference between the luminescent properties of these materials, we have performed an ab initio calculation to investigate the charge-transfer mechanism involving their first excited states. By using a representative cluster model, a crossing is found between the ground and the excited state of the ytterbium compound, though not so in the yttrium compound. This suggests that in the solid state, the luminiscence quenching can occur via a nonradiative transition, although luminescence at low temperature might thus be feasible.

AB INITIO STUDY OF ABSORPTION AND EMISSION PROPERTIES OF BGO AND BSO

Abstract Bismuth ortho-germanate (BGO) and bismuth ortho-silicate (BSO) crystals are described within an atomic cluster model. Electronic ground and first excited state for both crystal structures are calculated through unrestricted Hartree–Fock and configuration interaction methods, where the associated cluster geometries are optimized. The theoretical transition energies corresponding to the absorption and emission processes are very revealing. The displacement of one of the oxygen ions away from the bismuth in the excited state, together with the distributed spin density clearly shows that the excitation process cannot be understood as a deformed bismuth ion excitation. Instead, a molecular-like excitation is proposed.

Electronic calculations on fluorides and oxides of Zr, Hf and Th

Abstract The band gaps of the oxides and fluorides of zirconium, hafnium and thorium are calculated by means of two quantum chemical methods. Through the first the gap is estimated as a one-electron energy given by the HUMO–LOMO splitting, while through the second it is obtained as the energy difference between electronic potentials of crystal clusters computed at their experimental configuration. Doping effects for these compounds are also analyzed via substitutional impurities on Pr4+ sites.

Density Functional Study of the Structural Properties of Copper Iodide: LDA vs GGA Calculations

We have performed first principles total energy calculations to investigate the structural properties of copper iodide (CuI) in its sodium chloride, cesium chloride, zincblende and wurtzite structures. Calculations are done using the density functional theory. We employ the full potential linearized augmented plane wave method as implemented in the wien2k code. The exchange and correlation potential energies are treated in the generalized gradient approximation (GGA), and the local density approximation (LDA). Optical absorption experiments and x-ray diffraction measurements have shown that zincblende is the ground state of CuI. Our calculations find that in the GGA formalism wurtzite and zincblende have similar total energies, while in the LDA formalism the lowest minimum corresponds to zincblende. Results show that the energy difference between the wurtzite and the zincblende structures, as calculated within the GGA formalism is 2 meV, and within the LDA formalism, is 31 meV. These results may suggest a coexistence of both wurtzite and zincblende structures in the ground state of CuI. Structural parameters are correctly reproduced by the GGA calculations. We obtain that under the application of external pressure the atomic configuration may transform into the NaCl structure. At higher pressures it is possible to have a phase transition to the CsCl geometry.

Uranyl complexes formed with a para-t-butylcalix[4]arene bearing phosphinoyl pendant arms on the lower rim. Solid and solution studies

Abstract The current interest in functionalized calixarenes with phosphorylated pendant arms resides in their coordination ability towards f elements and capability towards actinide/rare earth separation. Uranyl cation forms 1:1 and 1:2 (M:L) complexes with a tetra-phosphinoylated p-tert-butylcalix[4]arene, B4bL4: UO2(NO3)2(B4bL4)n· xH2O (n = 1, x = 2, 1; n = 2, x = 6, 2). Spectroscopic data point to the inner coordination sphere of 1 containing one monodentate nitrate anion, one water molecule and the four phosphinoylated arms bound to UO22+ while in 2, uranyl is only coordinated to calixarene ligands. In both cases the U(VI) ion is 8-coordinate. Uranyl complexes display enhanced metal-centred luminescence due to energy transfer from the calixarene ligands; the luminescence decays are bi-exponential with associated lifetimes in the ranges 220 μs <τs <250 μs and 630 μs <τL < 640 μs, pointing to the presence of two species with differently coordinated calixarene, as substantiated by a XPS study of U(4f5/2,7/2), O(1s) and P(2p) levels on solid state samples. The extraction study of UO22+ cation and trivalent rare-earth (Y, La, Eu) ions from acidic nitrate media by B4bL4 in chloroform shows the uranyl cation being much more extracted than rare earths.

Structure Study of ZnO:Eu with the Supercell Method

One of the interests on the study of doped materials with rare earths in their bulk or nanoscale size is owing to the enhancement of the intensity of light in their photoluminescence when a lanthanide exists in a receptor material, as ZnO in our case. Until now, one of the most useful theories for calculations of electronic properties in molecular and solid state systems is the Density Functional Theory (DFT), which is not capable to manage well the presence of high localized electrons, as in lanthanide compounds in general and the doped case in particular. We propose to study these materials with super cell model using some correction to the standard calculations. For this goal, we employ the WIEN2k [1] code using the LDA+U approximation to take into account the strong correlation of the f electrons coming from the lanthanide. We emphasise the study of deformation due to the presence of Eu ion in the structure of host material, optimizing the position of neighboring Oxygen atoms. This deformation has been related to Kondo Resonance [2] appearing around the Fermi Energy of the compound, due to hybridization [3] among the f electrons from rare earth and neighboring oxygen levels.

Atom-in-the-lattice description of Ce3+ impurity centers in alkaline-earth fluorides

Abstract We apply the Perturbed Ion method, amenable to the description of ionic crystal structures within an atom-in-the-lattice scheme, to analyze some electronic properties of Ce-doped BaF 2 and CaF 2 crystals. In addition, we compare with other ab initio molecular orbital methods for model clusters. We find that the most stable structure consists of a substitutional Ce 3+ combined with an interstitial F − to compensate the excess positive charge.

Dopamine and Caffeine Encapsulation within Boron Nitride (14,0) Nanotubes: Classical Molecular Dynamics and First Principles Calculations

Classical molecular dynamics (MD) and density functional theory (DFT) calculations are developed to investigate the dopamine and caffeine encapsulation within boron nitride (BN) nanotubes (NT) with (14,0) chirality. Classical MD studies are done at canonical and isobaric-isothermal conditions at 298 K and 1 bar in explicit water. Results reveal that both molecules are attracted by the nanotube; however, only dopamine is able to enter the nanotube, whereas caffeine moves in its vicinity, suggesting that both species can be transported: the first by encapsulation and the second by drag. Findings are analyzed using the dielectric behavior, pair correlation functions, diffusion of the species, and energy contributions. The DFT calculations are performed according to the BLYP approach and applying the atomic base of the divided valence 6-31g(d) orbitals. The geometry optimization uses the minimum-energy criterion, accounting for the total charge neutrality and multiplicity of 1. Adsorption energies in the dopamine encapsulation indicate physisorption, which induces the highly occupied molecular orbital-lower unoccupied molecular orbital gap reduction yielding a semiconductor behavior. The charge redistribution polarizes the BNNT/dopamine and BNNT/caffeine structures. The work function decrease and the chemical potential values suggest the proper transport properties in these systems, which may allow their use in nanobiomedicine.

Effects of electron doping on the stability of the metal hydride NaH

Alkali and alkali-earth metal hydrides have high volumetric and gravimetric hydrogen densities, but due to their high thermodynamic stability, they possess high dehydrogenation temperatures which may be reduced by transforming these compounds into less stable states/configurations. We present a systematic computational study of the electron doping effects on the stability of the alkali metal hydride NaH substituted with Mg, using the self-consistent version of the virtual crystal approximation to model the alloy Na1-x Mg x H. The phonon dispersions were studied paying special attention to the crystal stability and the correlations with the electronic structure taking into account the zero point energy contribution. We found that substitution of Na by Mg in the hydride invokes a reduction of the frequencies, leading to dynamical instabilities for Mg content of 25%. The microscopic origin of these instabilities could be related to the formation of ellipsoidal Fermi surfaces centered at the L point due to the metallization of the hydride by the Mg substitution. Applying the quasiharmonic approximation, thermodynamic properties like heat capacities, vibrational entropies and vibrational free energies as a function of temperature at zero pressure are obtained. These properties determine an upper temperature for the thermodynamic stability of the hydride, which decreases from 600u2009K for NaH to 300u2009K at 20% Mg concentration. This significant reduction of the stability range indicates that dehydrogenation could be favoured by electron doping of NaH.