Electronic and magnetic structure of LaMnO3 from hybrid periodic density-functional theory

The electronic and magnetic structures of the LaMnO3 compound have been studied by means of periodic calculations within the framework of spin polarized hybrid density-functional theory. In order to quantify the role of approximations to electronic exchange and correlation three different hybrid functionals have been used which mix nonlocal Fock and local Dirac-Slater exchange. Periodic Hartree-Fock results are also reported for comparative purposes. The A-antiferromagnetic ground state is properly predicted by all methods including Hartree-Fock exchange. In general, the different hybrid methods provide a rather accurate description of the band gap and of the two magnetic coupling constants, strongly suggesting that the corresponding description of the electronic structure is also accurate. An important conclusion emerging from this study is that the nature of the occupied states near the Fermi level is intermediate between the Hartree-Fock and local density approximation descriptions with a comparable participation of both Mn and O states.

Coherent tunnelling in Cu2+- and Ag2+-doped MgO and CaO: Cu2+ explored through ab initio calculations

The observation of coherent tunnelling in Cu2+ - and Ag2+ -doped MgO and CaO:Cu2+ was a crucial discovery in the realm of the Jahn-Teller (JT) effect. The main reasons favoring this dynamic behavior are now clarified through ab initio calculations on Cu2+ - and Ag2+ -doped cubic oxides. Small JT distortions and an unexpected low anharmonicity of the eg JT mode are behind energy barriers smaller than 25 cm-1 derived through CASPT2 calculations for Cu2+ - and Ag2+ -doped MgO and CaO:Cu2+ . The low anharmonicity is shown to come from a strong vibrational coupling of MO610- units (M=Cu,Ag) to the host lattice. The average distance between the d9 impurity and ligands is found to vary significantly on passing from MgO to SrO following to a good extent the lattice parameter.

First principles LDA+U and GGA+U study of cerium oxides: Dependence on the effective U-parameter

The electronic structure and properties of cerium oxides (CeO2 and Ce2O3) have been studied in the framework of the LDA+U and GGA(PW91)+U implementations of density functional theory. The dependence of selected observables of these materials on the effective U parameter has been investigated in detail. The examined properties include lattice constants, bulk moduli, density of states, and formation energies of CeO2 and Ce2O3. For CeO2, the LDA+U results are in better agreement with experiment than the GGA+U results whereas for the computationally more demanding Ce2O3 both approaches give comparable accuracy. Furthermore, as expected, Ce2O3 is much more sensitive to the choice of the U value. Generally, the PW91 functional provides an optimal agreement with experiment at lower U energies than LDA does. In order to achieve a balanced description of both kinds of materials, and also of nonstoichiometric CeO2¿x phases, an appropriate choice of U is suggested for LDA+U and GGA+U schemes. Nevertheless, an optimum value appears to be property dependent, especially for Ce2O3. Optimum U values are found to be, in general, larger than values determined previously in a self-consistent way.

Ab initio study of MF2 (M=Mn, Fe, Co, Ni) rutile-type compounds using the periodic unrestricted Hartree-Fock approach

The ab initio periodic unrestricted Hartree-Fock method has been applied in the investigation of the ground-state structural, electronic, and magnetic properties of the rutile-type compounds MF2 (M=Mn, Fe, Co, and Ni). All electron Gaussian basis sets have been used. The systems turn out to be large band-gap antiferromagnetic insulators; the optimized geometrical parameters are in good agreement with experiment. The calculated most stable electronic state shows an antiferromagnetic order in agreement with that resulting from neutron scattering experiments. The magnetic coupling constants between nearest-neighbor magnetic ions along the [001], [111], and [100] (or [010]) directions have been calculated using several supercells. The resulting ab initio magnetic coupling constants are reasonably satisfactory when compared with available experimental data. The importance of the Jahn-Teller effect in FeF2 and CoF2 is also discussed.

Ionic-covalent transition in titanium oxides

The nature of the chemical bond in three titanium oxides of different crystal structure and different formal oxidation state has been studied by means of the ab initio cluster-model approach. The covalent and ionic contributions to the bond have been measured from different theoretical techniques. All the analysis is consistent with an increasing of covalence in the TiO, Ti2O3, and TiO2 series as expected from chemical intuition. Moreover, the use of the ab initio cluster-model approach combined with different theoretical techniques has permitted us to quantify the degree of ionic character, showing that while TiO can approximately be described as an ionic compound, TiO2 is better viewed as a rather covalent oxide.

Electronic structure and properties of AlN

The electronic structure of the wurtzite-type phase of aluminum nitride has been investigated by means of periodic ab initio Hartree-Fock calculations. The binding energy, lattice parameters (a,c), and the internal coordinate (u) have been calculated. All structural parameters are in excellent agreement with the experimental data. The electronic structure and bonding in AlN are analyzed by means of density-of-states projections and electron-density maps. The calculated values of the bulk modulus, its pressure derivative, the optical-phonon frequencies at the center of the Brillouin zone, and the full set of elastic constants are in good agreement with the experimental data.

Extent and limitations of density functional theory in describing magnetic systems

The performance of density-functional theory to solve the exact, nonrelativistic, many-electron problem for magnetic systems has been explored in a new implementation imposing space and spin symmetry constraints, as in ab initio wave function theory. Calculations on selected systems representative of organic diradicals, molecular magnets and antiferromagnetic solids carried out with and without these constraints lead to contradictory results, which provide numerical illustration on this usually obviated problem. It is concluded that the present exchange-correlation functionals provide reasonable numerical results although for the wrong physical reasons, thus evidencing the need for continued search for more accurate expressions.

La biblioteca electronica en Colima-Mexico

Un sueño se volvió realidad. La tarea gratificadora de colocar a disposición de los usuarios de recursos tecnológicos de información en el ámbito interno e foráneo, emprendida por la Universidad de Colima, en uno de los estados de México. En 1983, la Universidad de Colima creó la Dirección General de Desarrollo Bibliotecario con el objetivo fundamental de estructurar el sistema de bibliotecas universitárias para dar apoyo a la enseñanza e a la investigación. Fue un proyecto que envolvia un arduo trabajo de capacitación de personal, sistematización de información y captación de recursos financieros. La automatización de las bibliotecas posibilitó sistematizar el acervo de las bibliotecas de Colima, además de servir como base para la organización de la primera biblioteca electrónica de America Latina. La biblioteca electrónica de ciencia fué inagurada el día 1° de Junio de 1995. Ofrece servicio on-line de información sobre documentos de su propia biblioteca así como de otras extranjeras. El edifício fué proyectado para acomodar equipos modernos e rápidos, conectados a la Internet. Ocupando un área de 3000 metros, el complejo abriga áreas de consulta automatizada, espacio destinado a textos electrónicos; CD-ROMs, colecciones, fonoteca para 20 usuários; 44 cabinas individuales y 10 colectivas, equipadas con Internet y um auditório con capacidad de 250 persoas para congresos. La biblioteca electrónica despertó un gran interés en los usuários, aumentando los servicios con que atiende desde a una ama de casa como a pesquisadores universitários. Los desafios y las perspectivas de esta nueva experiencia son los de cómo contar con recursos inovadores, personal y usuários capacitados y equipo adecuado.

Identificación y significado de la calcita secundaria en cerámicas arqueológicas

La presencia de calcita como fase secundaria en cerámicas arqueológicas ha sido citada en diversos trabajos. Sin embargo, su identificación e interpretación han sido siempre problemáticas. La calcita secundaria se presenta bajo diferentes hábitos que deben reconocerse por microscopia óptica de luz transmitida o por microscopia electrónica de barrido. Más problemática resulta su interpretación según se trate de un aporte completamente alóctono, parcialmente alóctono o la concurrencia de ambos factores. En el presente trabajo se exponen dos ejemplos en los que se identifica calcita secundaria, pero en los que es interpretada de manera diferente. La interpretación depende de cada caso de estudio y requiere el concurso de diversas técnicas.

Pisa arcaica i ceràmica vidriada del segle XIII a Barcelona. Un estudi arqueomètric

L"estudi arqueomètric d"un lot de pises arcaiques, vaixella verda i ceràmiques comunes vidriades trobades a l"excavació arqueològica del carrer de Sant Honorat ha permès la identificació, ja en el segle XIII, d"una producció local d"aquest tipus de vaixelles a la ciutat de Barcelona, especialment gràcies a la seva similitud amb les peces produïdes al forn del carrer de l"Hospital. Les peces estudiades han estat analitzades mitjançant la fluorescència de raigs X (FRX) i la difracció de raigs X (DRX). Un grup reduït de peces ha estat seleccionat per al seu estudi mitjançant la microscòpia electrònica de rastreig (MER). Els resultats permeten la identificació de 3 grans grups químics considerats com a barcelonins, els quals engloben la major part de les ceràmiques analitzades. A més, s"ha identificat una segona producció també barcelonina, com també tres produccions de les quals ara per ara no es pot proposar una provinença segura. Tecnològicament, s"han estudiat les diferències existents entre les produccions de pisa arcaica i les ceràmiques de vaixella verda.