Adsorption of atomic and molecular oxygen on the LaMnO3(001) surface: ab initio supercell calculations and thermodynamics

We present and discuss the results of ab initio DFT plane-wave supercell calculations of the atomic and molecular oxygen adsorption and diffusion on the LaMnO(3) (001) surface which serves as a model material for a cathode of solid oxide fuel cells. The dissociative adsorption of O(2) molecules from the gas phase is energetically favorable on surface Mn ions even on a defect-free surface. The surface migration energy for adsorbed O ions is found to be quite high, 2.0 eV. We predict that the adsorbed O atoms could penetrate the electrode first plane when much more mobile surface oxygen vacancies (migration energy of 0.69 eV) approach the O ions strongly bound to the surface Mn ions. The formation of the O vacancy near the O atom adsorbed atop surface Mn ion leads to an increase of the O-Mn binding energy by 0.74 eV whereas the drop of this adsorbed O atom into a vacancy possesses no energy barrier. Ab initio thermodynamics predicts that at typical SOFC operation temperatures (approximately 1200 K) the MnO(2) (001) surface with adsorbed O atoms is the most stable in a very wide range of oxygen gas pressures (above 10(-2) atm).     

Fractality of materials based on silica gel

Based on the analysis of the isotherms of N₂ adsorption, the fractal dimension and other texture geometric parameters were determined for silica gel, modified silica gel, and relevant composite materials. The correlations between the texture parameters were found. A criterion for the estimation of the efficiency of using the composite adsorbent volume was proposed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 781–785, May, 2000.     

Atomic, electronic and thermodynamic properties of cubic and orthorhombic LaMnO3 surfaces

We studied in detail the atomic and electronic structure of the LaMnO₃ surfaces, in both cubic and orthorhombic phases, combining GGA-plane wave approach, as implemented into the VASP-4.6.19 computer code, with a slab model. These studies are complemented by a thermodynamic analysis of the surface stability at different gas pressures and temperatures. The obtained results are compared with similar studies for other ABO₃-perovskites.     

Fast oxygen exchange kinetics of pore-free Bi(1-x)Sr(x)FeO(3-δ) thin films

The oxygen incorporation/extraction kinetics of the potential solid oxide fuel cell (SOFC) cathode material Bi(1-x)Sr(x)FeO(3-δ) with x = 0.5 and 0.8 was studied by electrochemical impedance spectroscopy on geometrically well-defined pore-free thin film electrodes. The oxygen exchange rate was found to be higher than that of La(1-x)Sr(x)FeO(3-δ) and-among cobalt-free perovskites-only surpassed by Ba(1-x)Sr(x)FeO(3-δ) which is however known to be unstable in a SOFC environment.     

Influence of nonadsorbable component on the dynamics of water sorption on composite adsorbent

Experimental data are analyzed on water adsorption in the presence of a nonadsorbable component on the composite adsorbent silica gel impregnated with calcium chloride for different partial pressures of water and air. It is established that the mass-transfer mechanism inside the grain is Knudsen diffusion. It is shown that a dynamic structure is formed in the system, with increased concentration of the nonadsorbable component close to the external surface of the grain, causing externally diffusive resistance. The coefficients of heat exchange between the grain and the gas phase are defined.     

The formulae for calculation of g-tensor shifts in third-order perturbation theory

The analytical formulae have been derived which allow to carry out calculations of the g-tensor of paramagnetic systems in the molecular-orbital approximation. It is based on third-order perturbation theory and is valid for arbitrary values of the total electron spin of the system.     

The energy-momentum spectrum in theP(ϕ)2 quantum field theory

TheP()₂ interaction with the periodic boundary conditions is considered. It is shown that the energy-momentum spectrum lies in the forward light cone. As a consequence, this result implies that theP()₂ theory in the infinite volume with the periodic boundary conditions is Lorentz invariant.     

The energy-momentum spectrum in the Yukawa2 quantum field theory

We prove that the Yukawa₂ quantum field theory with periodic boundary conditions satisfies the spectral condition, i.e., the joint spectrum of the energy operatorH and the momentum operatorP is contained in the forward cone. In addition, the -bound is obtained.     

Ab initio Hartree-Fock calculations of LaMnO3 (110) surfaces

We present the results of ab initio Hartree-Fock calculations of the LaMnO₃ (110) surface. Using seven-plane slabs, periodic in 2D and containing three formula units, we compare the properties of a stoichiometric surface with oxygen vacancies, and non-stoichiometric, defect-free surfaces, analyze the dispersion of the effective charges near the surface, and calculate the surface energy before and after relaxation, for both ferromagnetric and antiferromagnetic spin orderings in a slab.