TiO2-rich reconstructions of SrTiO3(0 0 1): a theoretical study of structural patterns

We have recently reported structure solutions for the (2 × 1) and c(4 × 2) reconstructions of SrTiO₃(0 0 1) based on high-resolution electron microscopy, direct methods analysis of diffraction data and density functional theory. Both reconstructions were found to be TiO₂-rich and feature a single overlayer of TiO₂ stoichiometry on top of a bulk-like TiO₂ layer. Qualitatively, the two reconstruction geometries differ in the cation sub-lattice of the overlayer only, where Ti atoms occupy half of the fivefold cation sites. In the present work we use density functional theory to generate a number of variations of this structural motif in search of patterns of stability. We find a reliable predictor for the reconstruction energy in the ability of oxygen atoms to relax vertically out of the overlayer plane to minimize non-bonded oxygen-oxygen repulsions. Out-of-plane relaxation of oxygen atoms in turn is modulated by the number and relative position of coordinating Ti atoms, which yields simple empirical rules as to how cations are distributed in low energy reconstructions.     

Method for computing the anisotropy of the solid-liquid interfacial free energy.

We present a method to compute accurately the weak anisotropy of the solid-liquid interfacial free energy, a parameter which influences dendritic evolution in materials with atomically rough interfaces. The method is based on monitoring interfacial fluctuations during molecular dynamics simulation and extracting the interfacial stiffness which is an order of magnitude more anisotropic than the interfacial free energy. We present results for pure Ni with interatomic potentials derived from the embedded atom method.     

Crystallizable trans-tactic copolyalkenamers

Random, essentially trans copolyalkenamers, obtained by ring opening copolymerization of cyclo-olefins in the presence of a metathesis catalyst, exhibit X-ray crystallinity. Interpretation of the X-ray fibre spectra indicates mutual isomorphous replacement of trans alkenamer units of widely different length in the crystal lattice, resulting in lowering of melting temperatures and degrees of crystallinity of the copolymers as compared to the corresponding homopolyalkenamers.     

Spectroscopic evidence of monomeric aluminium phthalocyanine tetrasulphonate in aqueous solutions

Aqueous solutions of aluminium phthalocyanine tetrasulphonate (AlPcS(4)) were investigated by means of absorption and fluorescence spectroscopy. The absorption spectrum of AlPcS(4) is independent of concentration in a wide range (from 10(-8) to 10(-4) M). The fluorescence spectrum measured with a standard setup is strongly dependent on AlPcS(4) concentration, and the fluorescence maximum is gradually red-shifted with increasing concentration. Calculations that take into account reabsorption of fluorescence (inner-filter effect) fit the experimental observations at low concentrations (up to 10(-6) M). Disagreement between the calculations and spectra recorded at higher concentrations (above 10(-5) M) shows that the reabsorbed light may be reemitted as fluorescence. The influence of inner-filter effects on the spectral shape was demonstrated by the experiments where a fibre-optic front-face fluorescence setup was applied: Under such conditions the shape of the fluorescence spectra for a high concentration (10(-3) M) coincided with that of a low concentration (10(-8) M). In conclusion, the present spectroscopic results show that AlPcS(4) does not form aggregates and is a very stable compound in aqueous solutions.     

Temperature effect on accumulation of protoporphyrin IX after topical application of 5-aminolevulinic acid and its methylester and hexylester derivatives in normal mouse skin

Significant amounts of protoporphyrin IX (PpIX) are formed after 6 min of topical application of 5-aminolevulinic acid (ALA) and its hexylester derivative, whereas PpIX is formed after 10 min of topical application of ALA-methylester derivative in normal mouse skin at 37 degrees C. Lowering the skin temperature to 28-32 degrees C by the administration of the anesthetic Hypnorm-Dormicum reduces the PpIX fluorescence by a factor of 2-3. Practically no PpIX was formed as long as the skin temperature was kept at 12-18 degrees C. At around 30 degrees C PpIX fluorescence appears later after application of ALA-ester derivatives (14-20 min) than after application of ALA (8 min), indicating differences in their bioavailability (delayed penetration through the stratum corneum, cellular uptake, conversion to ALA, PpIX production) in mouse skin in vivo. The difference in lag time in the PpIX formation after application of ALA and ALA-esters may be partly related to deesterification of the ALA-ester molecules. The temperature dependence of PpIX production may be used for improvement of photodynamic therapy with ALA and ALA-ester derivatives, where accumulation of PpIX can be selectively enhanced by increasing the temperature of the target tissue.     

NMR Crystallography: Evaluation of Hydrogen Positions in Hydromagnesite by 13C{1H} REDOR Solid‐State NMR and Density Functional Theory Calculation of Chemical Shielding Tensors

Solid‐state NMR measurements coupled with density functional theory (DFT) calculations demonstrate how hydrogen positions can be refined in a crystalline system. The precision afforded by rotational‐echo double‐resonance (REDOR) NMR to interrogate ¹³C–¹H distances is exploited along with DFT determinations of the ¹³C tensor of carbonates (CO₃^2?). Nearby ¹H nuclei perturb the axial symmetry of the carbonate sites in the hydrated carbonate mineral, hydromagnesite [4 MgCO₃?Mg(OH)₂?4 H₂O]. A match between the calculated structure and solid‐state NMR was found by testing multiple semi‐local and dispersion‐corrected DFT functionals and applying them to optimize atom positions, starting from X‐ray diffraction (XRD)‐determined atomic coordinates. This was validated by comparing calculated to experimental ¹³C{¹H} REDOR and ¹³C chemical shift anisotropy (CSA) tensor values. The results show that the combination of solid‐state NMR, XRD, and DFT can improve structure refinement for hydrated materials.     

Atomic-Scale Simulation Study of Equilibrium Solute Adsorption at Alloy Solid-Liquid Interfaces

Equilibrium structural properties of solid-liquid interfaces in Cu-Ni alloys are studied by Monte-Carlo simulations employing interatomic potentials based on the embedded-atom method. We describe a thermodynamic-integration approach used to derive bulk concentrations and densities for solid and liquid phases in two-phase thermodynamic equilibrium. These results are used as a basis for constructing three-dimensional supercell geometries employed in Monte-Carlo-simulation studies of solid-liquid interface properties for {100} and {111} crystallographic orientations. At a temperature of 1750 K (four percent below the calculated melting point of pure Ni) equilibrium density and concentration profiles have been derived, allowing a calculation of the relative Gibbsian adsorption, , of Cu (solute) relative to Ni (solvent) at solid-liquid interfaces in Ni-rich alloys. We derive absorption values of and –0.23 ± 0.50 atoms/nm² for {100} and {111} interfaces, respectively. These results are discussed in the context of available experimental measurements and continuum-theory results for adsorption at heterophase interfaces.     

Electrodeposition of Mo-Se thin films from a sulfamatic electrolyte

Mo-Se thin films have been electrodeposited on conducting tin oxide (SnO₂) coated glass substrates from a sulfamatic solution containing Na₂MoO₄ and H₂SeO₃ under potentiostatic conditions. The deposition potential varied from –0.6 V to –0.9 V, at a deposition temperature of 20–40 °C and pH 6.5. X-ray diffraction analysis revealed that the overall composition of the films deposited is consistent with the formation of MoO₂ and MoSe₂. The lattice parameters of the as-deposited MoSe₂ are a=b=3.2340 Å and c=13.2859 Å, which fits a hexagonal structure.