A High-Pressure Praseodymium Fluoride Borate Linking Multiple Structural Features of Apatite-Type Compounds

Pr₅(BO₄)_3−x(BO₃)_x(F,OH)_2.67O_0.28 (x≈1.6), a boron-containing fluoride-oxoapatite-like compound, was obtained by the application of high-pressure/high-temperature synthesis. It exhibits a superstructure of the apatite type with a tripled c lattice parameter (space group P6₃/m) and shows complex anion disorder along the 6₃ screw axis and occupation of distorted octahedra, as well as almost trigonal planar sites, by oxygen and fluorine atoms. Furthermore, a distinct BO₄/(BO₃+F) group disorder is found; 46 % of the sites being occupied by BO₄ groups and 54 % by BO₃ groups, with a fluoride ion located near the missing oxygen atom. The rare earth cations in the 4f sites exhibit a specific distorted tricapped trigonal prismatic coordination with a mean metaprism twist angle of 21.3°. The crystal structure of Pr₅(BO₄)_3−x(BO₃)_x(F,OH)_2.67O_0.28 (x≈1.6) shows much “flexibility” resulting in split and off-site positions of all other rare earth cations. The title compound therefore combines many structural features of apatite-like compounds, for example biologically highly-important carbonated apatites, shedding more light onto the complex structural chemistry of apatites.     

Filamentous Supramolecular Structures

Summary: The formation of four filament forming proteins was studied mainly by light scattering as a function of the scattering angle. Besides the molar mass M_w and the radius of gyration R_g the contour length L, was determined. The corresponding structure parameters are compared with those of the Aβ-amyloid. A minimum cross-sectional diameter of 2 nm appeared to be necessary for filament stabilization. Bundle and network formation are often observed and are tentatively explained by thermodynamic arguments. The local conformation of the unimer proteins in the filaments remained largely unexplored. Only in one example CD and IR spectroscopy was applied. The analysis disclosed a β-sheet/α-helix transition on de-naturation, as was conjectured before as reason for the Aβ-amyloid formation.     

Critical association and thermoreversible gelation of some selected polymers

The study of associating polymers is complicated by the fact that at a finite concentration only an apparent molar mass is directly measured. This apparent molar mass deviates from the true one because of thermodynamic interaction. In a good solvent the repulsive interaction can be fully described in terms of the second virial coefficient, which can be measured at very low concentrations, and a correction can be made for the true molar mass. The technique has been successively applied to β-galactosidase in a phosphate buffer and to cellulose 2,5-acetate in acetone. The gel point could be determined, and critical behavior was found in very good agreement with percolation prediction. Two other methods were employed and tested. One is based on the condition that the longest relaxation time must diverge at the gel point, and correspondingly the translational diffusion coefficient should go to zero. The second criterion consists of the prediction that power law behavior should be observed for the time correlation function of dynamic light scattering as well as for the frequency dependent storage and loss moduli. These predictions were indeed found to hold for two polysaccharides which form thermoreversible gels in water at a fairly low salt concentration. The two polysaccharides are the Tamarind seeds polysaccharide in 1M Na₂SO₄ and an exopolysaccharide from Rhizobium leguminosarum, strain 8002 in 0.1M NaCl solutions respectively.     

Molecular characterization of starch polysaccharides

Light scattering and rheologic measurements have been carried out with native starch in DMSO and with degraded amylopectin of a starch hydrolysate in water at different temperatures. The measurements of dilute solutions in DMSO were made to get a reference for the molecularly dispersed state. In water association take place which under certain conditions leads to gelation. An aqueous solution of about 11.5% (w/w) degraded amylopectin was investigated in detail. Structural change was observed during cooling the solution below 20°C with both techniques. A fractal dimensions of about 2.5 was found at temperatures above 20°C indicating non-swollen random aggregates. Light scattering proved to be the more sensitive method than rheology, but in both experiments a structural collapse was detectable. A fractal dimension of about 1 was now obtained which is characteristic for rigid, elongated supra-structures. A certain time after this collapse an increase of loss and storage module and particle growth in light scattering were obtained. The crossing of the G' and G” was taken as point of gelation and the same gelling time was estimated from light scattering. Hence, the combination of oscillatory rheology with light scattering is an appropriate procedure for a thorough investigation of such complex, structure forming systems.     

The role of hydrogen in low-temperature MOVPE growth and carbon doping of In0.53Ga0.47As for InP-based HBT

Hydrogen radicals are decisive for the low-temperature growth and carbon doping of In_0.53Ga_0.47As in LP-MOVPE. This is demonstrated for the growth of highly p-type doped In_0.53Ga_0.47As layers with CCl₄ as dopant source. Perturbed angular correlation measurements (PAC) were used to investigate the passivation of acceptors by hydrogen in low-temperature grown In_0.53Ga_0.47As. Based on the above analysis an InP-based layer stack is developed which employs low-temperature growth of the base layer, high-temperature growth of the remaining HBT layers, and an in situ post-growth annealing under TMAs/N₂ ambient.