R. Böttcher

Study of the tetragonal-to-cubic phase transition in PbTiO3 nanopowders

The CPP (combined polymerization and pyrolysis) preparation route, in its enhanced liquid-precursor-based version, was combined with consecutive soft milling. For studies of temperature- and size-dependent structural changes occurring in ferroelectric lead titanate, this combined route yields a nanopowder series covering the relevant particle-size region at target quality. This material basis enables consistent SEM, TEM, XRD, Raman, EPR and dielectric measurements, which furnish a comprehensive picture of the cooperation between temperature rise and size reduction to eliminate tetragonality and concomitant ferroelectricity. Our previous original EPR studies on nanosized barium titanate are now extended to the lead titanate case. Furthermore, as compared to the pertinent literature standard, the materials basis is extended to powder samples of smaller mean particle sizes, comprising the critical size at which a PbTiO3 particle undergoes a transition into cubic paraelectric phase. Thus, the size-driven phase transition can be observed in a direct way (at 7 nm, which compares to 40 nm for BaTiO3), and the EPR data suggest a much less spacious gradient shell at the particle surfaces (thickness ≈ 2 nm) than in previous analogous investigations on BaTiO3 (15 nm).

The effects of hyaluronan and its fragments on lipid models exposed to UV irradiation

The effects of hyaluronan and its degradation products on irradiation-induced lipid peroxidation were investigated. Liposomal skin lipid models with increasing complexity were used. Hyaluronan and its fragments were able to reduce the amount of lipid peroxidation secondary products quantified by the thiobarbituric acid (TBA) assay. The qualitative changes were studied by mass spectrometry. To elucidate the nature of free radical involvement electron paramagnetic resonance (EPR) studies were carried out. The influence of hyaluronan and its fragments on the concentration of hydroxyl radicals generated by the Fenton system was examined using the spin trapping technique. Moreover, the mucopolysaccharides ability to react with stable radicals was checked. The quantification assay of 2,2-diphenyl-1-picrylhydrazyl hydrate (DPPH) showed no concentration changes of the stable radical caused by hyaluronan. Hyaluronan was found to exhibit prooxidative effects in the Fenton assay in a concentration dependent manner. A transition metal chelation was proposed as a mechanism of this behavior. Considering human skin and its constant exposure to UV light and oxygen and an increased pool of iron in irradiated skin the administration of hyaluronan or its fragments in cosmetic formulations or sunscreens could be helpful for the protection of the human skin.

Role of Ascorbic Acid in Stratum Corneum Lipid Models Exposed to UV Irradiation

AbstractPurpose. The effects of ascorbic acid on Stratum corneum lipid models following ultraviolet irradiation were studied adding iron ions as transition metal catalysts. Methods. Lipid peroxidation was quantified by the thiobarbituric acid assay. The qualitative changes were studied on a molecular level by mass spectrometry. To elucidate the nature of free radical involvement we carried out electron paramagnetic resonance studies. The influence of ascorbic acid on the concentration of hydroxyl radicals was examined using the spin trapping technique. Moreover, we checked the vitamins ability to react with stable radicals. Results. Ascorbic acid was found to have prooxidative effects in all lipid systems in a concentration dependent manner. The degradation products of ascorbic acid after its prooxidative action were detected. The concentration of the hydroxyl radicals in the Fenton assay was decreased by ascorbic acid. The quantification assay of 2,2-diphenyl-1-picrylhydrazyl hydrate showed reduced concentration levels of the stable radical caused by ascorbic acid. Conclusions. Considering human skin and its constant exposure to UV light and oxygen, an increased pool of iron ions in irradiated skin and the depletion of co-antioxidants, the administration of ascorbic acid in cosmetic formulations or in sunscreens could unfold adverse effects among the Stratum corneum lipids.

Preparation of barium titanate ultrafine powders from a monomeric metallo-organic precursor by combined solid-state polymerisation and pyrolysis

A recipe has been elaborated for preparing barium titanate (BaTiO3) particles in a nucleation route which is mediated by thermal decomposition of polymeric barium titanium methacrylate. Adjustment of particle size d in the range from 10 nm to 1.5 μm is easily done by choosing appropriate reaction temperatures and tempering atmospheres. In particular, doping with paramagnetic probe ions such as Mn2+, Gd3+ or Cr3+ can be readily accomplished by just adding the corresponding metal acetates to the monomeric precursor. In addition to well approved standard techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC), the complementary spectroscopic methods electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), X-ray absorption near edge structure (XANES) and FT-Raman are applied to characterise the micro- and nanocrystalline BaTiO3 powders prepared and to study the phase transition behaviour in dependence on the mean particle size. In contrast to expectation from literature, the dimensionality effect does not manifest itself in a temperature shift of the ferroelectric phase transition but, instead, the tetragonal-to-cubic phase transition is smeared out at reduced particle size and an increasing tetragonal-to-cubic phase admixture is detected.

3C-6H phase transition in BaTiO3 induced by Fe ions : an electron paramagnetic resonance study

X-ray diffraction (XRD) patterns and electron paramagnetic resonance (EPR) powder spectra (9 and 34 GHz) of BaTiO3+0.04 BaO+0.5xFe2O3 (0.0005≤x≤0.02) ceramics were studied to investigate the development of the hexagonal phase (6H modification) of Fe-doped material in dependence upon doping level x and sintering temperature Ts. Three axially symmetric EPR spectra assigned to Fe3+ ions substituted at Ti lattice sites corresponding to the different distorted octahedra in tetragonal and hexagonal modification are observed at room temperature. The presence of a hexagonal phase is shown by the XRD pattern and the EPR spectra. The 6H modification begins to occur at a nominal Fe concentration of between x = 0.005 and 0.01 and increases with increasing sintering temperature. BaTiO3 ceramics with x = 0.02 sintered at Ts = 1400 °C is hexagonal. As discussed in the case of other 3d ions we propose the Jahn–Teller (JT) distortion as the driving force for the cubic–hexagonal transition at high temperatures whereas a sufficiently high concentration of oxygen vacancies is the second condition for this transformation process. In the case of Fe-doped BaTiO3 a minimum concentration of Fe4+ in the ceramic grains could trigger the transmutation into the hexagonal phase.

Q-Band single-crystal EPR study and molecular orbital calculations of [(C6H5)4As][ReVINCl4/ReVOCl4]

Abstract A Q -band single-crystal EPR study of tetraphenylarsoniumtetrachloro-nitridorhenate(VI), [(C 6 H 5 ) 4 As][Re VI NCl 4 ], diamagnetically diluted by the isoelectronic oxorhenate(V) complex is reported. The EPR spectra are typical of an ion with 5d 1 ( S =1/2) configuration and are influenced by large rhenium hyperfine coupling constants and nuclear quadrupole interactions. They are characterized by well-resolved 185,187 Re hyperfine patterns with almost equal spacings and the occurrence of “forbidden” transitions ( Δm I =±1, Δm I =±2). The 185,187 Re hyperfine parameters as well as the data obtained from density functional theory (DFT) and the extended Huckel theory (EHT) molecular orbital calculations are used to analyze the spin density distribution in the system under study. Thereby, the negative sign found for the spin density at the nitrogen from DFT is mainly determined by spin polarization. In addition, both MO methods are used to calculate the electric field gradient being responsible for the 185,187 Re nuclear quadrupole coupling.

Local order-disorder behaviour in Cr3+-doped dimethylammonium aluminium sulphate hexahydrate (DMAAS) studied by electron paramagnetic resonance

Electron paramagnetic resonance (EPR) investigations of the ordering behaviour in single crystals of Cr3+-doped dimethylammonium aluminium sulphate hexahydrate (DMAAS) are presented at the transition from the ferroelastic to the ferroelectric phase. The measurements on chromium-doped DMAAS show that the Cr3+ probe substitutes at aluminium sites. Remarkable line-shape changes and line splittings are observed in the EPR experiment in contrast to the findings of former 27Al nuclear magnetic resonance studies. The EPR measurements confirm the order-disorder character of the ferroelectric phase transition at 151 K on a microscopic level and give information about the local order parameter of the dimethylammonium reorientation and the relation of the dimethylammonium reorientational motion to the critical dynamics.

Physico-chemical modification effects of nanoparticles in radiation-cured polymeric composites

Abstract For studying nanoglobular modification effects in radiation-cured polymeric composites, we prepared polymerisation-active metallo-organic cerium(IV) sorbate (CeSo) and ferroelectric barium titanate (BaTiO3) nanoparticles. Through electron beam curing (EBC) of nanopowder/acrylate dispersion films the latter particles can impart ferroelectricity on thin polymeric nanocomposite foils which are of potential importance in piezo- and pyrosensorics. The CeSo nanoparticles, through the high redox potential of the metal ions, can undergo pronounced radiation-induced solid-state polymerisation. In radiation-cured polymeric nanocomposites the CeSo nanoparticles form crosslinks, thus efficiently modifying the viscoelastic properties. In the event, quite analogous silico-organic nanoparticles were used to apply this modification scheme to novel scratch and abrasion resistant coatings.

Preparation of lead titanate ultrafine powders from combined polymerisation and pyrolysis route

Lead titanate (PbTiO3) nanopowders were prepared from a monomeric metallo-organic precursor through combined solid-state polymerisation and pyrolysis (CPP). This makes it possible to adjust the mean particle size in a wide range by just choosing the appropriate reaction temperature. This particular preparation route was studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The size effects in the resulting PbTiO3 nanopowders were investigated by standard methods such as X-ray diffraction (XRD) and FT-Raman spectroscopy. By using the double Voigt method in analysing the XRD results, smaller mean particle size and narrower size distributions were found for powders prepared at lower reaction temperatures, with the tetragonality being reduced. Preliminary electron paramagnetic resonance (EPR) measurements demonstrate that paramagnetic chromium probe ions incorporate very well into the PbTiO3 lattice, particularly enabling corresponding high-field EPR measurements which have proven exceedingly informative in our previous investigations on Mn2+-doped barium titanate (BaTiO3) nanopowders. Moreover, the potential of the CPP route is enhanced to prepare perovskitic stoichiometric solutions for advanced practical applications.

Structural phase transitions in partially deuterated betaine phosphite crystals studied by dielectric and electron paramagnetic resonance methods

Abstract Partially deuterated betaine phosphite (DBPI) crystals were investigated by measurements of the dielectric constant and by means of electron paramagnetic resonance (EPR). Two phase transitions could be detected in the temperature range from 100 K to 380 K: a high-temperature phase transition at Tc1 = (355 ± 3) K and a ferroelectric phase transition at Tc2 = (297 ± 3) K. The EPR studies show that the ferroelectric phase transition is related to the simultaneous ordering of the protons and deuterons in the O-H/D… O bonds.