Frank Kubel

Structure of a Ferroelectric and Ferroelastic Monodomain Crystal of the Perovskite BiFeO3

A ferroelec. and ferroelastic monodomain single crystal of BiFeO3 at space group R3c is rhombohedral, with a 5.57874(16), c 13.8688(3) .ANG.; arh 5.6343 .ANG., arh 59.348 Deg; d. (calcd.) = 8.337 for Z = 6. Final R = 2.4, Rw = 1.9% for 422 reflections. The structure can be described as a rhombohedrally distorted simple cubic perovskite cell. The O octahedron is distorted with min. and max. O-O distances of 2.710(7) and 3.015(9) .ANG., resp., and rotated by about +- a = 13.8(3) Deg around the 3-fold axis. The Fe atom is shifted away from the center of the deformed octahedron by .apprx.0.134(7) .ANG. along the 3-fold axis. The Bi atom is shifted with respect to 2 neighboring octahedron centers by .apprx.0.540(7) .ANG. along the 3-fold axis. The ferroelec. poling mechanism was analyzed: atom displacements were calcd. for 60, 120 and 180 Deg switching of Ps and found to be 0.44, 0.62 and 0.82 .ANG. for Fe. Av. oxygen displacements for +a (-a) rotated octahedra were 0.87 (0.89) 1.13 (1.24) and 1.34 (1.53) .ANG.. Av. Bi shifts were <0.06 .ANG.. As the displacements increase in the order 60, 120 and 180 Deg switching of Ps, the 180 Deg reversal must have a very high activation energy and will be avoided.

Oral bioavailability of a poorly water soluble HIV-1 protease inhibitor incorporated into pH-sensitive particles: effect of the particle size and nutritional state.

The new chemical entity CGP 70726, a very poorly water-soluble HIV-1 protease inhibitor, was incorporated into pH-sensitive nanoparticles and microparticles made of the poly(methacrylic acid-co-ethylacrylate) copolymer Eudragit((R)) L100-55. The particles were characterized in terms of morphology, size distribution, drug loading, production yield and dispersion state of the drug inside the polymeric matrices. Aqueous dispersions of the particles were administered orally to Beagle dogs against a suspension of free drug (control formulation) all at a dose of 100 mg/kg. Oral administration was conducted in the absence and presence of food. Plasma concentrations and pharmacokinetic parameters were determined within 8 h post-dose. While no measurable absorption of the drug resulted after administration of the control formulation, substantial systemic exposure to the compound was obtained with both kinds of pH-sensitive formulations. The selective release of CGP 70726 in a highly dispersed/amorphous state and creation of high concentrations close to its absorption site was thought to account for this positive result. The largest areas under the plasma concentration-time curve (AUC) were obtained in the fasted state, with slightly better performance of the microparticles over the nanoparticles, in both nutritional states (7.8+/-1.5 versus 5.8+/-0. 8 micromol.h/l in the fasted state; 4.4+/-1.4 versus 2.00+/-0.5 micromol.h/l in the fed state). With these results, the potential of pH-sensitive particles for the oral delivery of HIV-1 protease inhibitors with low water solubility was confirmed.

X-ray room temperature structure from single crystal data, powder diffraction measurements and optical studies of the aurivillius phase Bi5(Ti3Fe)O15

Abstract The room temperature structure of the ferroelastic and presumably ferroelectric Aurivillius type compound Bi5(Ti3Fe)O15 (Mr = 1484.4, Dx = 8.065 Mg/m3 Z = 4) was refined by X-rays on a weakly twinned crystal prepared in a Bi2O3/B2O3 flux with TiO2/Fe2O3 in stoichiometrical proportions. The lattice constants were found to be orthorhombic with a = 5.4318(6), b = 41.149(4) and c = 5.4691(12)A. Refinement in space group Fmm2 converged at R(Rw). = 4.6 (4.2%) with 538 averaged reflections. As the crystal showed <5% of twin domains, a supplementary powder refinement was performed to confirm space group Fmm2. The refinement gave confidential values R(Rw) = 3.1 (4.3%) and R Bragg = 7.4%. No superstructure reflections were found. The structure is built up of layers, composed of two slightly and two strongly distorted perovskite units. These layers are separated by oxygen layers. Birefringence, Δn, was restudied at room temperature on a wedge shaped crystal and found to be 0.080 ± 4. When studied as a funct...

Injection-molding versus extrusion as manufacturing technique for the preparation of biodegradable implants

Polylactic acid (PLA) is a biocompatible and biodegradable material with wide utility for many applications, including the design of controlled-release systems for pharmaceutical agents. The factors determining the degradation kinetics of these systems include the composition and the molecular mass of the polymer, the morphology and the structure of the device, and the influence of thermal processes. The processing of the polymer determines the structure and design of the device, and influences to a high degree its morphology, namely its microporous structure, polymeric chain orientation and crystallinity.In this work, we aimed to compare the influence of two different implant manufacturing techniques, extrusion and injection-molding, on the in vitro degradation of the polymeric matrix. Both kinds of implants were loaded with a somatostatin analogue. Decrease in molecular weight, and polydispersity evolution during an accelerated in vitro degradation test were studied by size exclusion chromatography. Morphological changes in the polymeric matrix during degradation were followed after defined time intervals by means of scanning electron microscopy. Crystallinity studies were performed by differential scanning calorimetry and by X-ray analysis. Peptide stability in the polymeric matrix after both manufacturing methods was evaluated. Peptide release profiles, obtained in vitro during a week dissolution test, from both implant samples, were studied. It was shown that both molecular weight and polydispersity decreased after extrusion or injection-molding. This decrease was more pronounced with the latter technique. Crystallinity studies demonstrated that the crystalline network was not destroyed after both manufacturing methods. Peptide release profiles obtained in vitro were in good accordance with scanning electron microscopy. It was found that both manufacturing techniques had to be considered, although the extruded implants degraded more rapidly in vitro than the injection-molded ones.

Growth, twinning and etch figures of ferroelectric/ferroelastic dendritic BiFeO3 single domain crystals

Abstract Ferroelectric/ferroelastic dendritic single domain crystals of BiFeO 3 were grown in the ferroelectric phase in a Bi 2 O 3 /B 2 O 3 / Fe 2 O 3 flux using a platinum crucible, placed in an evacuated sealed quartz tube. Etching of the crystals shows the non-centrosymetric polar character of the crystals. The single domains can be rendered ferroelastically polydomain by heating above the phase transition temperature of 1110 K and subsequent cooling or by mechanical twinning using stress components along the spontaneous polarization direction, which represents the longest pseudocubic space diagonal. Mechanical detwinning has also been observed. Etching of the twinned crystals at room temperature showed no 180° ferroelectric domains inside the ferroelastic domains.

Structural phase transition and high temperature phase structure of Friedels salt, 3CaO.Al2O3.CaCl2.10H2O

Friedels salt, the chlorinated compound 3CaO · Al2O3 · CaCl2 · 10H2O (AFm phase), presents a structural phase transition at about 30°C from a monoclinic to a rhombohedral phase. It has been studied by X-ray powder diffraction and optical microscopy in transmitted light with crossed polarisers on single crystals prepared by hydrothermal synthesis. The high temperature phase was determined at 37°C from X-ray single crystal diffraction data. The compound crystallises in the space group Rc with lattice parameters of . The refinement of 498 independent reflections with I > 2σ(I) led to a residual factor of 7.1%. The Friedels salt can be described as a layered structure with positively charged main layers of composition [Ca2Al(OH)6]+ and negatively charged layers of composition [Cl−,2H2O]. The chloride anions are surrounded by 10 hydrogen atoms, of which six belong to hydroxyl groups and four to water molecules. The structural phase transition may be related to the size of the chloride anions, which are not adapted to the octahedral cavity formed by bonded water molecules.

Large enhancement of magnetostriction due to compaction hydrostatic pressure and magnetic annealing in CoFe2O4

Polycrystalline CoFe2O4 was produced by a ceramic method. The heat-treated powder was pressed, varying the hydrostatic pressure between 87 and 278 MPa, and was heat-treated again at 1350 °C for 24 h. All magnetic parameters showed a clear dependence on this hydrostatic pressure. The saturation magnetization showed a minimum, and the coercivity, the anisotropy, and the magnetostriction showed a maximum at compaction pressures around 150 MPa. This pressure dependence of the magnetic parameters can be explained by a cation redistribution due to the hydrostatic pressure and heat treatment. Additionally, all samples were field-annealed in an external field of 10 T (at 300 °C for 3 h). The field-annealing process causes an induced uniaxial anisotropy, which results in a reduction of the coercivity (in the easy axis) as well as a dramatic increase in the magnitude of the magnetostriction along the hard axis. Maximum magnetostriction value of -400×10-6 was obtained. Additionally, dλ/dH is increased within a facto...

THE CRYSTAL STRUCTURES OF SRALF5 AND BA0.43(1)SR0.57(1)ALF5

Two members belonging to the pseudobinary phase diagram of Ba/SrF2 and AlF3 were synthesized. Single domain crystals of SrAlF5 and Ba0.43(1)Sr0.57(1)AlF5 were prepared from the corresponding metal fluorides. The mixed compound Ba0.43(1)Sr0.57(1)AlF5 has been synthesized and characterized in detail for the first time. The structure of SrAlF5 was reinvestigated; a superstructure was found which is absent in Ba0.43(1)Sr0.57(1)AlF5. The compounds crystallize at room temperature in the centrosymmetric tetragonal space groups I41/a and I4/m. Lattice parameters are a = b= 1988.22(14), c = 1432.24(19) and a = b = 1431.32(14), c = 722.83(7) pm. Both structures differ mainly in the AlF6 arrangement situated in the cavities of the matrix structure. In SrAlF5, ordered dimer [Al2F10]4– units are found, whereas in the Ba2+ mixed sample a linear chain of AlF6 octahedra is observed. For both compounds no partial occupation was observed. Sm2+ doped samples are further characterized by luminescence measurements. Four independent sites of alkaline earth atoms were found in the Sm2+ luminescence of the host SrAlF5. Die Kristallstrukturen von SrAlF5 and Ba0.43(1)Sr0.57(1)AlF5 Zwei Verbindungen des pseudobinaren Phasendiagramms Ba/SrF2 and AlF3 wurden synthetisiert. Eindomanenkristalle von SrAlF5 und Ba0.43(1)Sr0.57(1)AlF5 wurden aus den entsprechenden Metallfluoriden hergestellt. Die gemischte Verbindung Ba0.43(1)Sr0.57(1)AlF5 wurde synthetisiert und zum ersten Mal detailliert charakterisiert. Die Struktur von SrAlF5 wurde neu bestimmt; eine Uberstruktur wurde gefunden, welche bei der Verbindung Ba0.43(1)Sr0.57(1)AlF5 nicht auftritt. Die Verbindungen kristallisieren bei Raumtemperatur in den zentrosymmetrischen Raumgruppen I41/a bzw. I4/m. Gitterkonstanten sind a = b = 1988.22(14), c = 1432.24(19) bzw. a = b=1431.32(14), c = 722.83(7) pm. Beide Strukturen unterscheiden sich hauptsachlich in der Anordnung der AlF6 Oktaeder, welche sich in den Hohlraumen der Hauptstruktur befinden. In SrAlF5 befinden sich geordnete dimere [Al2F10]4–-Einheiten, wahrend in der mit Barium substituierten Verbindung Ketten von AlF6 Oktaedern beobachtet wurden. Bei beiden Verbindungen wurden keine partiellen Besetzungen beobachtet. Sm2+ dotierte Verbindungen wurden durch Lumineszenzmessungen zusatzlich charakterisiert. Das Lumineszenzspektrum von Sm2+ ergab vier unabhangige Atomlagen im Wirtsgitter von SrAlF5.

Bulk and surface characterization of In2O3(001) single crystals

A comprehensive bulk and surface investigation of high-quality In2O3(001) single crystals is reported. The transparent-yellow, cube-shaped single crystals were grown using the flux method. Inductively coupled plasma mass spectrometry (ICP-MS) reveals small residues of Pb, Mg, and Pt in the crystals. Four-point-probe measurements show a resistivity of 2.0 +/- 0.5 x 10(5) Omega cm, which translates into a carrier concentration of approximate to 10(12) cm(-3). The results from x-ray diffraction (XRD) measurements revise the lattice constant to 10.1150(5) angstrom from the previously accepted value of 10.117 angstrom. Scanning tunneling microscopy (STM) images of a reduced (sputtered/annealed) and oxidized (exposure to atomic oxygen at 300 degrees C) surface show a step height of 5 angstrom, which indicates a preference for one type of surface termination. The surfaces stay flat without any evidence for macroscopic faceting under any of these preparation conditions. A combination of low-energy ion scattering (LEIS) and atomically resolved STM indicates an indium-terminated surface with small islands of 2.5 angstrom height under reducing conditions, with a surface structure corresponding to a strongly distorted indium lattice. Scanning tunneling spectroscopy (STS) reveals a pronounced surface state at the Fermi level (E-F). Photoelectron spectroscopy (PES) shows additional, deep-lying band gap states, which can be removed by exposure of the surface to atomic oxygen. Oxidation also results in a shoulder at the O 1s core level at a higher binding energy, possibly indicative of a surface peroxide species. A downward band bending of 0.4 eV is observed for the reduced surface, while the band bending of the oxidized surface is of the order of 0.1 eV or less.

Influence of manganese substitution on the microstructure and magnetostrictive properties of Co1−xMnxFe2O4 (x = 0.0–0.4) ferrite

Manganese substituted cobalt ferrite, Co1−xMnxFe2O4 (x = 0.0–0.4), was prepared by a ceramic method. The heat-treated powders were pressed at hydrostatic pressure of 167 MPa, and were annealed at 1350 °C for 24 h. These samples present a single-phase cubic spinel structure and the compositional mass ratios are close to the empirical formula. The lattice constant determined from XRD increases with the increase of Mn content, whereas SEM study reveals that Mn substitution changes the microstructure and cause pores within the grains, which reduces the bulk density of the samples. The magnetocrystalline anisotropy constant, coercive field, and magnetostriction were observed to decrease with increasing Mn substitution; however, the strain derivative (dλ/dH) reaches a maximum value for x = 0.3. The observed variation in strain derivative in the Mn substituted cobalt ferrite is correlated to the microstructure whereas the reduced anisotropy of the system plays only a minor role.