Jekabs Grins

Anomalous Magnetic Properties of Nanoparticles Arising from Defect Structures: Topotaxial Oxidation of Fe1–xO|Fe3−δO4 Core|Shell Nanocubes to Single-Phase Particles

Here we demonstrate that the anomalous magnetic properties of iron oxide nanoparticles are correlated with defects in their interior. We studied the evolution of microstructure and magnetic properties of biphasic core|shell Fe(1-x)O|Fe(3-δ)O4 nanoparticles synthesized by thermal decomposition during their topotaxial oxidation to single-phase nanoparticles. Geometric phase analysis of high-resolution electron microscopy images reveals a large interfacial strain at the core|shell interface and the development of antiphase boundaries. Dark-field transmission electron microscopy and powder X-ray diffraction concur that, as the oxidation proceeds, the interfacial strain is released as the Fe(1-x)O core is removed but that the antiphase boundaries remain. The antiphase boundaries result in anomalous magnetic behavior, that is, a reduced saturation magnetization and exchange bias effects in single-phase nanoparticles. Our results indicate that internal defects play an important role in dictating the magnetic properties of iron oxide nanoparticles.

An Open-Framework Silicogermanate with 26-Ring Channels Built from Seven-Coordinated (Ge,Si)10(O,OH)28 Clusters

We report a new open-framework silicogermanate SU-61 containing 26-ring channels with a low framework density. It can be seen as a crystalline analogue to the mesoporous silica MCM-41. The structure is built from the assembly of (Ge,Si)10(O,OH)28 clusters. It is the first time that silicon has been successfully introduced in the Ge10 cluster in significant amounts ( approximately 21% of the tetrahedral sites). Five- and six-coordinated Ge10 clusters have previously been observed in other germanate compounds leading to either dense or open structures. In SU-61, the seven-coordinated clusters fall onto yet another underlying net, the osf net. SU-61, along with other Ge10 based frameworks, shows the versatility of the germanate system to adopt defined topologies playing on the connectivity of the clusters following the principles of net decoration and scale chemistry.

Strong hierarchically porous monoliths by pulsed current processing of zeolite powder assemblies.

Binderless hierarchically porous monoliths have been produced from silicalite-1 and ZSM-5 zeolite powders by a rapid and facile powder processing method where the zeolite powders are assembled in a graphite die and subjected simultaneously to a compressive pressure and a pulsed current. Pulsed current processing (PCP) or, as it is commonly called, spark plasma sintering, enables rapid thermal processing of zeolite powder assemblies with heating and cooling rates at 100 degrees C/minute or more, which results in the formation of strong powder bodies without any addition of secondary binders. Nitrogen adsorption measurements show that it is possible to form strong zeolite monoliths by PCP that maintain between 85 and 95% of the surface area of the as-received silicalite-1 and ZSM-5 powders. Line-broadening analysis of X-ray powder diffraction data by the Rietveld method and high-resolution electron microscopy showed that the formation of strong interparticle bonds is associated with a local amorphization reaction at the interfacial contact points between the zeolite particles. The PCP-treated binderless ZSM-5 monoliths display a high selectivity in xylene isomer separation.

Preparation and crystal structure of a new Sr containing sialon phase Sr2AlxSi12–xN16–xO2+x (x≈2)

A nitrogen-rich sialon phase containing Sr or Eu, named the S-phase, has been reported to form in the M′-Si-Al-O-N systems with M′=Sr and Eu. A sample with overall composition Sr 2 Al 2.5 Si 10 O 4 N 14.5 hot-pressed at 1800°C for 2 h contained approximately 85 vol% of the S-phase, and in addition the α- and β-sialon phases and an amorphous phase. Its structure was solved from X-ray synchrotron powder data (λ=1.1608 A), using direct methods, and was refined by the Rietveld method from 131 reflections in the 2θ range 10-59° to R F =2.7%, with the assumed composition Sr 2 Al 2 Si 10 O 4 N 14 , space group Imm2, a=8.2788(9), b=9.5757(9), c=4.9158(4) A, V=389.7 A 3 . The structure model was confirmed by its electron diffraction pattern and by high-resolution electron microscopy studies. The structure exhibits a tetrahedral network with high connectivity, each tetrahedron sharing corners with seven surrounding tetrahedra, and the Sr atoms, irregularly coordinated by eight O/N atoms, are found in tunnels extending along [001].

Precise control over shape and size of iron oxide nanocrystals suitable for assembly into ordered particle arrays

Abstract Here we demonstrate how monodisperse iron oxide nanocubes and nanospheres with average sizes between 5 and 27 nm can be synthesized by thermal decomposition. The relative importance of the purity of the reactants, the ratio of oleic acid and sodium oleate, the maximum temperature, and the rate of temperature increase, on robust and reproducible size and shape-selective iron oxide nanoparticle synthesis are identified and discussed. The synthesis conditions that generate highly monodisperse iron oxide nanocubes suitable for producing large ordered arrays, or mesocrystals are described in detail.

Triclinic oxy-hydroxyapatite

AbstractPartially dehydrated hydroxyapatite was characterised by X-ray powder diffraction (XRPD) using Guinier-Hägg films, diffractometer and high-resolution synchrotron data as a function of the degree of dehydration (DD). It is for the first time shown that the space group symmetry for partially dehydrated hydroxyapatite changes from hexagonal P63/m to triclinic when more than ca. 35% of the structurally bound water is removed. With increasing DD the a- and b-axes decrease and the c-axis increases. At the highest DD value attained (78%) before the onset of a decomposition of the apatite, the cell parameters were determined to be a = 9.40023 (3), b = 9.39704 (3), c = 6.89967 (2) Å, α = 90.0626 (2)°, β = 89.7478 (1)° and γ = 119.9971 (2)°. A structure refinement for this sample converged with RF = 3.7% in space group P

Synthesis, structure determination and X-ray photoelectron spectroscopy characterisation of a novel polymeric silver(I) nicotinic acid complex, H[Ag(py-3-CO2)2]

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Structural investigations of natural and synthetic chlorite minerals by X-ray diffraction, Mossbauer spectroscopy and solid-state nuclear magnetic resonance

, using synchrotron data (λ = 0.852790 Å), 795 reflections in the 2θ range 5–45° and 62positional parameters. The shifts of the atoms from their corresponding positions in hexagonal hydroxyapatite are small. A possible cause for the triclinic distortion is a tilting of the hydroxide ions in the apatite channels away from the channel axis.