Crystalline and magnetic properties of Er and Yb chromites modified by Mn and Ca cations

The structural and physical properties of some chromites–manganites solid solutions (RE,Ca)Cr_0.5Mn_0.5O₃, RE = Er, Yb, have been studied by X-ray diffraction and magnetic measurements. Powders were prepared by solid state reaction between the component oxides. Complete solid solutions were obtained, except for YbCr_0.5Mn_0.5O₃ in which traces of hexagonal YbMnO₃ remain through all the synthesis and sintering processes. Incorporation of Mn³⁺ cations does not alter the perovskite O-type structure of the chromites. Ca-containing solid solutions maintain the O-type structure.Magnetic measurements indicate an increase of the ferromagnetic behavior when Ca²⁺ is incorporated into the lattice, with an expected transformation of Mn³⁺ into Mn⁴⁺, and its associated ferromagnetic exchange interactions. Competition between ferromagnetic Mn³⁺–Mn⁴⁺, Mn³⁺–Cr³⁺ and antiferromagnetic Cr³⁺–Mn⁴⁺, Yb–TM interactions (TM: transition metal) leads to complex magnetic ordering in the ytterbium chromites, although double exchange Mn³⁺–Mn⁴⁺ interactions predominate and suppress the spin reversal expected for the pure YbCrO₃ material.     

Structural characterization of sputtered single-phase γ? iron nitride coatings

Single-phase γ?-FeN films were deposited by d.c. magnetron sputtering in a reactive Ar/N₂ atmosphere. The films were characterized by X-ray diffraction, scanning and transmission electron microscopies, electron energy-loss and Mössbauer spectroscopy. The average lattice parameter of the γ? cubic cell is 0.455 nm. Microstructure studies by electron microscopies revealed nanostructured columnar films with no preferential orientation. Diffraction peak intensities (X-ray and electron diffraction) are close to the theoretical values for a ZnS-type structure while profile analysis of the X-ray diffraction pattern using Rietveld refinement method demonstrated that the γ?-phase is of ZnS-type. The energy-loss near-edge structures of N K-edge of the γ?-phase is similar to those of the ZnS-type γ″-phase suggesting an identical local atomic environment for N atoms. On the contrary, Mössbauer spectra of both structures are different, which is understood as a consequence of numerous vacancies in the γ? structure. Investigations of magnetic properties showed that the γ?-FeN compound is paramagnetic from room temperature to 2 K. The main conclusion of this work is that the γ?-FeN phase is of ZnS-type, and not of NaCl-type as it is usually reported.     

Liquidus Projection of the B-Fe-U Diagram: The Fe-Rich Corner

The liquidus projection of the Fe-rich corner of the B-Fe-U phase diagram is proposed based on powder X-ray diffraction measurements, differential thermal analysis, and scanning electron microscopy observations complemented by energy dispersive X-ray spectroscopy. The present work demonstrates the existence of 10 ternary reactions together with 18 monovariant lines and establishes their location and approximate temperatures. The ternary compounds existing in this region of the B-Fe-U ternary phase diagram, UFe₃B₂, UFe₄B, and U₂Fe₂₁B₆, were confirmed to form by ternary peritectic reactions, yet UFe₄B and U₂Fe₂₁B₆ have minute primary crystallization fields, indicating a more challenging preparation of single crystals for these compounds when compared with UFe₃B₂.     

Thermo-reversible sol–gel transition of TiO2 nanoparticles with surface modified by p-toluene sulfonic acid

In this paper an unprecedent thermo-reversible sol–gel transition for titania nanoparticles dispersed in a solution of p-toluene sulfonic acid (PTSH) in isopropanol is reported. The sol formed by the thermo-hydrolysis at 60 °C of titanium tetraisopropoxide (Ti(OⁱPr)₄) reversibly changes into a turbid gel upon cooling to room temperature. Turbidimetric measurements performed for samples containing different nominal acidity ratios (A = [PTSH]/[Ti]) have evidenced that the gel transformation temperature increases from 20 to 35 °C as the [PTSH]/[Ti] ratio increases from 0.2 to 2.0. SAXS results indicate that the thermo-reversible gelation is associated to a reversible aggregation of a monodisperse set of titania nanoparticles with average gyration radius of ≈2 nm. From the different PTSH species evidenced by Raman spectroscopy and TG/DTA of dried gels we proposed that the thermo-reversible gelation in this systems is induced by the formation of a supramolecular network, in which the protonated surface of nanoparticles is interconnected through cooperative hydrogen bonds between –SO₃ groups of p-toluene sulfonic acid.     

Scanning tunneling microscopy investigations of electropolymerized tetra-arylporphyrin complexes

Scanning tunneling microscopy (STM) studies on poly(Zinc-tetraphenylporphyrin) (poly(ZnTPP)) and poly(Zn- or Pt-tetradimethoxyphenylporphyrins) (poly(Zn8OMeTPP) or poly(Pt8OMeTPP)) deposited through anodic oxidation on platinum surfaces are reported. STM investigations on these metalloporphyrin films were conducted to obtain informations on the supramolecular architecture of these polymers. These STM images indicate that poly(ZnTPP) covered the metallic electrode with a layered structure. For poly(Zn8OMeTPP) and poly(Pt8OMeTPP) a tendency to form regular molecular assemblies is obvious.     

Thermoregulated phase-transfer rhodium nanoparticle catalyst for hydroaminomethylation of olefins

Rh nanoparticles used as catalysts for hydroaminomethylation reaction is reported for the first time. An efficient and recyclable Rh nanoparticle catalyst stabilized by thermoregulated ligand Ph₂P(CH₂CH₂O)_nCH₃ (n = 16) was studied for the hydroaminomethylation of olefins in the aqueous/1-butanol biphasic system through thermoregulated phase-transfer catalysis, which allows not only for a homogeneous catalytic reaction, but also for an easy biphasic separation. Under the optimized conditions, the conversion of 1-octene and the product amine selectivity were as high as 99% and 97%, respectively. After reaction, the Rh nanoparticle catalyst can be separated from products by simple phase separation and recycled directly for the next run.     

Correlation between infrared,THz and microwave dielectric properties of vanadium doped antiferroelectric BiNbO4

Terahertz (THz) transmissivity and infrared (IR) reflectivity spectra of orthorhombic microwave (MW) ceramics Bi(Nb_1−xV_x)O₄ (0.002 < x < 0.032) were measured between 4 and 3000 cm⁻¹ (0.09–90 THz) at room temperature. A well underdamped mode, presumably the ferroelectric soft mode, was observed at 25 cm⁻¹. Complex permittivity spectra obtained from the fits to our data were extrapolated down to the MW range and compared with the dielectric data near 5 GHz. The linear extrapolation of dielectric losses from THz down to the MW range is in agreement with the experimental MW losses. Addition of 3.2% of vanadium reduces the sintering temperature to 850 °C and the dielectric properties (ɛ′ = 42.2, Q·f = 14,000 GHz, τ_f = +10 ppm/°C) remain at a level satisfactory for MW applications. Somewhat lower MW losses were observed in a sample sintered in the N₂ atmosphere.