An ultra-high-entropy rare earth orthoferrite (UHE REO): solution combustion synthesis, structural features and ferrimagnetic behavior.

A novel ultra-high-entropy rare earth orthoferrite (UHE REO) of Sc1/16Y1/16La1/16Ce1/16Pr1/16Nd1/16Sm1/16Eu1/16Gd1/16Tb1/16Dy1/16Ho1/16Er1/16Tm1/16Yb1/16Lu1/16FeO3 nominal composition was successfully synthesized for the first time through a simple and efficient solution combustion approach. PXRD, Raman, and 57Fe Mössbauer spectroscopy confirmed the high chemical and phase purity of the synthesized UHE REO (hereafter denoted as ΣREFeO3), which belonged to the Pnma space group, typical of the perovskite-like rare earth orthoferrites. Despite the fact that the main X-ray reflections, vibration modes, and spectral Mössbauer components unambiguously indicate the single-phase nature of the sample, the results of SEM and TEM make it possible to establish the presence of a main (about 50 nm) and a minor ultrafine (about 10 nm) fraction of ΣREFeO3 nanoparticles. The bimodal size distribution of nanoparticles was also reflected in the magnetic behavior of this substance: the presence of several sextet components in the Mössbauer spectra, the hard single-domain magnetic nature of the main fraction of 50 nm UHE REO nanoparticles, and the superparamagnetic state of the minor fraction of 10 nm UHE REO nanoparticles. Thus, the unusual features of nanostructured ΣREFeO3 can potentially be used for the creation of new generations of transformers, magnetic memory systems, magnetic screens, radio devices, etc.

Efficient Production of Segmented Carbon Nanofibers via Catalytic Decomposition of Trichloroethylene over Ni-W Catalyst.

The catalytic utilization of chlorine-organic wastes remains of extreme importance from an ecological point of view. Depending on the molecular structure of the chlorine-substituted hydrocarbon (presence of unsaturated bonds, intermolecular chlorine-to-hydrogen ratio), the features of its catalytic decomposition can be significantly different. Often, 1,2-dichloroethane is used as a model substrate. In the present work, the catalytic decomposition of trichloroethylene (C2HCl3) over microdispersed 100Ni and 96Ni-4W with the formation of carbon nanofibers (CNF) was studied. Catalysts were obtained by a co-precipitation of complex salts followed by reductive thermolysis. The disintegration of the initial bulk alloy driven by its interaction with the reaction mixture C2HCl3/H2/Ar entails the formation of submicron active particles. It has been established that the optimal activity of the pristine Ni catalyst and the 96Ni-4W alloy is provided in temperature ranges of 500-650 °C and 475-725 °C, respectively. The maximum yield of CNF for 2 h of reaction was 63 g/gcat for 100Ni and 112 g/gcat for 96Ni-4W catalyst. Longevity tests showed that nickel undergoes fast deactivation (after 3 h), whereas the 96Ni-4W catalyst remains active for 7 h of interaction. The effects of the catalyst's composition and the reaction temperature upon the structural and morphological characteristics of synthesized carbon nanofibers were investigated by X-ray diffraction analysis, Raman spectroscopy, and electron microscopies. The initial stages of the carbon erosion process were precisely examined by transmission electron microscopy coupled with elemental mapping. The segmented structure of CNF was found to be prevailing in a range of 500-650 °C. The textural parameters of carbon product (SBET and Vpore) were shown to reach maximum values (374 m2/g and 0.71 cm3/g, respectively) at the reaction temperature of 550 °C.

Comprehensive Study on the Reinforcement of Electrospun PHB Scaffolds with Composite Magnetic Fe3O4-rGO Fillers: Structure, Physico-Mechanical Properties, and Piezoelectric Response.

This is a comprehensive study on the reinforcement of electrospun poly(3-hydroxybutyrate) (PHB) scaffolds with a composite filler of magnetite-reduced graphene oxide (Fe3O4-rGO). The composite filler promoted the increase of average fiber diameters and decrease of the degree of crystallinity of hybrid scaffolds. The decrease in the fiber diameter enhanced the ductility and mechanical strength of scaffolds. The surface electric potential of PHB/Fe3O4-rGO composite scaffolds significantly increased with increasing fiber diameter owing to a greater number of polar functional groups. The changes in the microfiber diameter did not have any influence on effective piezoresponses of composite scaffolds. The Fe3O4-rGO filler imparted high saturation magnetization (6.67 ± 0.17 emu/g) to the scaffolds. Thus, magnetic PHB/Fe3O4-rGO composite scaffolds both preserve magnetic properties and provide a piezoresponse, whereas varying the fiber diameter offers control over ductility and surface electric potential.

Nonstoichiometry Defects in Double Oxides of the A2BO4-Type.

Double oxides with the structure of the Ruddlesden-Popper (R-P) layered perovskite An+1BnO3n+1 attract attention as materials for various electrochemical devices, selective oxygen-permeable ceramic membranes, and catalytic oxidative reactions. In particular, Sr2TiO4 layered perovskite is considered a promising catalyst in the oxidative coupling of methane. Our high-resolution transmission electron microscopy (HRTEM) studies of Sr2TiO4 samples synthesized using various methods have shown that their structure often contains planar defects disturbing the periodicity of layer alternation. This is due to the crystal-chemical features of the R-P layered perovskite-like oxides whose structure is formed by n consecutive layers of perovskite (ABO3)n in alternating with layers of rock-salt type (AO) in various ways along the c crystallographic direction. Planar defects can arise due to a periodicity violation of the layers alternation that also leads to a violation of the synthesized phase stoichiometry. In the present work, a crystallochemical analysis of the possible structure of planar defects is carried out, structures containing defects are modeled, and the effect of such defects on the X-ray diffraction patterns of oxides of the A2BO4 type using Sr2TiO4 is established as an example. For the calculations, we used the method of constructing probabilistic models of one-dimensionally disordered structures. For the first time, the features of diffraction were established, and an approach was demonstrated for determining the concentration of layer alternation defects applicable to layered perovskite-like oxides of the A2BO4 type of any chemical composition. A relation has been established between the concentration of planar defects and the real chemical composition (nonstoichiometry) of the Sr2TiO4 phase. The presence of defects leads to the Ti enrichment of particle volume and, consequently, to the enrichment of the surface with Sr. The latter, in turn, according to the data of a number of authors, can serve as an explanation for the catalytic activity of Sr2TiO4 in the oxidative coupling of methane.

The Influence of Argon Cluster Ion Bombardment on the Characteristics of AlN Films on Glass-Ceramics and Si Substrates.

In this paper, the influence of surface modification on the characteristics and properties of AlN thin films on Si and glass-ceramics substrates is investigated. The surface modification was made at various parameters of argon cluster ions. By using XRD and Raman spectroscopy, it was shown that the obtained AlN films have a hexagonal structure with a characteristic direction of texturing along the c axis and slight deviations from it. A comparison of the AlN surface morphology obtained by atomic force microscopy before and after cluster processing was demonstrated. This demonstrated that the cluster ions with low energy per atom (E/N = 10 eV/atom) have a high efficiency of surface smoothing. A decrease in the intensity of the Raman peaks and an increase in their full-width after bombardment with cluster ions were found, which may be caused by a change in the physicochemical state of the surface. The optical properties, the quality of the boundaries, and the distribution map of the thickness of the functional layer of AlN were investigated by the methods of spectral and spatial resolution ellipsometry. By using the cross-sectional SEM, the direction of crystallite texturing was demonstrated. The influence of argon cluster ion bombardment on the stoichiometry of samples was analyzed by EDX spectroscopy. The results obtained demonstrate the efficiency of the cluster ion smoothing of polycrystalline thin films for microelectronics, particularly when creating surface acoustic wave resonators.

Reduction of double manganese-cobalt oxides: in situ XRD and TPR study.

The work reported here was aimed at determining differences in redox properties of simple and double oxides. Comparison between the reduction of double oxides (Mn,Co)3O4 and simple oxides Co3O4 and Mn3O4 was performed using in situ X-ray diffraction (XRD), temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). The double oxides with a ratio of cations Mn : Co = 1 : 1 were prepared by the coprecipitation method and contained a mixture of 50% MnCo2O4 and 50% CoMn2O4. It was shown that the mechanism of reduction of double oxides with hydrogen differs significantly from the processes occurring on simple oxides. For simple cobalt and manganese oxides, transformations Co3O4→ CoO → Co and Mn3O4→ MnO are observed under a hydrogen atmosphere. The reduction of mixed-metal oxides occurs in two steps. In the first step, at 300-450 °C, (Mn,Co)3O4 transforms to (Mn,Co)O solid solutions. In situ XRD under isothermal conditions illustrates that Co-rich Co2MnO4 oxide starts to be reduced to Co0.6Mn0.4O first, and then Mn-rich Mn2CoO4 passes into Mn0.6Co0.4O. In the second step, at 450-700 °C, the reduction of solid solutions (Mn,Co)O to metallic cobalt Co and MnO proceeds. Again, the reduction begins with transformation of Co-rich oxide with the Co0.6Mn0.4O structure. The temperature of appearance of the intermediate phase (Mn,Co)O shifts to the higher values as compared to those observed for CoO, and to lower temperatures as compared to MnO during simple oxide reduction.

Highly Stable Single-Atom Catalyst with Ionic Pd Active Sites Supported on N-Doped Carbon Nanotubes for Formic Acid Decomposition.

Single-atom catalysts with ionic Pd active sites supported on nitrogen-doped carbon nanotubes have been synthesized with a palladium content of 0.2-0.5 wt %. The Pd sites exhibited unexpectedly high stability up to 500 °C in a hydrogen atmosphere which was explained by coordination of the Pd ions by nitrogen-containing fragments of graphene layers. The active sites showed a high rate of gas-phase formic acid decomposition yielding hydrogen. An increase in Pd content was accompanied by the formation of metallic nanoparticles with a size of 1.2-1.4 nm and by a decrease in the catalytic activity. The high stability of the single-atom Pd sites opens possibilities for using such catalysts in high-temperature reactions.