No 6 (2023)

Osmium compounds with the Os5d⁴ electron configuration and an octahedral environment of neighboring atoms attract much attention due to the influence of the spin-orbit interaction on the appearance of magnetic properties in materials. XANES spectroscopy makes it possible to obtain information about the magnitude of the spin-orbit interaction from measuring the intensity ratio of lines near the absorption edges. The influence of the spin-orbit interaction on the XANES OsL_2,3 spectra in osmium compounds having an octahedral halogen environment of osmium atoms has been studied. Two types of systems have been investigated: isolated osmium clusters in complex compounds and OsCl₄ compound containing polymeric chains of Os connected by bridging Cl atoms. Magnetic susceptibility measurements show a non-magnetic ground state and Van Vleck paramagnetism in the case of isolated clusters and a non-zero magnetic moment over the entire temperature range in OsCl₄. As a result of measurements of the XANES spectra, high values of the line intensity ratio near the OsL₃/L₂ absorption edges have been obtained, which is associated with a strong influence of the spin-orbit interaction on the electronic structure. Theoretical analysis of the XANES spectra of Os compounds with different ligands and outer-sphere cations shows that the electronic structure and magnetic properties depend on the spin-orbit interaction, the crystal field splitting, the electron pairing energy, and non-cubic distortions of the Os environment.

The results of a study of biphase ceramics SrTiO₃–TiO₂, previously proposed as a promising n-type thermoelectric material, obtained using synchrotron radiation techniques at the shared research center “Siberian Synchrotron and Terahertz Radiation Center”, are presented. In particular, it has been demonstrated by in-situ heating X-ray diffraction that the reaction between the powder components SrCO₃ (strontianite) and TiO₂ (anatase) to obtain SrTiO₃ (tausonite) is not the driving force in the preparation of ceramics by spark plasma sintering of the reaction mixture. For two spectral methods – X-ray luminescence and XANES spectroscopy, the spectrum of biphasic ceramics was compared with a model spectrum obtained from the spectra of single-phase ceramics as reference samples. The X-ray luminescence method revealed a shift to the high-energy region and a narrowing of the spectrum of biphase ceramics, which may indicate size quantization (the presence of a two-dimensional electron gas) in the system. Changes were found in the XANES spectrum of biphase ceramics in the region in which its shape can significantly depend on the symmetry of the nearest environment of Ti⁴⁺ atoms. However, it is difficult to interpret these data without numerical simulation.

Graphene layers on semiconducting substrates, modified using covalent and non-covalent chemical functionalization, can be utilized for fabrication of hybrid structures combining physical properties of graphene and organic molecules. In this paper the results of investigations of the atomic and electronic structure of ultrathin graphene layers on β-SiC/Si(001) wafers modified using phenazine dye Neutral Red are presented. Continuous graphene films consisting on several atomic layers were synthesized on β-SiC/Si(001) wafers using high-temperature annealing in ultrahigh vacuum. The synthesized graphene layers were chemically modified in a solution of diazonium salt of the Neutral Red dye under white light illumination. The results of the scanning tunneling microscopy and spectroscopy experiments demonstrate the formation of a composite phenazine/graphene structure with a large energy gap in all surface regions. The molecules can be oriented preferentially parallel and perpendicular to the graphene layers and form locally ordered structures with rectangular and oblique unit cells. The electronic energy spectrum and band energy gap in different surface areas depend on the local atomic structure and the molecule’s orientation relative to the surface. According to the density functional theory calculations, local modifications of the electronic structure and band energy gap can be related to deformations (compression or extension) of the phenazine dye molecules because of their interaction with the topmost graphene layer.

The influence of microarc oxidation process duration on the thickness, roughness and optical characteristics (solar absorbance α_s and emissivity ε) of thermal control white and black cosmic coatings formed on an aluminum alloy AMg6 is studied experimentally. It is found that α_s decreases with an increase in the thickness of coatings with increasing microarc oxidation treatment duration. For white coatings, a decrease in their roughness is accompanied by an increase in ε, and vice versa. For black coatings, the main role is played by the degree of blackness of the coating, which is determined by the content of vanadium oxide in it. A comparative analysis of the optical characteristics of thermally controlled coatings obtained using microarc oxidation process on various aluminum alloys showed that it is better to form white coatings of the “solar reflector” class on an aluminum alloy AMg3, and black coatings of the “true absorber” class – on an aluminum alloy AMg6 with MAO treatment duration of 25 min. The obtained experimental data can serve as a framework for the development of basic technology for the formation of thermal control cosmic coatings on products made of aluminum alloys.

The article presents the results of a study of vacuum-arc coatings obtained by sputtering Ti–B–Si–Ni cathodes, manufactured by the method of self-propagating high-temperature synthesis with simultaneous pressing. The techniques for manufacturing cathodes of the indicated composition were characterized; the modernized NNV 6.6-I1 device, the conditions and modes of coating deposition in an argon atmosphere and in a nitrogen + argon gas mixture in a ratio of 90/10 were described. To study the physical and mechanical properties of the resulting coatings, the hardness was measured at different loads on the indenter; the strength of adhesion to the base was assessed by the Rockwell method; the elemental composition of the cathodes and the (Ti–B–Si–Ni)N coating was determined by Auger spectroscopy and the phase composition of the (Ti–B–Si–Ni)N coating was determined by X-ray phase analysis; a study of the properties of the coating (Ti–B–Si–Ni) was carried out by the scratching method (scratch testing). As a result of a comprehensive analysis of the results obtained, it was concluded that the high hardness of the (Ti–B–Si–Ni)N coating (more than 40 GPa) is due to its composition, which includes both nitrides and highly hard titanium borides. The heterophasic nature of the structure of this coating can serve as a contribution to the stressed state of the material. The coating has a gradient-layered structure. The material contains a Ti–B–Si–Ni layer bonding with the substrate and the main functional layer (Ti–B–Si–Ni) N. The coating has both high hardness and sufficient adhesion to the substrate (adhesion) determined by the scratch testing. The combination of these properties makes the material promising for its practical application in the production of tools.

A series of MnO_x–CeO₂ catalysts with a molar ratio of Mn : Ce = 3 : 7 was prepared by co-precipitation method and futher calcination at temperatures ranged from 300 to 800°С. As prepared catalysts were characterized by powder X-ray diffraction, low-temperature nitrogen adsorption, X-ray photoelectron spectroscopy, and the catalytic activity in the CO oxidation reaction was tested for all samples. It has been shown that a (Mn,Ce)O₂ solid solution with the fluorite structure is formed for all catalysts. Based on the studies performed, a catalyst obtained at calcination temperature of 600°С was chosen for further studies of the effect of redox exsolution on the catalytic activity in the CO oxidation reaction by operando X-ray diffraction. The experiment was carried out sequentially in a stepwise manner: stepwise heating/cooling in the reaction mixture 1% CO + 2% O₂ at temperatures of 150–175–200–175–150°C (stages 1, 3, and 5); reduction of the sample in a mixture of 10% CO + He at 400°C (stage 2); reduction of the sample in a mixture of 10% H₂ + He at 400°C (stage 4). It was shown that the reductive pretreatment leads to phase segregation of the initial (Mn,Ce)O₂ solid solution and the appearance of dispersed manganese oxides on the surface. In turn, enrichment of the surface with manganese oxide leads to an increase of the activity in the CO oxidation reaction.

The charged particles motion in the crystal could be either regular or chaotic. In the quantum picture the chaos manifests itself in the statistical properties of the energy levels set. The systems with regular motion domains in the phase space separated by the chaotic one are of a particular interest. The possibility to tunnel between dynamically isolated domains of the phase space influences essentially on the energy levels statistics. This effect account leads to Podolskiy–Narimanov distribution function. The tunneling matrix elements for the transversal motion of 20 and 40 GeV positrons axially channeled in [100] direction of Si crystal are estimated in the present paper. The parameter of the Podolskiy–Narimanov distribution is derived from this estimation. It is demonstrated that this distribution successfully describes the level spacing statistics for the energy levels set of the positron transversal motion.

A comprehensive study of the local atomic structure of titanium compounds obtained by mechanical activation (Ti–Al–C, Ti₂AlC) and reference samples (Ti, TiH₂) using EXAFS and EXELFS spectroscopy has been carried out. An analysis of the local atomic structure of titanium hydride shows that the presence of hydrogen expands the crystal lattice and leads to a change in the parameters of the local atomic structure. This change is observed both in the EXAFS and EXELFS spectra. It is shown that after mechanical activation, the coordination numbers decrease, which may indicate the formation of a multiphase system. Further annealing leads to the formation of the Ti₂AlC compound, which is confirmed by the results of model calculations.