We present the results of experiments on the optical, electrical and magnetic properties and electronic structure and optical spectrum calculations of the Heusler alloys Fe 2 TiAl, Fe 2 VAl and Fe 2 CrAl. We find that the drastic transformation of the band spectrum, especially near the Fermi level, when replacing the Me element (Me = Ti, V, Cr), is accompanied by a significant change in the electrical and optical properties. The electrical and optical properties of Fe 2 TiAl are typical for metals. The abnormal behavior of the electrical resistivity and the optical properties in the infrared range for Fe 2 VAl and Fe 2 CrAl are determined by electronic states at the Fermi level. Both the optical spectroscopic measurements and the theoretical calculations demonstrate the presence of low-energy gaps in the band spectrum of the Heusler alloys. In addition, we demonstrate that the formation of Fe clusters may be responsible for the large enhancement of the total magnetic moment in Fe 2 CrAl.

We describe the design and show first results of a large solid angle x-ray emission spectrometer that is optimised for energies between 5.5 keV and 1.5 keV. The spectrometer is based on an array of eleven cylindrically bent Johansson crystal analysers arranged in a point-to-point Rowland circle geometry. The smallest achievable energy bandwidth is smaller than the core hole lifetime broadening of the absorption edges in this energy range. Energy scanning is achieved using an innovative design, maintaining the Rowland circle conditions for all crystals with only four motor motions. The entire spectrometer is encased in a high-vacuum chamber that allows fitting a liquid helium cryostat and provides sufficient space for in situ cells and operando catalysis reactors.

Cobalt-doped anatase Ti1–x Co x O2 (0 < x ≤ 0.04) nanopowders (with a particle size of 30–40 nm) were produced by the hydrothermal synthesis method. Morphology, structure, and thermal stability of the synthesized compounds were examined using transmission electron microscopy, infrared spectroscopy, and X-ray diffraction analysis. Using X-ray photoelectron spectroscopy, cobalt ions are shown to have an oxidation state of 2+, with titanium ions having a tetravalent state of Ti4+. In the as-prepared state, all investigated compounds of Ti1–x Co x O2 are paramagnetic, with the value of paramagnetic susceptibility growing in proportion to cobalt content; with the spin of cobalt ion equal to S = 3/2. Analysis of the electron paramagnetic resonance spectra reveals that doping TiO2 with cobalt (up to 2%) is accompanied by a significant increase in the concentration of F+ centers. Further growth of the cobalt content results in a relatively wide line (nearly 600 Oe) in the spectrum, with a g-factor of about 2.005, demonstrating exchange-coupled regions being formed, the fraction of which increases with cobalt content, while the intensity of F+-center signals is reduced appreciably. Annealing of Ti0.96Co0.04O2 in vacuum at 1000 K is shown to have resulted in the substantial localization of cobalt atoms in the subsurface layers, resulting in an approximately 3-fold increase in the Co atoms content on the surface of nanoparticles as compared with that in the bulk. This is shown to be accompanied by appearance of spontaneous magnetization at room temperature, the value of which depends on the cobalt content in TiO2 nanopowders. The value of magnetic moment per Co atom decreases monotonically down to a value of ≃1 μB with cobalt content increasing. A core–shell model proposed to be the most adequate for describing the magnetic properties of TiO2:Co after the reducing annealing. A hypothesis is put forward suggesting that the defect surface enriched with Co atoms and vacancies is described with itinerant type magnetism, allowing for the delocalized nature of electrons near vacancies.

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X-ray spectroscopy using high-energy-resolution fluorescence detection (HERFD) has critically increased the information content in X-ray spectra. We extend this technique to the tender X-ray range and present a study at the L3-edge of molybdenum. We show how information on the oxidation state, phase composition, and local environment in molybdenum-based compounds can be obtained by analyzing the HERFD L3 X-ray absorption near-edge structure (XANES). We demonstrate that the chemical shift of the L3-edge HERFD spectra follows a parabolic dependence on the oxidation state and show that a qualitative analysis of high-resolution spectra can help to estimate parameters such as distortion of a ligand environment and radial order of atoms around the absorber. In certain cases, the spectra allow disentangling the contributions from bond lengths and angles to the distortion of the ligand polyhedron. Comparison of the high-resolution spectra with theoretical simulations shows that the single-electron approximation is able to reproduce the spectral shape. The results of this work may be useful in every branch of physics, inorganic and organometallic chemistry, catalysis, materials science, biochemistry, and mineralogy where observed changes in performance or chemical properties of Mo-based compounds, accompanied by small changes in spectral shape, are to be related to the details of electronic structure and local atomic environment.

We report the first four magnetic representatives of the trigonal layered A2M­(4+)­TeO6 (here, M = Mn) family. Na2MnTeO6 was synthesized from NaMnO2, NaNO3, and TeO2 at 650–720 °C, but analogues for which A = Li and K could not be obtained by direct synthesis. However, those for which A = Li, Ag, and Tl (but not K) were prepared by exchange reactions between Na2MnTeO6 and the corresponding molten nitrates. The oxygen content was verified by redox titration. According to the X-ray diffraction Rietveld analysis, the four new compounds are isostructural with Na2GeTeO6, trigonal (P3̅1c), based on ilmenite-like layers of edge-shared oxygen octahedra occupied by Mn­(4+) and Te­(6+) in an ordered manner. These layers are separated by cations A, also in a distorted octahedral coordination. However, off-center displacement of Tl+ is so strong, due to the lone-pair effect, that its coordination is better described as trigonal pyramid. Each MnO6 octahedron shares two opposite faces with AO6 octahedra, whereas TeO6 octahedra avoid sharing faces. Besides this double-layered structure, Na2MnTeO6 was often accompanied by a transient triple-layered rhombohedral polytype. However, it could not be prepared as a single phase and disappeared on annealing at 700–720 °C. All A2MnTeO6 samples (A = Ag, Li, Na, or Tl) revealed the unusual phenomenon of hidden magnetic order. Low-field magnetic susceptibility data exhibit a Curie–Weiss type behavior for all samples under study and do not show any sign of the establishment of long-range magnetic order down to 2 K. In contrast, both the magnetic susceptibility in sufficiently high external magnetic fields and the zero-field specific heat unambiguously revealed an onset of antiferromagnetic order at low temperatures. The frustration index f = Θ/T N takes values larger than the classical values for three-dimensional antiferromagnets and implies moderate frustration on the triangular lattice.