A ferromagnetic contribution to the magnetic susceptibility of the multiferroic compound CuCrSe2 is related to the ordering of spin frustrations in the triangular antiferromagnetic Cr sublattice. To enhance this contribution, Cr atoms have been partially replaced by non-magnetic Zr atoms. It has been established that the Zr atoms in CuCr1-xZrxSe2, within the concentration range of 0 < x <= 0.1, occupy the Cu sites in preference to the Cr sites. Lattice electroneutrality is maintained when Cr3+ is replaced by Zr4+ through the formation of vacancies in the Cu sublattice. Zirconium, acting as an acceptor impurity, captures thermally generated electrons from the conduction band. This property ensures the semiconductor characteristics of the material are preserved across a broad range of temperatures. The Cr by Zr substitution has been shown to enhance the ferromagnetic contribution rather due to the presence of additional phases than by diluting the chromium sublattice.

The crystal and electronic structure of ZrxTi1-xSe2 (0 < x < 1) compounds and their electrical resistivity have been studied in detail for the first time. A combination of soft x-ray spectroscopic methods (XPS, XAS, and ResPES) was used to investigate the electronic structure. The lattice parameters as a function of the metal concentration x obey Vegard's law. It was shown that the substitution of Ti by Zr results in an increase in the Fermi energy, attributed to the lower binding energy of Zr 4d compared to Ti 3d in the ZrxTi1-xSe2 valence band. Given that the oxidation states of both Ti and Zr are +4, and the concentration of free charge carriers remains unchanged upon substitution, the observed effect is explained by a reduced density of electronic states near the Fermi level. The influence of temperature on the Ti 2p-3d and Zr 3p-4d ResPES spectra is interpreted in terms of pseudodoping occurring with the substitution of Ti by Zr.

Nanostructured materials play a key role in modern technologies adding new functionalities and improving the performance of current and future applications. Due to their nature resulting in diffused heterogeneous structures (chemical and electronic composition typically organized in phases or building blocks) characterizing these materials needs state of the art technologies which combine nanometer spatial resolution, environmental reliability, and operando capabilities. Scanning photoelectron spectromicroscopy (SPEM) is one of the characterization tools that combine high spectral resolution X-ray photoelectron spectroscopy with submicron spatial resolution. In particular, the SPEM equipment hosted at the ESCA microscopy beamline at Elettra is capable of in situ and operando analysis regardless of sample morphology. The review presents three different case studies illustrating the capabilities of SPEM in the investigation of catalytic materials in different conditions and processes.

Substitution solid solutions CuCr1-xZrxSe2, x = 0-0.2 have been synthesized for the first time. The crystal structure and phase equilibria of these samples were studied in the temperature range of 100-700 degrees C. It was found that the delafossite-like crystal structure is stable in the whole temperature and concentration range studied. It was found that parasitic phases, binary copper and chromium selenides and spinels CuCr2Se4 and Cu (Cr1-xZrx)2Se4, are formed during the synthesis. Their appearance is because doping with zirconium does not affect the crystal structure. Instead, it affects the thermodynamic stability of the CuCr1-xZrxSe2 solid solution with a delafossite-like structure.

The chemical and electrochemical extraction of copper from CuCrSe2 was carried out at room temperature (RT). The compositions corresponding to the boundaries the homogeneity region of CuyCrSe2 and Cu1-xCrSe2 were determined. X-ray diffraction (XRD) analysis demonstrated that within the composition range of y from 0 0.29, the phase with the structure of CrSe2 with space group P 3 m 1 is stable. Furthermore, within the composition range of x from 0 to 0.33, the phase with the structure of delafossite CuCrSe2 (space group R3m) is stable.

The effective valence of copper in the region of its concentration up to x = 0.25 in CuxZrSe2 has been determined based on an analysis of binding energies of the Zr3d and Se3d core levels and the electromotive forces from an electrochemical experiment for CuxZrSe2, as a function of Cu concentration x. It has been shown that the domination of covalent bonding between copper and the ZrSe2 host lattice within this concentration range results in a notable reduction in the effective valence of copper in comparison to its nominal value.

Transmission spectra of TiS3 single crystal whiskers in the infrared and optical bands have been studied at various light polarizations in the temperature range from room to liquid helium. The absolute absorption coefficients have been determined. Previously unknown phonon modes have been observed in the far infrared region. The study of the temperature dependence of the band gap width ℰ_g has shown that it increases by 50 meV under cooling from 300 to 5 K. The comparison of spectra measured at different polarization directions indicates that the optical gap is anisotropic. A 1.28-eV absorption peak has been detected below 150 K and has been attributed to the excitation of excitons.

New intercalation compounds CrxZrSe2 were synthesized and thoroughly studied. Cr atoms were found to occupy the positions both tetrahedrally and octahedrally coordinated by the Se atoms in the interlayer gap. The magnetic properties and electrical resistivity were studied in the temperature ranges of 2.4-300 K and 80-340 K, respectively. The compounds change their behavior from semiconducting (x = 0.1) to metallic (x > 0.1). The magnetic interaction strongly depends on the Cr content and temperature. The spin-glass state with antiferromagnetic interaction exists at T < T-crit for CrxZrSe2 with x <= 0.2, while at x >= 0.3 ferromagnetic contribution arises as well. The single crystals of CrxZrSe2 were grown to study the electronic structure of the materials. A combination of the X-ray photoelectron spectroscopy (including that across Cr 2p-3d and Zr 3p-4d resonance) and X-ray absorption spectroscopy methods allowed to propose the location of the Cr 3d-states in the Se 3p-Zr 4d energy gap.

Transient processes in the photoconductivity of TiS3 single crystals exposed to rectangular light pulses in the wavelength range of λ = 405–940 nm are investigated. It is established that the decay in the photoconductivity after switching the light off at temperatures from 78 to 180 K is described by a logarithmic law in the range from 10–3 to 102 s (i.e., when time changes by five orders of magnitude). This means that the relaxation process is characterized by times ranging, at least, from several tens of microseconds to several tens of minutes. It is shown that the relaxation is basically successive: the recombination barrier increases while the conductivity approaches the equilibrium value.

The crystal structure of the Ni0.5TiSe2 compound has been studied in situ in the temperature range of 25-1000 degrees C using synchrotron radiation X-ray diffraction. The previously known order-disorder transition in the Ni sublattice at similar to 100 degrees C was found to be a second-order phase transition and belongs to the 3D Ising universality class. Reversible extraction of nickel selenides was observed in the temperature range of 275-975 degrees C. It was explained in terms of Ni extraction from Ni0.5TiSe2 due to the thermal widening of the Ni 3d/Ti 3d/Se 4p impurity band. The Ni-TiSe2 phase diagram follows the typical pattern of MxTiSe2 (M - transition metal) compounds.

Cu x CrSe 2 materials can exist in the form of delafossite with a layered structure and exhibit multiferroic properties, or in the form of spinel and exhibit ferromagnetic properties, depending on the copper concentration. The stability of both forms of Cu x CrSe 2 has been studied with regards to defectivity in the copper sublattice. The boundaries of the Cu x CrSe 2 homogeneity regions for the delafossitelike structure and for the cubic spinel CuCr 2 Se 4 were determined by coulometric titration at room temperature. Analysis of the time dependence of cell polarization allowed us to estimate the diffusion coefficients in these materials.

An experimental study of the crystal and electronic structure and properties of the quasi-two-dimensional CuxZrSe2 compound in the region of copper concentration, in which Cu atoms are tetrahedrally coordinated by the Se atoms, has been performed. A study of the magnetic, kinetic, and optical properties of CuxZrSe2 revealed that the tetrahedral coordination of the Cu by Se atoms results in the formation of a covalent bond with a probable Cu 4s/Se 4p hybridization. A combination of soft X-ray spectroscopic methods (X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and resonance photoelectron spectroscopy) with optical spectroscopy, which was used to investigate the electronic structure, confirmed this assumption. The Cu 4s/Se 4p hybrid band falls into the energy gap between the Zr 4d-derived conduction band and the Se 4p-derived valence band.

The crystal structure of CuCrSe2 in the temperature range 25 -1000 degrees C was studied by synchrotron X-ray diffraction. It was found that heating leads to the decomposition of stoichiometric CuCrSe2 into two phases of composition CuCr2Se4 and Cu1}xCrSe2 with increased Cu content. It was found that copper enrichment of CuCrSe2 during heating occurs up to the temperature of 800 degrees C and the limiting composition is close to the compound Cu4Cr3Se6. It was found that heating equalizes the copper distribution at the alpha-and beta-tetrahedrally coordinated chalcogen positions, but full equality of their occupancy at temperatures up to 1000 degrees C was not achieved. The low temperature limit of CuCr2Se4 excretion was established to be approximately 300 degrees C and the decomposition temperature of CuCr2Se4 to be approximately 1050 degrees C.

Министерство науки и высшего образования Российской Федерации Российское химическое общество им.Д.И.Менделеева Секция по химической термодинамике и термохимии Научного совета РАН по физической химии Сибирское Отделение Российской Академии Наук Институт неорганической химии им.А.В.Николаева СО РАН

The electronic structure of the single-crystalline solid solutions Nb1-xVxSe2 has been studied using soft X-ray photoelectron, resonant photoelectron, and absorption spectroscopy. A charge transfer between VSe2 and NbSe2 sublattices was observed. This charge transfer makes the octahedral coordination of the Nb atoms by Se atoms preferable as compared to the trigonal-prismatic coordination in undoped NbSe2.