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The crystal structure of the Ni 0.5 TiSe 2 compound has been studied in situ in the temperature range of 25-1000 °C using synchrotron radiation X-ray diffraction. The previously known order-disorder transition in the Ni sublattice at ∼100 °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 °C. It was explained in terms of Ni extraction from Ni 0.5 TiSe 2 due to the thermal widening of the Ni 3d/Ti 3d/Se 4p impurity band. The Ni-TiSe 2 phase diagram follows the typical pattern of M x TiSe 2 (M - transition metal) compounds.

The work is devoted to the study of the possibility of determining the critical points of phase diagrams by measuring the EMF of electrochemical cells relative to a metal that is not part of the test substance. The measurement results of the concentration dependences of the EMF for the Cu x TiS 2 system in the electrochemical cells Na|Na + |Cu x TiS 2 and Cu|Cu + |Cu x TiS 2 in the concentration range 0 < x < 0.7 are compared. It was determined that the chemical potential of the ion is independent of x in a satisfactory approximation. The EMF difference in the given cells is close to the standard electrode potentials difference for respective metals. It is concluded that the dependences E ( x ) for both types of cells are similar and correlated. Therefore, one can choose any convenient metal for measurements of this kind. Using the example of measuring EMF in electrochemical cells for the Fe x TiSe 2 system concerning sodium and lithium, we showed that the elastic contribution to free energy is constant over the entire concentration range 0 < x < 0.5.

The effect of atomic ordering in the cobalt sublattice on the electronic structure of the Co x TiSe 2 intercalation compounds has been studied using a complex of spectral techniques - XPS, XAS, and ResPES, along with theoretical calculations of the total and partial densities of states. It has been found that the cobalt intercalation significantly affects the character of the chemical bond inside the host lattice. No signs of the direct overlapping of the Co 3d orbitals in the ordered material were observed. It has been found that the reason for the ordering of the cobalt atoms is a Co 3d/Ti 3d/Se 4p hybridization.

A thorough study of the chemical bonding between intercalated copper and host lattice TiCh 2 (Ch = S, Te) was performed. In order to separate the contributions of the copper, titanium, and chalcogen states into the electronic structure of the valence band, photoelectron spectroscopy in nonresonant and resonant (Cu 3p-3d and Ti 2p-3d) excitation modes was used. It is shown that the ionicity of the chemical bond between copper and host lattice is decreased in the TiS 2 → TiSe 2 → TiTe 2 row. In Cu x TiS 2 , copper atoms form the chemical bond with TiCh 2 host lattice, while in Cu x TiTe 2 directly with tellurium atoms.

The thermal stability of the layered modification of the Cu 0.5 ZrTe 2 polycrystalline intercalation compound, synthesized at room temperature, has been studied in the temperature range 25-900 °C. A change in the occupation of the octahedral and tetrahedral coordinated sites in the interlayer space of the zirconium ditelluride was observed using in-situ time-resolved synchrotron X-ray powder diffraction experiments. The formation of the rhombohedral CuZr 2 Te 4 phase, which is stable in the temperature range 300-700 °C, has been observed. The copper intercalation at room temperature leads to the formation of a phase in which the Cu atoms occupy only octahedral sites in the interlayer space. At temperatures above the decay temperature of the rhombohedral CuZr 2 Te 4 , a layered phase with Cu atoms uniformly distributed between octahedral and tetrahedral sites in the interlayer space is stable. The changes in the crystal structure independent of temperature are in agreement with the previously proposed model, according to which the stability of the layered or the rhombohedral phase is determined by the entropy factor associated with the distribution of the intercalated atoms between the octahedral and tetrahedral sites in the interlayer space.

The electronic structure of the ZrX 2 (X = Se, Te) compounds has been studied using photoelectron, resonant photoelectron and X-ray absorption spectroscopy, theoretical calculations of the X-ray absorption spectra, and density of electronic states. It was found that the absorption spectra and valence band spectra are influenced by the chalcogen type. The results of the multiplet calculation of the Zr 4+ atom show that the change in the splitting in the crystal field, which is described by the 10Dq parameter, is due to the change in the ratio of covalent and ionic contributions to the chemical bond. The resonance band near the Fermi level in the valence band spectra is observed for ZrTe 2 in the Zr 3p-4d resonant excitation mode. The extent of photon energy indicates the charge localization on the Zr atom. Similar resonance band for ZrSe 2 is absent; it indicates the presence of a gap at the Fermi level.

The phase diagram of the quasi-binary Cu-TiS 2 system has been studied in situ in the temperature-concentration plane for the first time. Phase transitions between 2D (layered) and 3D (cubic) phases in Cu x TiS 2 (x = 0-0.5) intercalation compounds have been studied by the X-ray diffraction technique in the temperature range 20-1000 °C. It has been found that the stability of the layered phase is determined by the distribution of copper atoms between the octahedral and tetrahedral crystallographic sites. The occupation of octahedral sites dominates at low temperatures. Upon heating, tetrahedral site occupation is limited due to elastic lattice distortion and the layered phase becomes unstable and transforms to the spinel. Further heating allows the distribution of copper between octahedral and tetrahedral sites; the layered phase becomes stable again.

An experimental study of the electronic structure of V x Ti 1-x Se 2 system in a wide range of vanadium concentrations (x = 0.06-0.9) using x-ray photoelectron spectroscopy and resonant photoelectron spectroscopy has been performed. The partial charge transfer from the VSe 2 to TiSe 2 structural fragments is experimentally observed, and the most part of the charge is localized on the vanadium atoms in the VSe 2 structural fragments.

The electronic structure of the Ni x TiSe 2 intercalation compounds with disordered and ordered Ni atoms is studied using photoelectron, resonant photoelectron and X-ray absorption spectroscopy, theoretical calculations of the X-ray spectra and density of electronic states. The increase in the covalent component of the chemical bond of Ni and Ti with the nearest neighbour was found with increasing nickel concentration and its ordering. A significant charge transfer from the Ni atoms to the host lattice resulting in the transformation of Ti 4+ into Ti 3+ occurs, while the symmetry of the crystal lattice is lowered from the hexagonal to the monoclinic one.

An experimental study of the electronic structure of copper intercalated titanium dichalcogenides in a wide range of copper concentrations (x = 0.04-0.8) using x-ray photoelectron spectroscopy, resonant photoelectron spectroscopy, and x-ray absorption spectroscopy has been performed. Shift towards low energies of the Ti 2p and Se 3d core level spectra and a corresponding decrease in the photon energy of Ti 2p absorption spectra with the increase in copper concentration have been found. These sign-anomalous shifts may be explained by the shielding effect of the corresponding atomic shells as a result of the dynamic charge transfer during the formation of a covalent chemical bond between the copper atoms and the TiSe2 matrix.

Resonant valence band x-ray photoelectron spectra (ResPES) excited near the 2p(3/2) core level energies, 2p x-ray photoelectron spectra (XPS) and L(3,2) x-ray absorption spectra (XAS) of Ti and Mn in single crystals of 1T-Mn(0.2)TiSe(2) were studied for the first time. The ionic-covalent character of the bonds formed by the Mn atoms with the neighboring Se atoms in the octahedral coordination is established. From the XPS and XAS measurements compared with the results of atomic multiplet calculations of Ti and Mn L(3,2) XAS, it is found that the Ti atoms are in the ionic state of 4 + and the Mn atoms are in the state of 2 +. In ResPES of Mn(0.2)TiSe(2) excited near the Ti 2p(3/2) and Mn 2p(3/2) absorption edges the Ti 3d and Mn 3d bands at binding energies just below the Fermi level are observed. According to theoretical calculations of E(k) the Ti 3d states are localized in the vicinity of the Γ point and the Mn 3d states are localized along the direction K-Γ-M in the Brillouin zone of the crystal.