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The equilibrium boundary for the layered VTiSesolid solution was determined according to a set of experimental data of synchrotron radiation X-ray photoelectron spectroscopy and electromotive force of the Li|Li|VTiSeelectrochemical cells. This boundary appears to be 20 mol % of V (x = 0.2). The formation of the VSestructural fragments, which are tightly bounded with VTiSelayers, appears at x > 0.2. A combined analysis of the Fermi-surface maps and the atomic force microscopy images reveals that the (001) planes in VSeand VTiSestructural fragments are parallel to each other. At the same time, these fragments demonstrate turbostratic disordering within the basal plane. The formation of such pseudo-homogeneous materials is associated with the contribution of the free surface energy, which is relatively high in the quasi-two-dimensional materials.

The electronic structures of V-intercalated TiSe2 and substitutionally doped dichalcogenides Ti1-xVxSe2 have been studied using soft X-ray photoelectron, resonant photoelectron, and absorption spectroscopies. In the case of the substitution of Ti by V, the formation of coherently oriented structural fragments VSe2 and TiSe2 is observed and a small charge transfer between these fragments is found. Intercalation of the V atoms into TiSe2 leads to charge transfer from the V atoms to the Ti atoms with the formation of covalent complexes Ti-Se-3-V-Se-3-Ti.

The effect of atomic ordering in the cobalt sublattice on the electronic structure of the CoxTiSe2 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.

New intercalation compounds CrxZrTe2 were synthesized in the Cr concentration range of x = 0–0.3. A thorough study of the crystal and electronic structure has been performed. It was found that there is competition in the distribution of the Cr atoms over the octa- and tetrahedral sites in the van der Waals gap, depending on the Cr content. The ordering of the Cr atoms was found at x ≥ 0.25; at the same time, the lattice symmetry decreases from trigonal P-3m1 to monoclinic F2/m. This ordering stabilizes the octahedral coordination of the Cr atoms by Te atoms. The analysis of the experimental data on the electronic structure and DOS calculations showed that the Cr 3d states are spin-split. However, these Cr states are still overlapped by non-spin-split Zr and Te states. © 2021 Elsevier B.V. The research was carried out within the state assignment of Minobrnauki of Russia (theme “Electron” No. AAAA-A18-118020190098-5, theme “Spin” No. AAAA-A18-118020290104-2 and theme “Quantum” No. AAAA-A18-118020190095-4) and with partial financial support of the RFBR (project 20-03-00275). This work has been done using facilities of the Shared Service Centre “Ural-M”, Institute of Metallurgy of the Ural Branch of the Russian Academy of Sciences. The sample synthesis were performed within the RSF grant (project No.17-73-10219). I.P., and S.N. acknowledge funding from EUROFEL project (RoadMap Esfri). This project has received funding from the EU-H2020 research and innovation program under grant agreement No 654360 having benefitted from the Access provided by IOM-CNR in Trieste (Italy) within the framework of the NFFA-Europe Transnational Access Activity. We thank Federica Bondino and Elena Magnano for the kind support and we acknowledge the Elettra Sincrotrone Trieste for providing access to its synchrotron radiation facilities.

The CuxZrSe2 intercalation single crystals have been synthesized and studied in a concentration range of x = 0-0.3, in which the semiconductor-metal transition was observed. The evolution of the electronic structure of CuxZrSe2 as a function of the copper content has been studied experimentally using the XPS and XAS methods. The obtained results indicate the gradual shift of the Fermi level with increasing copper content, which does not allow the metal-insulator transition to be associated with charge transfer to the conduction band. This transition is associated with the formation of the covalent bond between copper and the nearest selenium atoms and of the p-d hybridized states inside the Zr 4d/Se 4p band gap. First-principle calculations were performed in order to obtain a theoretical description of the density of states near the Fermi level and confirm this conclusion.

The effect of atomic ordering in the cobalt sublattice on the electronic structure of the Co

Crystal and electronic structure, optical absorption and transmission spectra, temperature dependences of conductivity and magnetic susceptibility were studied for copper intercalated ZrSe(2)in the copper concentration range of 0

The thermal stability of the layered modification of the Cu0.5ZrTe2 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 CuZr2Te4 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 CuZr2Te4, 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.

A low-temperature layered phase CuxTiS2 (0 ≤ x ≤ 0.7) has been prepared. The occupation of both octahedral and tetrahedral sites by the copper atoms in the interlayer gap of the TiS2 lattice is revealed. The presence of two concentration regions of 0 ≤ x ≤ 0.3 and 0.5 ≤ x ≤ 0.7, which differ in the concentration dependence of the unit cell volume, has been observed. In the first region the unit cell volume increases with x, in the second region it decreases. This difference may be associated with the competition between Cu-Ti and Cu-S covalent centres. The electrical and magnetic properties of the materials are in a good agreement with this assumption. The electronic structure of CuxTiS2 in the low copper concentrations region has been studied using X-ray photoelectron, resonant photoelectron and absorption spectroscopy techniques. It was found that, in the low copper concentrations region, the charge transfer from Cu to the conduction band of TiS2 along with the formation of covalent centres is observed.

The influence of the selective distortion of the Se and Ti sublattices on the stability of the charge-density-wave (CDW) state has been studied. The selective distortion has been obtained in two ways: by Mn or Cr intercalation, which differ in the selective hybridization between Mn 3d/Se 4p and Cr 3d/Ti 3d orbitals, and by the plastic uniaxial deformation of the TiSe2 single crystals. Plastic deformation generates defects in the Se sublattice, which create the difference between the nearest neighbor positions. It is found that the distortions of the Se sublattice make impact up to 1000 times more on the CDW state stability than those of the Ti sublattice. The effect is related to the distortion of the spatial coherence of the Se 4p-orbitals. The critical size of the coherence area, below which the CDW becomes unstable structure and loses a long-range-order, has been estimated. It is found that such a kind of suppression does not change the temperature of the CDW transition but gradually decreases its amplitude.

The ability to manipulate the electronic transport properties and electronic states in transition metal chalcogenides greatly expands the range of their application. Here, we investigate the effect of applied stress on the electrical conductivity of quasi-two-dimensional ZrTe2 and several intercalated AgxZrTe2 and CuxZrTe2 crystals using pressure-dependent measurements of their Seebeck coefficients (thermoelectric powers) at room temperature. The pristine ZrTe2 crystal is characterized by p-type semimetal conductivity, and the pressure dependence of its thermopower exhibits a broad wave-like maximum at about 2 GPa. Potentially, this feature is related to a charge density wave transition. The electrical conductivity in all the intercalated crystals is of n-type, but an applied pressure of about 1.5–3 GPa, dependent on crystal composition, reversibly inverts it to p-type. At pressures above 6 GPa, thermopower curves for all samples demonstrate a nearly identical and degenerate pressure behavior, thereby suggesting metallization of the samples. We verify the reproducibility of the above features by multiple pressure cycling of the samples up to 9 GPa. We, therefore, experimentally show that slightly intercalated ZrTe2 crystals with n-type conductivity can be utilized as stress-controlled n-/p-switches of the conductivity type. We give and discuss simple examples of how the above n–p conductivity inversions could be potentially implemented in various modules for next-generation nanoelectronic devices.

The comparison of the electronic structure modifications of intercalation materials at intercalated metal ordering has been performed depending on the degree of metal d-shell filling using the Fe x TiSe 2 and V x TiSe 2 . The crystal and electronic structure of the V x TiSe 2 compounds has been studied for the first time. The electronic structure based on experimentally obtained atomic parameters has been calculated by the first principle approach. Obtained results are discussed taking into account an order of both intercalated atoms and vacancies. It has been shown that ordering leads to the formation of additional bands, the energy and orbital symmetry of which are determined by the electronic configuration of the intercalated metal ion.

The electronic structure of the NixTiSe2 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 Ti4+ into Ti3+ occurs, while the symmetry of the crystal lattice is lowered from the hexagonal to the monoclinic one.

Changes in the crystal structure, which are accompanied by iron intercalation and release in the layered intercalation compound TiSe2, demonstrating the retrograde solubility in the completely solid state, are considered. Various concentration regions not exceeding, exceeding, and corresponding to the leakage limit of the overlap of titanium orbitals coordinated by iron are analyzed. It is shown that at low temperatures (below 400°С) the behavior of iron in the TiSe2 lattice is governed by a covalent bond of iron with the lattice whereas at high temperatures (above 1000°С) iron becomes an ionic impurity. In the intermediate temperature range, iron atoms are involved in either covalent or ionic bond with the lattice. When the concentration of iron in the form of the ionic impurity increases, an increase in the cell parameter in the direction perpendicular to the layers is accompanied by a compression of the Se–Ti–Se sandwich and an increase in the van der Waals gap. When the covalent bond forms, there is a decrease in the lattice parameter in the direction perpendicular to the layers, which is accompanied by an increase in the width of the Se–Ti–Se sandwich and a decrease in the van der Waals gap.

Decomposition of a homogeneous intercalation compound with the formation of inclusions in the interlayer space of a matrix lattice has been directly observed. In full compliance with previously advanced theoretical concepts, it has been shown that the decomposition is accompanied by metallic iron extraction, which then gradually transforms into iron selenide due to the interaction with gaseous selenium.The hierarchy of diffusion mobilities of various defects in FexTiSe2 intercalation compound has been determined. It has been found that the largest mobility is inherent to intrinsic defects of the TiSe2 lattice, i.e., vacancies in Ti and Se sublattices. The phenomenon of dissociation pressure oscillations as a function of time has been detected. This phenomenon has been explained by the existence of a slow decomposition stage, i.e., intercalated iron diffusion during the formation and dissolution of its inclusions as an individual phase.

The introduction of 3d transition metals into the van der Waals gaps of titanium dichalcogenides with layered structures gives rise to an interesting family of intercalation compounds, the physical properties of which differ substantially from those of the host compound due to “host–guest” and “guest–guest” interactions, resulting in ordering of the intercalated metal. FexTiSe2 is one of the most intriguing of such intercalation compounds due to its special electronic and magnetic properties. In this paper, the influence of the iron concentration on the crystal structure, electronic structure and chemical bond in the FexTiSe2 intercalation compounds is studied across a wide composition range (0 ≤ x ≤ 0.50) within the homogeneous region of the system. Photoelectron, resonance photoelectron and X-ray absorption spectroscopy methods were combined with thermodynamic electromotive force (EMF) measurements for the Li|Li+|FexTiSe2 cell in order to monitor the Fermi energy concentration dependence. The electronic structure changed substantially as the Fe content increased. A proposed model of the FexTiSe2 electronic structure transformation implies that iron atoms form Ti–Fe–Ti bonds at x < 0.40, but that Fe–Fe bonds prevail at x ≥ 0.40 when nanoscale iron chains appear in the van der Waals gap.

The crystal structure of Fe0.5TiSe2 has been studied in-situ by means of X-ray diffraction technique using synchrotron radiation upon heating in the temperature range 20–1000 °C. It was found that the iron sublattice undergoes a second order transition at a temperature of about 420 °C without affecting the TiSe2 sublattice. This phase transition has been described using an order parameter, the behavior of which is close to that for a 2D model. It was established that this transition is not related to the first-order transition, associated with the extraction of metallic iron.

The comparison of the specifics of the guest-host chemical bonding in the materials with (FexTiSe2) and without (FexTiTe2) ordering of the iron atoms was performed. For this purpose the electronic structure of the materials were studied using X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, resonant X-ray photoelectron spectroscopy, and theoretical calculations (total density of states, partial density of states, and multiplet calculations). For the iron-intercalated TiTe2 compound iron-chalcogen bonds are formed, whereas the formation of iron-iron bonds is most typical for the iron-intercalated TiSe2 compound. This leads to an increase in the lifetime of electrons on the titanium atoms and does not allow the formation of atomic chains of intercalated metal. © 2018 American Chemical Society

The electronic structure of the ZrX

The thermal stability of the layered modification of the Cu

The phase diagram of the quasi-binary Cu-TiS

An experimental study of the electronic structure of V

The electronic structure of the Ni

Using static tensimetry, the selenium pressure during dissociation of intercalated compounds Fe x TiSe2 has been measured in a temperature range of stability of the homogeneous material and in the region of decomposition caused by thermal expansion of the polaron band. It has been shown that covalent centers in the stability region of the homogeneous state, which are stabilized by the polaron state of conduction electrons, behave as the effective oxidants lowering the Fermi level. Broadening of the polaron band in the region of a high iron concentration leads to weakening the oxidizing influence of polarons. Iron selenides are formed during the decomposition of intercalated compounds, which leads to an essential non-quasi-binarity of the Fe-TiSe2 system.

The photoelectron valence band spectra of TiS2, TiSe2, and TiTe2 dichalcogenides are investigated in the Ti 2p-3d resonance regime. Resonance bands in the vicinity of the Fermi energy are found for TiS2 and TiTe2. The nature of these bands is analyzed based on model calculations of the density of electronic states in TiS2, TiSe2, and TiTe2 compounds intercalated by titanium atoms. Analysis of experimental data and their comparison with model calculations showed that these bands have different origins. It is found that the resonance enhancement of an additional band observed in TiS2 is explained by self-intercalation by titanium during the synthesis of this compound. The resonance enhancement in TiTe2 is caused by occupation of the 3d band in Ti.

An experimental study of the electronic structure of copper intercalated titanium dichalcogenides in a wide range of copper concentrations (x = 0.05 - 0.6) using the x-rays photoemission spectroscopy, resonant photoemission and x-rays absorption spectroscopy has been performed. Negative energy shifts of the Ti 2p and Se 3d core levels spectra and a corresponding decrease of the photon energy in Ti 2p absorption spectra with increasing concentration of copper have been found. Such 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., Comment: 12 pages, 6 figures

Single crystals FeSe x were grown by the vapor-transport reaction method using a polycrystalline material, a sealed quartz tube, and iodine I2. The single crystals grown are 0.1–0.5 mm in size and are characterized by hexagonal prismatic, tetragonal prismatic, platelike square, and hexagonal faceting. Measurements of the electron-transport and magnetic properties of the FeSe x single crystals indicate the existence of a superconducting transition at 24 K.

Phase diagram has been refined by the quenching method; the phase composition has been monitored by NGR spectroscopy. The experimental data agree well with theoretical predictions on the relation of the phase diagram with the character of a chemical bond. It has been shown that the system is essentially nonquasi-binary because of the selenium exchange between the precipitated iron and the matrix lattice.

The electronic structure and the chemical bond in titanium dichalcogenides TiX2 (X = S, Se, Te), which are promising electrode materials for lithium batteries, are studied experimentally and theoretically. It is found that the X-ray photoelectron spectra of the valence bands and the core levels of titanium and its X-ray L 2, 3 absorption spectra demonstrate a change in the ionic and covalent components of the chemical bond in these compounds. The densities of states in these compounds are calculated by the full-potential augmented-plane-wave method, and multiplet calculations of the X-ray L 2, 3 absorption spectra of titanium are performed. It is shown that, in the row TiS2-TiSe2-TiTe2, the covalence increases, the ionicity of the chemical bond decreases, and the effect of the crystal field of a ligand is weakened.

The electronic structure of the ZrX2 (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 Zr4+ 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 ZrTe2 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 ZrSe2 is absent; it indicates the presence of a gap at the Fermi level. © 2018 Author(s).

The phase diagram of the quasi-binary Cu-TiS2 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 CuxTiS2 (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 VxTi1-xSe2 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 VSe2 to TiSe2 structural fragments is experimentally observed, and the most part of the charge is localized on the vanadium atoms in the VSe2 structural fragments.

A comprehensive study of the crystal and electronic structure of TiSe2 intercalated with two 3d metals, Cu and Ni, is presented. It is found that the intercalation of Cu into NixTiSe2 at room temperature leads to the displacement of the Ni atoms from the octahedral to tetrahedral sites with respect to the Se sublattice. This effect is explained by the charge transfer from Cu to the TiSe2-derived conduction band, which makes the chemical bond of Ni with the lattice more stable. This is because the hybridization between Ni 3d and Se 4p, 4s states is stronger than that between Ni 3d and Ti 3d in mono-intercalated NixTiSe2. Therefore, it is possible to control the type of the coordination of the intercalated transition metal atom by changing the concentration of doped electrons.