A. N. Titov

Structure and properties of the intercalation compound CuxTiSe2

Compounds in the pseudobinary Cu-TiSe2 intercalation system are directly synthesized from elements. The phase diagram of the system is investigated, the solubility limit of copper is measured, and the structure of the material is determined. In the copper concentration range up to 60 mol %, single crystals are grown and the temperature dependence of the electrical resistance is measured. It is demonstrated that, in the concentration range under investigation, the intercalation of the system with copper gives rise to a set of phenomena observed upon intercalation of alkali metals.

Fast ionic transport in AgxTiS2

The enthalpy of the subsystem of silver ions in the intercalation compounds AgxTiS2 has been calculated from the electrochemically measured thermodynamic functions of the silver subsystem. The ionic conductivity and the coupled chemical diffusion coefficients for silver in the intercalation compound have been measured. The activation energy for diffusion of silver ions is determined and the obtained value is interpreted from analyzing the concentration dependence of the enthalpy of the ionic subsystem. The conclusion has been drawn that the high diffusion mobility is associated with the competition between the covalent and elastic interactions, which decreases the activation energy for diffusion of ions.

Resonant photoemission at the L3 absorption edge of Mn and Ti and the electronic structure of 1T-Mn0.2TiSe2

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.

The influence of intercalation on the phonon spectrum of titanium dichalcogenides

A systematic investigation of the inelastic neutron scattering spectra is carried out for intercalation titanium diselenide compounds of the general formula MxTiSe2 (M = Cr, Fe, Ni, Ag). It is shown that the effect of intercalation on the phonon spectrum of the material is determined primarily by the modification of the host lattice due to the formation of Ti-M-Ti covalent centers. It is established that the lattice can undergo a substantial softening when the impurity band coincides with the Fermi level.

Structural features of Fe x TiSe 2 materials with the retrograde solubility in the solid state

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.

Phase Diagram and Thermodynamic Equilibrium in the Fe x TiSe 2 System

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.

Decay of the homogeneous state in FexTiSe2

The stability of a homogeneous state in materials with a more than half-filled polaron band has been investigated for the FexTiSe2 system used as an example. It has been demonstrated that the factor limiting the stability of the homogeneous state of these materials is a change in the degree of filling of the conduction band due to the thermal broadening of the polaron band. This factor becomes substantial when the top of the polaron band intersects the Fermi level. It has been revealed that the decay of the homogeneous state leads to the precipitation of the intercalant. The morphology and structure of the precipitates formed in this decay in FexTiSe2 single crystals have been studied by transmission electron microscopy.

Synthesis, single-crystal growth, and superconducting properties of Fe-Se system

Single crystals FeSex 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 FeSex single crystals indicate the existence of a superconducting transition at 24 K.

Shift of the Fermi level during cointercalation of copper and iron into TiSe2

The influence of copper intercalation into the TiSe2 compound on the Fermi energy has been investigated using two independent methods. The first method is based on the analysis of the amplitude of the deformation (induced by impurity intercalation and providing the formation of polarons) as a function of the binding energy of the band of polaron states. For this purpose, the FexCuyTiSe2 system has been synthesized for the first time by cointercalation of copper and iron into the TiSe2 lattice and studied. The second method consists in measuring the electromotive force of an open-circuit electrochemical cell with respect to a metal reference electrode. Both methods lead to consistent results in the range of moderate copper contents. However, considerable discrepancies have been observed in the copper concentration range that corresponds to the beginning of filling of Cu/Ti hybrid states. These discrepancies are explained by the softening of the lattice due to an increase in the density of states at the Fermi level.

Chemical bond in FexTiSe2 intercalation compounds: dramatic influence of Fe concentration

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.