STRUCTURAL FEATURES OF Fe<i><sub>x</sub> </i>TiSe<sub>2</sub> MATERIALS WITH A RETROGRADE SOLUBILITY IN THE SOLID STATE
Changes in the crystal structure, which are accompanied by iron intercalation and release in a layered intercalate TiSe<sub>2</sub> compound demonstrating retrograde solubility in the complete 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 TiSe<sub>2</sub> 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.