Solid-state electronic structures of intercalation compounds: The system Mg(Ti3S4)2

The electronic band structure of the intercalation system Mg(Ti3S4)2 is investigated by a three-dimensional (3D) self-consistent-field (SCF) Hartree-Fock (HF) crystal orbital (CO) approach based on an INDO (intermediate neglect of differential overlap) all-valence Hamiltonian. The band-structure properties of Mg(Ti3S4)2 are correlated with those of the binary matrix as well as the dichalcogenide TiS2 where the transition-metal is in a higher oxidation-state. The calculated electronic-structure data of Mg(Ti3S4)2 and TiS2 are compared with available results of experimental measurements. For TiS2 the present computational findings are furthermore compared with previous band-structure calculations. The formation of charge-density-wave (CDW) solutions in Mg (Ti3S4)2 and Ti3S4 are analyzed. The CDW condensation is caused by fluctuating valences (i.e. deviations of an orbital occupancy around its average value) in the Ti sublattice. Some associated many-body effects are discussed. Mg(Ti3S4)2 is a system where electronic and ionic conductivity are both operative. A first-order phase-transition of the insulator-semimetal type is predicted for the ternary system as a function of the spatial localization of the intercalant-atoms in the 1D van der Waals channels. Electronic reorganization-processes between the intercalant and the Ti3S4 matrix accompanying this phase-transition are analyzed. It is shown, that the rigid band-model is nonvalid for the interpretation of the band-structure properties of the intercalation compound. The results of frequently used methods in band-structure theory of intercalated transition metal chalcogenides are critically reviewed. The present findings render possible a reinterpretation of some electronic consequences of intercalation. Variations of the one-electron energies in intercalated materials were incorrectly predicted by methods employed conventionally in solid-state theory.