Band Structures of Intercalation Compounds. The System (NH4)(NH3)3(TiS2)4

Abstract

The electronic band structure of the intercalation system ammonia TiS2 has been investigated by a semiempirical self-consistent-field (SCF) Hartree-Fock (HF) crystal orbital (CO) formalism supplemented by an INDO (intermediate neglect of differential overlap) Hamiltonian. A two-dimensional (2D) model for the title system with stoichiometry (NH4)(NH3)3(TiS2)4 has been selected on the basis of available experimental data. The model is defined via a TiS2 monolayer coupled to the intercalant monolayer. The corresponding band-structure properties are compared with bandstructure calculations of monolayered TiS2 and bulk TiS2 . For TiS2 available experimental data and numerical results of conventional band-structure approaches are reported. The interaction between the guest-molecules and the host lattice has the character of a redox-process; i.e. one electron per formula unit has been transferred from the intercalant to the TiS2 layer. One consequence of this transfer is a semiconductor-to-metal transition upon intercalation; an additional consequence is a remarkable electronic reorganization in the TiS2 host. The surplus of electronic charge is predominantly localized at the S centers. The electronic states at the Fermi-level are of Ti 3d character. Two electronic configurations of the title system have been investigated. The mean-field ground state is of a charge density wave type with respect to the TiS2 sublattice. A “symmetry adapted” (SA) configuration is predicted at higher energy.