Sodium doped TiOCl as a realization of a multiband ionic Hubbard model*

Using first principles molecular dynamics simulations, we investigate the effect of Na intercalation in the layered Mott insulator TiOCl. In agreement with recent photoemission experiments, we find that the system remains insulating for all studied Na concentrations. Our calculations also explain the evolution of the spectral weight upon Na doping. This is possible due to a prediction of the Na-doped superlattice structures. In the search for alternatives to metallize TiOCl, we also carried out simulations of substitutions of O by F, Cl by S, or Ti by V (or Sc), respectively, but all resulting structures turn out to be insulating. We propose a description in terms of a multiorbital ionic Hubbard model in a quasi-two-dimensional lattice and discuss the nature of the insulating state under doping. Finally, a different route for metallizing TiOCl by doping is proposed.