Born charge differences of TiO2 polytypes:: Multipole expansion of Wannier charge densities -: art. no. 121101

The bonding Wannier functions (WF's) associated with the O 2s/2p valence states of TiO2 have been calculated for both the rutile and the anatase forms. They are similar in the two polymorphs, and correspond to three distorted O sp(2)-like hybrids in the plane of the Ti-3\ coordination triangle, and one p-like orbital perpendicular to this plane. For both structures, the WF's show that the OTi3 triangular complex is the relevant structural unit for understanding TiO2 electronic properties. WF's differences between the two phases are quantitatively measured with a multipole expansion of the static charge density of the fourfold WF complex. These differences reflect the opposite distortion from equilateral shape that the isosceles Ti-3 triangle has in the two structures, and correlate quantitatively with the opposite anisotropy of the dynamical Born charge tensors calculated for the two polytypes.