CHEMICAL BONDING IN RUTILE-TYPE COMPOUNDS

We present an ab initio study of the bonding in the rutile (TiO2) structure, where the five members TiO2, VO2, CrO2, RuO2, and ZnF2 are investigated by means of band structure calculations, from which we derive electron densities, densities of states, total energies, and structure factors. We present simple molecular orbital schemes in order to interpret the main features of the calculated densities of states. Our results show that a MX6 cluster model (with M = Ti, Zn, V, Cr, or Ru and X = O or F) accounts only for part of the bonding mechanism. In the crystalline environment, the anionic ligands reduce their amount of pi-bonding in the MX6 cluster at the expense of an increased sigma-bonding with all three nearest metal neighbors in the M3X units. This reduction of anionic pi-bonding character causes an increase in the metal-metal bonding across the shared edges of MX6 octahedra. The observed lattice geometry can be seen as the optimum balance between a maximal stabilization of the MX6 and the M3X units. In addition, we determine for one example, namely TiO2, the internal coordinate u by minimizing the corresponding total energy and obtain a theoretical u of 0.3064 in excellent agreement with the experimental value of 0.305, thus demonstrating the high accuracy of our approach. All calculations are carried out using the full-potential linearized augmented plane wave (LAPW) method where exchange and correlation effects are treated by the local-density approximation.