Motional activation enthalpies were calculated (table 1), for interstitial cations in the rutile structure, using a minimum energy path approach. Coulombic, overlap repulsion and polarization energy terms were included in the potential energy expression. Pre-exponential factors in the overlap repulsion terms were evaluated by minimizing the total electrostatic energy, and permanent dipole moments of the anion were treated using an iterative method. The most favorable diffusion path was found to be parallel to the c-axis in the ½,0,z direction in TiO2. The motional activation enthalpies increased with increasing interstitial cation radius. The path was independent of radius and material. A strongly anisotropic conductivity was predicted.

Calculation of Motional Activation Energies for Interstitial Ions in the Rutile Structure using a Minimum Energy Path Model. Ajayi, O.B., Nagel, L.E., Raistrick, I.D., Huggins, R.A.: Journal of Physics and Chemistry of Solids, 1976, 37[2], 167-72

Table 1

Activation energy for interstitial cation diffusion in TiO2