The evidence for the dimer started with results in 1981 showing thermal donor formation rates decreased towards the sample surface. Interpreting this as oxygen out-diffusion gave oxygen diffusivities enhanced by more than four orders of magnitude .
There are 17 thermal donors which are likely to involve increasing numbers of oxygen atoms. The growth rates of these donors have been monitored and it is found that they grow with an activation energy around 1.8 eV - much lower than the energy for a single Oi hop . There have been suggestions that VO2 complexes could diffuse rapidly  but the evidence is not conclusive. The early suggestion  that oxygen pairs could diffuse rapidly has been supported by SIMS measurements of the out-diffusion of oxygen  between 500 and 700C, and recent experiments [143,144] studying carefully the loss of Oi from solution at temperature as low as 350C. These show that the process is second order below 400C but of higher orders, up to 8, as T increases to 500C. The low temperature process must be controlled by the formation of oxygen dimers. The change in order of the process is then explained by an instability of the dimer above about 450C and thus larger numbers of oxygen atoms are required to diffuse together to form a stable precipitate.
A plane wave supercell calculation  gave a binding energy for the oxygen dimer of 1.0 eV. However other calculations suggest that dimers will not bind . Semi-empirical CNDO/S techniques showed that the dimer has a binding energy of 0.1 eV and the saddle point for its migration is 1.36 eV . The saddle point for dimer diffusion was found to lie close to an over-coordinated oxygen square defect - rather similar to the known structure of the nitrogen pair  (see Chapter 7). However this was obtained from total energy calculations of manually selected intermediate structures, and re-examination of this problem using ab initio methods with constrained optimisation would be desirable.