At 370C where Oi is not mobile, there is a gradual decrease in the 1012 cm-1 dimer band from its residual levels, accompanied with an associated increase in the 975-1006 cm-1 bands. These changes have been directly correlated , and are consistent with a thermal donor model constructed from dimers (see Chapter 9). There is no notable change in the Oi levels, suggesting the available dimers are becoming depleted as they form higher order complexes, but the lack of Oi migration means that new dimers are not being formed and hence the dimer concentration is dropping.
At 450C when oxygen is mobile (Figure 6.11b), the story is very different. There is a rapid initial drop in [Oi], accompanied with a rapid rise in dimer concentration. Meanwhile there is also an increase in the other modes. The dimer concentration then tails off, before reaching a steady state growth along with a similar growth in the 975-1006 modes, and an associated drop in [Oi]. This can be understood in terms of an initial glut of dimer formation due to Oi atoms which are close to the capture radius required to form a dimer. However once these are exhausted, there is then a drop to steady state Oi diffusion from a more homogenous background level; the behaviour of the 975-1006 modes is then simply a knock-on from the fluctuation in dimer levels (note that this is different from the explanation given in Reference ).