Annealing OV centres at 300C or above leads to their breakdown and the formation of VO2. However, a set of careful annealing experiments by Londos et al after neutron bombardment [97,98] showed that at 272-340C in long time anneals (4-5 hours), a pair of LVMs at 914 and 1000 cm-1 appear. As the 889 cm-1 VO2 line grows, these two peaks anneal out. These peaks are only seen when the material is initially neutron bombarded; electron irradiation does not lead to these pre-cursors.
They speculated that these modes were due to a metastable intermediate structure consisting of a VO centre with a neighbouring Oi, which forms when VO diffuses through the lattice to a Oi atom. This then has to overcome an energy barrier to form VO2. We modelled the proposed structure with Oi in a Si-Si bond neighbouring the Si-O-Si of the VO centre, in the same 110 plane. This is the natural bond site for the Oi to adopt if it is not yet in the vacancy, since the bond is dilated and also highly polarised due to the neighouring Os. The shared Si atom is able to move into the vacancy towards Os, so that the Si-O bond lengths of both O atoms are very similar (this behaviour also occurs in VO3). The Oi has an outer Si-O bond of 1.73 Å and a shared Si-O bond of 1.64 Å, Os also has an outer bond of 1.73 Å but its shared bond is slightly longer at 1.68 Å. Both have Si-O-Si bond angles of 138. The resultant vibrational modes are given in Table 5.2.
As can be seen, the modes are in reasonable agreement with experiment. However, the LVMs are highly sensitive to the position of the shared central Si atom sitting between the two O atoms. Moving this Si atom along -0.053,0.011,-0.011 Å, less than 0.08 Å towards the Oi, causes the modes to shift into excellent alignment with experiment, as can be seen from the table. There is currently no isotopic shift data available on these peaks.
|4lShifted Shared Si|