The OV-O_i pre-cursor to the VO₂ centre

Annealing OV centres at 300$^\circ$C 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 272-340$^\circ$C 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.

  

Figure 5.5: The OV-O_i centre in Si, pre-cursor to the O₂V. The box indicates the [100] directions.

They speculated that these modes were due to a metastable intermediate structure consisting of a VO centre with a neighbouring O_i, which forms when VO diffuses through the lattice to a O_i atom. This then has to overcome an energy barrier to form VO₂. We modelled the proposed structure with O_i in a Si-Si bond neighbouring the Si-O-Si of the VO centre, in the same $\langle$110$\rangle$ plane. This is the natural bond site for the O_i to adopt if it is not yet in the vacancy, since the bond is dilated and also highly polarised due to the neighouring O_s. The shared Si atom is able to move into the vacancy towards O_s, so that the Si-O bond lengths of both O atoms are very similar (this behaviour also occurs in VO₃). The O_i has an outer Si-O bond of 1.73 Å and a shared Si-O bond of 1.64 Å, O_s 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$^\circ$. 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 $\langle$-0.053,0.011,-0.011$\rangle$  Å, less than 0.08 Å towards the O_i, 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.

 

¹⁶O_i-¹⁶O_s ¹⁶O_i-¹⁸O_s ¹⁸O_i-¹⁶O_s ¹⁸O_i-¹⁸O_s 4lRelaxed

926 17 9 41

886 26 33 45

656 25 3 30 4lShifted Shared Si

994 3 35 42

905 39 10 46

656 24 4 31 4lExperimental 1000 914