The relaxed structure is shown in Figure 9.13 with eigenvalues in Figure 9.10a. When unconstrained, the central Si atom moves 0.05 Å off the C2 axis to form a structure somewhere between the Snyder-Stavola and an oxygen square with a flanking Oi, saving 0.063 eV in the process. This is consistent with calculations by Chadi who found an energy saving of 0.07 eV for the same process. This is presumably the diffusion barrier for reorientation of the defect between the two off-site configurations, and hence it would appear time averaged C2v.
The vibrational modes of the Snyder-Stavola structure are given in Table 9.5. The high frequency mode is due to out-of-plane wag of the core trivalent oxygen atom, and seems remarkably high for such a motion. The modes are not in good agreement with either TD1/TD2, which exhibit two modes each at 975/988 and 716/724 cm-1. The only possibility for correlation is if the calculated modes at 978.9 and 771.6 cm-1 are those that have been observed, and there are additional experimental modes in the 850-900 cm-1 range which have not yet been observed. The TD2 988 cm-1 mode drops by 43 cm-1 when 16O is replaced with 18O, which is not very good agreement with the 978.9 cm-1 mode shift of 46.9 cm-1.
We therefore conclude on the basis of the vibrational modes that the Snyder-Stavola model is unlikely to be TD2, but may be TD1 if there are further vibrational modes which have not yet been detected by experiment.
|3cLocal Vibrational Modes (cm-1)||Dipole moment squared|
|16O||17O||18O||for 16 O|