Support for the VO2 assignment of the 889 cm-1 defect comes from its IR-absorption intensity which is approximately proportional to [Oi]2 when VO has disappeared. However, there is no loss of Oi, as measured by IR-absorption, during the slow stage when the 889 cm-1 defect is being formed . This initially provided the necessity for the third formation mechanism given above, however recent studies by Londos et al  show two distinct activation energies in the slow process, with a switch in dominance at about 360C. In addition they showed that above 380C VO2 absorption continues to increase, even though VO levels have almost stabilised.
Further, early uniaxial stress studies on the 889 cm-1 defect  concluded that its symmetry was lower than D2d; unless the symmetry was D2d, two O-related LVMs would be expected. However, prompted by our theoretical investigations, Bech Neilsen et al  re-examined the defect with uniaxial stress and showed it to possess either C2v or D2d symmetry. Although it is difficult to distinguish between the two, they have recently performed a range of stress-induced dichroism experiments that were able to unambiguously show D2d symmetry  (see below). Finally, for mixed 16O-18O samples, only two LVMs were detected and not three [84,85], with isotopic shifts indicative of a single oxygen atom. This shows that the two O-modes in this model for the 889 cm-1 centre are decoupled even though their separation can only be a few Å. This objection is not fatal as earlier modelling studies  found that the O atoms were decoupled in the LVMs.
The difficulties  described here have led to an alternative assignment of the 889 cm-1 LVM to V3O . This defect would possess only one LVM and possess C1h symmetry. Presumably, in this model vacancies are thermally released from VO or Vn complexes and form stable divacancies which subsequently trap VO. EPR studies have suggested  that complexes such as V2O and V3O, with S= 1, form and anneal out around 300C although Davies et al  suggest that V2O may be responsible for an LVM at 1005 cm-1 (10 K). This defect anneals out around 450C.