Czochralski-grown Si samples were irradiated by fast neutrons, at room
temperature, with the aim of studying the identity of the defects pro
duced, using infrared spectroscopy. Two localized vibrational modes at
914 and 1000 cm(-1) were considered as intermediate defect stages bet
ween VO and VO2 complexes. We express the view that they may arise fro
m a [VO+O-i] defect where an interstitial oxygen atom is trapped near
a VO pair. Another two peaks at 1032 and 1043 cm(-1) were attributed t
o the VO4 defect and a tentative model of its structure is presented.