It was noted that the Ci and Cs-Ci defects exhibited several unexplained features. In the neutral charge state, the Ci defect had 2, almost degenerate, vibrational modes. This suggested that it was a trigonal defect, in disagreement with the C2v symmetry which had been deduced from other experiments. The B-form of the second defect was believed to consist of a Si interstitial which was located near to a BC site between two Cs atoms. This appeared to conflict with the results of photoluminescence experiments, which showed that the C-related vibrational modes were de-coupled. The structures and vibrational modes of both defects were analyzed by using LDF cluster theory. The degeneracy of the modes of Ci was attributed to an almost D3h structure, with a 3-fold axis along [011¯]. The modes of the di-C interstitial solved a problem concerning the almost-zero shifts, due to mixed isotopes, in the 580 and 543/cm local modes which were observed in photoluminescence studies.

R.Jones, S.Oberg, P.Leary, V.Torres: Materials Science Forum, 1995, 196-201, 785-90