It is possible construct a thermal donor consisting of 4 O_{i} atoms
in a different way to the di-y-lid. This can be done by removing
O_{i} from the 5 O_{i} thermal donor described above. Removing the
oxygen atom on the C_{2} axis leads to the di-y-lid model, however
removing one of the outer oxygen atoms, simply shortens the chain and
removes some compression from the defect core.

The defect relaxed by rotating the core around to give an oxygen
`square' flanked by two standard divalent O_{i} atoms. This has
*C*_{2h} rotational symmetry, the same as that of the N_{2i}
pair. The eigenvalues are given in Figure 9.10c and its
double donor character can be seen. Distortion in the structure was
caused by cluster assymetry, so the calculations were repeated in a
*C*_{2h} symmetry constrained 180 atom cluster, .

The structure is shown in Figure 9.12. The bonding in
this defect is quite different to that of the di-y-lid, having no core
reconstructed Si-Si bond. Whereas the di-y-lid can be thought of as
two dimers held apart by two empty Si-Si bonds, this square-based
structure could be formed from two dimers with only a single empty
Si-Si bond between them. Such a structure does not have the required
C_{2v} symmetry, and thus this could only be a candidate for TD1.

The vibrational modes of this structure are given in
Table 9.4, along with their isotope shifts and their
absorption intensities. This model has two strong absorptions at 904
and 742 cm^{-1}. The experimental TD1 modes lie at 975 and
716 cm^{-1}, unfortunately their ^{18}O isotope shifts have not
yet been reported[239]. Thus the calculated modes are
in roughly the right range, and without isotopic shift data these LVMs
are not sufficient to exclude this as a model for TD1. The primary
absorptions for TD1 are still quite weak, so it is unlikely that the
mode at 811 cm^{-1} will be observable. In the assymetric cluster
this structure was nearly 1 eV more stable than the di-y-lid and was
the lowest energy 4O donor structure, although the error on such a
figure will be large, and also influenced by cluster surface effects.

Symmetry analysis of TD1 would be a useful tool to distinguish between
these models since this structure has well defined *C*_{2h} symmetry.
Note that if this is TD1 it would add weight to the suggestion that
the di-y-lid is TD2. This is because TD1 and TD2 are believed to be
isomeric, since their respective electrically inactive forms show
vibrational absorption at the same frequency[239]. In
conclusion, symmetry excludes this `flanked square' structure as
anything except TD1, but the calculations would be consistent with
what is currently known of TD1. Further experimental data is required
to either confirm the assignment or eliminate it from enquiries.

3cLocal Vibrational Modes (cm^{-1}) |
Dipole moment squared | ||

^{16}O |
^{17}O |
^{18}O |
for ^{16} O |

946 | 921 | 898 | 0.000 |

904 |
882 |
862 |
0.179 |
---|---|---|---|

854 | 831 | 811 | 0.000 |

811 | 791 | 773 | 0.100 |

742 |
725 |
711 |
0.206 |

741 | 723 | 706 | 0.000 |