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An alternative structure at which molecules might be sited, would be a micro-void in the lattice. Such voids are typically around 8 nm in diameter in crystalline silicon, this is somewhat large for ab initio modeling techniques, but some insight into the behaviour of hydrogen in such defects can be obtained by simulation of smaller structures. The smallest regular hydrogenated void that offers a more open environment than the crystal lattice is the passivated deca-vacancy. The vibrational frequencies of the molecule in such a void lie at 4324.6 cm-1 for H2, 3745.9 and 3743.8 for HD, and 3057.9 for D2. The modes of the hydrogen passivating the void fall into three bands at 2205-2191, 2141-2129 and 2108-2099 cm-1 (experimentally bands at ~2125, ~2087, and ~2073 cm-1 are observed). The molecular bond length is 0.759 Å, and the distance of the centre of the structure to a H atom on the void surface is 3.36 Å.

Antonio Resende