Bond angle disorder

Hofmann and Thomas (31) have discussed the effect of ion bombardment on thermally grown silicon dioxide. XPS showed that small changes in the surface chemistry were observable. The Si 2p and 0 Is peaks were seen to broaden due to ion damage trtilch presumably Increased bond-angle disorder resulting in charge redistribution. 

It may occasion surprise that an amorphous material has well-defined energy bands when it has no lattice planes, but as Street s book points out, the silicon atoms have the same tetrahedral local order as crystalline silicon, with a bond angle variation of (only) about 10% and a much smaller bond length disorder . Recent research indicates that if enough hydrogen is incorporated in a-silicon, it transforms from amorphous to microcrystalline, and that the best properties are achieved just as the material teeters on the edge of this transition. It quite often happens in MSE that materials are at their best when they are close to a state of instability. 

Bemtsen et al. [84, 85] have separated the effect of hydrogen content and bond-angle variation. The structural disorder causes broadening of the valence and conduction bands and a decrease of the bandgap by 0.46 eV. Hydrogenation to 11 at.% results in an independent increase of the bandgap with 0.22 eV. 

Figure 2. (a) Crystal structure ofCClfSb(OTeFs)f (b) A view of the CClf cation, with key bond lengths and bond angles, showing the two-fold positional disorder around the crystallographic inversion center.82... 

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