Intermediate range order, network voids and stress

2 Intermediate range order, network voids and stress 

The presence of the hydrogen imposes a distinction between the atomic and the network coordination. All the silicon atoms are fourfold coordinated, apart from the small concentration of coordination defects. However, the network coordination is reduced by the presence of hydrogen which forms a single bond to the silicon and so does not help to link the network together. The average network coordination is. 

The major source of the disorder energy is the bond strain within the random network. Phillips (1979) proposed a model to explain the relation between network coordination and disorder. A four-fold continuous random network is overcoordinated, in the sense that there are too many bonding constraints compared to the number of degrees of freedom. The constraints are attributed to the bond stretching and bending forces, so that for a network of coordination Z , their number, NciZJ is. 

The factors of one half arise because each bond constrains two atoms. The first term is for bond stretching and the second term is from the 

Ball and stick models contain only bond length and angle constraints and substantiate this result. It is easy to see that such a model of a twofold coordinated material will collapse because there is nothing holding the chains apart. (In reality, of course, additional constraints are provided by the weaker van der Waals forces between chains.) On the other hand the common experience of those who have constructed amorphous silicon models is that a large bond strain tends to accumulate in the process of satisfying each silicon atom s four bonds. 

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