Static Approach to the Mechanical Stability

The total number of constraint on an atom is given by n = Cbs + Cbb, with Cbs the number of BS constraints, that is equal to Z/2, with Z the coordination number, and Cbb the number of independent rigid angles around the considered atom. 

The situation is often more complex, as for the Ge environments in GST124 (see Fig. 18.12, right panel). The distance distributions averaged over all Ge atoms are bimodal, which reflects the existence of two distinct atomic environments. As shown previously, these are those of tetrahedrally bonded Ge (Ge4) and octahedrally bonded Ge (Ge3). Indeed, the distance distributions for Ge4 and Ge3 atoms plotted in Fig. 18.12 are totally different, which corresponds to = 4 for Ge4 and Cbs = 3 for Ge3, as expected. 

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