Violation of Vegard's law in covalent semiconductor alloys

Anomalous $S$-shaped deviations from Vegard's law in some semiconductor alloys are attributed to the effect of bond-bending forces. This idea is substantiated by numerical calculations on two-dimensional model alloys. A criterion is found which separates the occurrence of usually observed concave bowing and $S$-shaped deviations. In addition, our results show that the distribution of bond lengths in an alloy is bimodal.