Comment on bond 'Bond volumes in crystals and glasses and a study of the germanate anomaly' by H.-J. Weber [J. Non-Cryst. Solids 243 (1999) 220]

A recent paper [J. Non-Cryst. Solids 243 (1999) 220] introduced the concept of bond volume V-b = V-0 + alpha*d(3)/N, where d is bond length, N is coordination number, and V-0 is free volume. It was found empirically that alpha* = 12.25 +/- 0.25 for compact structures. The concept of bond volume was used to analyse free volume in germanate glasses, and hence investigate the germanate anomaly. The present comment gives an explicit derivation for the value of alpha* in common binary compounds. The common feature of structures with alpha* approximate to 12 is not that they have space-filling by coordination polyhedra, but rather that they have a dense packing of anions.