To order or not to order: that is the question

Jul 19, 2010

The general public is fascinated by shining, well-shaped crystals and much less thrilled by dim, shapeless glasses. Some may know that the shining shape of crystals is due to the fact that atoms inside them are arranged in a periodic pattern over long-range distances and this is not the case with glasses. However, only a few are aware that it is the lack of periodicity that renders glasses unique for important applications ranging from house windows to blue-ray DVDs. A recent study on an archetypal binary GeSe₂ glass resurrects an old debate: are binary AB₂-type glasses non-periodic yet fully chemically ordered or not?This is an important question since good knowledge of the atomic-scale structure is a prerequisite to understanding and, hence, gaining control over properties of glasses.

Using high-energy x-ray diffraction and computer simulations the study reaffirms that, at the atomic scale, GeSe₂ glass, just like SiO₂ and GeO₂ glasses, may be viewed as a continuous random network of tetrahedral units that are fully chemically ordered, meaning that A and B type atoms (A = Ge, Si; B = O, Se) are bonded only to unlike atoms. The difference between chalcogenide and oxide AB₂glasses is that in the latter the oxygen-based tetrahedra are very rigid and share only vertices while in the former the selenium-based tetrahedra are quite floppy and share both vertices and edges (see figure).

Now scientists can step back and enjoy nature’s ingenuity with a little bit more understanding. The present study provides strong evidence to show that nature likes keeping its building blocks intact as much as possible. In the case of binary AB₂ systems the building A-(B_1/2)₄ blocks seem to stay intact and so couple in fully chemically ordered networks. The networks may be periodic (as in crystals) or non-periodic (as in glasses) endowing the respective systems with a very different functionality.

[Source IOPscience]