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Although chalcogenide glasses, i.e. glasses containing one or more chalcogenide elements, sulfur, selenium, and tellurium, but no oxygen, are dealt with in Chapter 6.5, it is useful to discuss them briefly here because their random network structures differ from those of oxides by contravening Zachariasen's rules for glass formation.

Oxide glasses are completely ordered chemically, so that bonding occurs only between unlike atoms, i.e. oxygen anions only bond to cations, and cations only bond to oxygens. This is not the case for chalcogenide glasses, in which like atoms can bond. For example, in Ge─Se glasses, homopolar Ge─Ge or Se─Se bonds coexist with heteropolar Ge─Se bonds. In contrast to the single stoichiometric SiO₂ composition of silica glass, chalcogenide glasses form over a wide compositional range, as exemplified by Ge_xSe_1−x glasses, which form in the range 0 < x < 0.42. Some homopolar bonds are found even at the stoichiometric composition GeSe₂, as depicted in Figure 14 where the connectivity of a fragment of a Ge─Se network is sketched: Ge atoms are tetrahedrally bonded to four other Se or Ge atoms, whereas Se atoms are bonded to two other Ge or Se atoms. Nevertheless, there is a strong preference for chemical order so that the measured coordination numbers for the Ge─Ge and Se─Se homopolar bonds in GeSe₂ are 0.25 and 0.20, respectively [26]. In Figure 14, the number of homopolar bonds has thus been exaggerated for the purpose of illustration.

Another difference from oxide glasses is that edge‐sharing may occur between the structural units. For example, in Ge─Se glasses, a significant number of GeSe_4/2 tetrahedra share an edge with another tetrahedron (see Figure 14), the proportion being 34% in GeSe₂ glass [26]. A noteworthy consequence is the existence of unusually short Ge─Ge distances, which are readily observed by diffraction and also, thanks to a specific vibrational mode, by Raman spectroscopy. Furthermore, some chalcogenide glasses contain molecular units as well as network material. For example, As─S glasses with high S content contain sulfur rings, such as S₈, while As─S glasses with high As content (~40 at. % As) contain entities such as the As₄S₄ molecule that constitutes the mineral realgar, and others.

Figure 14 Network connectivity for a Ge─Se glass with a composition close to GeSe₂. Pair of edge‐sharing GeSe_4/2 tetrahedra shown at the top of the figure. Homopolar Ge─Ge and Se─Se bonds represented by a double line.

This variable composition of chalcogenide glasses leads to variations in the connectivity of the network, and hence in the rigidity of the network. It is predicted by constraint theory (see Chapter 2.8) that the network undergoes a transition from a floppy state to a rigid state when the average coordination number increases through a value of about 2.4 [27], with a major influence on the physical properties.