Chalcogenide Glasses
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Structural and Vibrational Properties The model most often used to describe the structure of the chalcogenide glasses is the continuous random network (CRN) model 3 where the basic structural building blocks, for example, AsS 3 pyramidal units in glassy As2S3, are considered to be linked together in a continuous random network. The existence of well-defined building blocks ensures that the short-range, or nearest-neighbor, order is well preserved in these glasses. There is, of course, no periodic long-range order as there is in crystalline solids. On the other hand, in simple chalcogenide glasses whose stoichiometrics match those of known crystalline phases, there is considerable evidence that some type of intermediate range order 4 5 persists in the glassy phase. For
example, in glassy As 2 S 3 it has been suggested that remnants of the layer structure of crystalline As 2 S 3 , such as the twelvem e m b e r e d - r i n g s , persist in the glass. 4 There have even been very specific suggestions as to how these layer segments are terminated and connected together. 5 The details of these descriptions of the intermediate-range order remain controversial. Because there is no long-range order, some of the vibrational properties of the chalcogenide glasses differ from those commonly observed in crystals. Figure 1 shows 6 the optical absorption (absorption coefficient, a, expressed in cm'1) in glassy As2S3 over a very wide range of energies. In the region between 102 and 103 cm"1 there are a series of peaks which are due to one phonon absorption. Similar peaks, albeit with different selection rules, are observed in Raman spectroscopy. The
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