Glass-forming region in the system As-S-Tl has been re-examined because the procedure of Flaschen et al. is doubtful on the point of homogeneous melting free from oxygen contamination. The As-S-Tl glass-forming system was found to be substantially comparable with the As-S-Ag glass-forming system reported already (Fig. 1).
Such properties as density, thermal expansion and infrared absorption have been measured on As2S3-Tl2S and As2S3-Ag2S glasses (Figs. 2 to 6). Both glasses exhibited some new absorption peaks in their infrared spectra in addition to the peak at 698cm-1 observed for As2S3 glass and these peaks were most distinct at the composition TlAsS2 or AgAsS2. On the basis of the crystal structure of As2S3 (orpiment), TlAsS2 (rolandite), AgAsS2 (trechmannite or smithite) and Ag3AsS3 (xanthoconite or proustite), the assignment of the peaks observed for As2S3, TlAsS2 and AgAsS2 glasses was made using the equation v=A(r-B)-3/2, where v, r, A and B are wave number, bond length and constants, respectively (Fig. 7). All the peaks observed may be assigned to the vibration of As-S bond with different bond length and, on the other hand, the absorption peaks calculated for the compounds As2S3, TlAsS2 and AgAsS2 agreed very closely with those observed experimentally for the corresponding glasses (Fig. 8). Consequently the above each glass may be considered to have the structure similar to that of the corresponding compound. Furthermore, it may be assumed that the absorption peak would be observed nearly at 696cm-1 if a glass could be obtained at the Ag3AsS3 composition (Table 1).
In view of the above facts the most reasonable conclusions to be drawn from the infrared absorption and density data are the followings: The structure of skeleton composed of As and S changes successively as Tl2S or Ag2S is added to the As2S3 glass. In the case of the As2S3-Tl2S glasses orpiment-like (AsS3/2)n sheet structure transforms to (AsS2)n chain structure found in rolandite and then to short chain structure. On the other hand, in the case of the As2S3-Ag2S glasses (AsS3/2)n sheet structure transforms to discrete As3S6 group structure found in smithite or trechmannite and then to isolated AsS3 pyramid structure found in xanthoconite or proustite.