The effect of Fe impurities on the properties of As//2S//3 glass has been examined. In line with previous workers the optical absorption below the principle edge at 2. 3 ev is increased when Fe is added, indicating impurity states are generated. However we find that the ac conductivity is insensitive to doping with this metal. On the other hand both Debye-Scherrer measurements and electron microscopy demonstrate pyrrhotite (FeS) readily crystallizes from heavily doped glasses (few wt%). These techniques are insensitive to crystallinity at lower concentrations but we demonstrate how X-ray absorption spectroscopy can be used to detect the characteristic Fe-S distribution of pyrrhotite down to 0. 5 wt%. At lower concentrations this gradually disappears leaving the Fe predominantly occupying octahedral sites in the glassy matrix. It is argued that the occupancy of the Fe 3d states will leave the position of the Fermi level fixed in the As//2S//3 gap as the ac conductivity results suggest.
