Temperature Dependence of Local Order in the Layer-Type Liquid Semiconductors ${\mathrm{As}}_2$${\mathrm{Se}}_3$ and ${\mathrm{Tl}}_2$Se·${\mathrm{As}}_2$${\mathrm{Te}}_3$

Far-infrared vibrational modes in vitreous ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ and ${\mathrm{Tl}}_2$Se·${\mathrm{As}}_2$ ${\mathrm{Te}}_3$ have been observed above the glass transition temperatures ($T_g$) and are an indication of the preservation of local order in the liquid phase. In ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ these modes exist above the melting point of the crystalline material. At a well-defined temperature $T_s=480\mathrm{}$ K ($>T_g$) the vibrational modes disappear in ${\mathrm{Tl}}_2$Se·${\mathrm{As}}_2$ ${\mathrm{Te}}_3$. Similar behavior is predicted for all layertype semiconducting glasses, and $T_s$ is proposed as a characteristic temperature for an abrupt change of local order in these materials.