THE OPTICAL-PROPERTIES OF LIQUID CHALCOGENS

The temperature and pressure variations of the optical reflectivity of liquid Te, Se, Se80Te20 and Se80S20 have been measured in the energy range of 0.5-5.4 eV. The optical conductivity, sigma(omega), has been derived by using an oscillator fitting analysis. The sigma(omega) curve of liquid Te, which has a stacked planar zigzag geometry, has a broad peak around 2.2 eV and a Drude term on the low-energy side. The peak can be mainly ascribed to the transition from the bonding sigma band to the antibonding sigma* band and from the filled sigma* band to the unfilled sigma* band. On the other hand, the sigma(omega) curve of liquid Se at 500-degrees-C has a broad peak around 3.5 eV, maintaining a semiconducting character. The peak is assigned to the transition from the lone-pair band to the sigma* band. With increasing temperature, the peak shifts to the lower energy side. The optical gap closes between 1100 and 1300-degrees-C and a Drude term appears. This implies that the structure of liquid Se changes from the spiral chain geometry to the geometry of metallic liquid Te. The temperature of the semiconductor-metal transition is lowered by replacing Se with Te.