X-ray diffraction and small-angle x-ray scattering measurements on expanded fluid selenium

To investigate the structural change in the semiconductor-to-metal transition in expanded fluid selenium x-ray diffraction and small-angle x-ray scattering (SAXS) measurements at high temperatures and pressures up to 1650 degrees C and 1500 bar have been carried out. The results of the x-ray diffraction study show that the twofold-coordinated structure is largely preserved in the metallic fluid. The covalent bond becomes short at around 1200 degrees C, where the metallic character starts to be enhanced. These results suggest that the local structure of chain molecules changes with the transition. Since the semiconductor-to-metal transition occurs near the liquid-gas critical point, it is important to study how the critical density fluctuations are concerned with the transition. The SAXS spectra at 400 bar show that a broad peak appears at around 0.12 Angstrom(-1) above 1200 degrees C, which may suggest density fluctuations with a correlation length of about 50 Angstrom in the metallic fluid.