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

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Abstract
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 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 , 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 above , which may suggest density fluctuations with a correlation length of about 50 Å in the metallic fluid.

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