Semiconductor-to-metal transition in fluid selenium at high pressure and high temperature: An investigation using x-ray-absorption spectroscopy

In the vicinity of the critical point (1620 °C, 385 bars) the semiconducting properties of fluid selenium give way to a metallic state, as evidenced by conductivity and optical measurements. The need for accurate local information on the structure of fluid Se near its critical point moved us to perform x-ray-absorption measurements in its semiconducting and metallic phases in the pressure and temperature range up to 600 bars and 1550 °C. The good quality of the x-ray-absorption fine-structure (EXAFS) data, obtained thanks to the simultaneous use of a special cell developed for x-ray-absorption studies of liquids under those extreme conditions and of the high brightness of the focused x-ray beam available on the BM32 beam line at the ESRF, allowed us to use a nonharmonic analysis of the EXAFS data by using the cumulant expansion method, at least for the first coordination shell. Such an analysis gives reliable values of the coordination numbers, and moreover knowledge of the cumulant values up to sixth order permits us to reconstruct the shape of the first peak in the radial distribution function with a fairly good accuracy. Our EXAFS results clearly support the shortening of the Se chains approaching the metallic fluid phase at high temperatures. The reconstructed real distribution function $\rho \mathrm{}\left(r\right),$ related to the twofold coordination along the chains, is quite asymmetric for $T>~800°\mathrm{C},$ with a tail extending down to the interatomic distance of Se dimers. In the metallic fluid state near the critical point, we find that the interatomic distance along the chains is strongly reduced in comparison to the semiconductor phase. This result is in agreement with the model suggested by Tamura et al. of a change from helical to planar chains at the phase transition. This planar geometry allows the lone pair orbitals to become π orbitals, which may overlap and form a strong covalent bond, resulting in a decrease of the interatomic distance. In addition, a main structural change in going from the semiconductor to the metallic fluid state is the appearance of a new bond at a significantly larger distance about 2.9 Å. It most probably implies the presence of a third neighbor for a large fraction up to 30–50 % of the Se atoms, which should be related to interchain interactions and form a network in the metallic phase.