Local structure of expanded fluid mercury using synchrotron radiation: From liquid to dense vapor

X-ray-diffraction measurements for fluid mercury up to the supercritical region have been carried out using synchrotron radiation at SPring-8. We obtained the structure factor $S(Q)$ with good signal-to-noise ratio, and the pair distribution functions $g(r)$ for expanded fluid Hg from the liquid to the dense vapor. To see the volume dependence of the local structure in fluid Hg in detail, we tried to reproduce 4πn₀r²g(r) of fluid Hg using Gaussian peaks. The optimized parameters show that the asymmetrical first peak in $g(r)$ of liquid Hg consists of atomic distributions at short and long distances of about 3 and 3.5. Volume dependence of the parameters suggests that decrease of the atoms coordinated at the short distance is strongly correlated with the metal-nonmetal transition in fluid Hg.