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 4pin(0)r(2)g(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 Angstrom and 3.5 Angstrom. 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.