Structural studies of expanded fluid mercury up to the liquid-vapor critical region

The energy-dispersive x-ray-diffraction measurements for expanded fluid Hg were carried out in the density region from 13.55 to 6.6 gcm^-3 including the metal-nonmetal (M-NM) transition region. We developed a new type of high-pressure vessel and a new sample cell made of single-crystal sapphire for the x-ray diffraction measurements under high temperature and pressure up to 1530 ^∘{}C and 1980 bars using the energy-dispersive method. We obtained the interference functions and the pair-distribution functions, the termination error of which was corrected using the method by Kaplow et al. The obtained local structure, such as interatomic distance r₁ and coordination number N₁, was discussed in relation to the M-NM transition in liquid Hg. It was found that the volume expansion of liquid Hg in the metallic region is not accompanied by a uniform increase of r₁, but mainly caused by a decrease of N₁. When the transition region is crossed, the rate of decrease of N₁ becomes small and r₁ starts to elongate. We discussed some previous band calculations for expanded fluid Hg using the obtained N₁ values and concluded that it is essential for explaining the M-NM transition in liquid Hg to take the fluctuation of N₁ into account. Such a microscopic inhomogeneity may be much more enhanced in the lower-density region of the present experiments. The minimum of $S(k)$ found in the small-k region below 1 ^-1 at ρ<~8.5gcm^-3 indicates a large enhancement of the density fluctuation and the $g(r)$ data strongly suggest that small clusters exist near the critical range.