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 ${\mathrm{g}\mathrm{}\mathrm{cm}}^{-3}$ including the metal-nonmetal $(M-\mathrm{NM})$ 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 $r_1$ and coordination number $N_1,$ 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 $r_1,$ but mainly caused by a decrease of $N_1.$ When the transition region is crossed, the rate of decrease of $N_1$ becomes small and $r_1$ starts to elongate. We discussed some previous band calculations for expanded fluid Hg using the obtained $N_1$ values and concluded that it is essential for explaining the M-NM transition in liquid Hg to take the fluctuation of $N_1$ into account. Such a microscopic inhomogeneity may be much more enhanced in the lower-density region of the present experiments. The minimum of $S\left(k\right)\mathrm{}$ found in the small-k region below 1 ${\AA }^{-1}$ at $\rho <~8.5{\mathrm{g}\mathrm{}\mathrm{cm}}^{\mathrm{-}3}$ indicates a large enhancement of the density fluctuation and the $g\left(r\right)\mathrm{}$ data strongly suggest that small clusters exist near the critical range.