Medium-Range Fluctuations Accompanying the Metal-Nonmetal Transition in Expanded Fluid Hg

We have carried out small angle x-ray scattering experiments of expanded fluid Hg in the metal-nonmetal (M-NM) transition region around $9.0\mathrm{g}{\mathrm{cm}}^{-3}$. Increase of small angle scattering intensity following the Ornstein-Zernike equation is clearly observed in the M-NM transition region as well as near the liquid-vapor critical point at $5.8\mathrm{g}{\mathrm{cm}}^{-3}$. The short-range correlation length, $R$, becomes twice as large in the M-NM transition region as in the critical region. The enhancement of $R$ in expanded fluid Hg suggests a new type of fluctuations reflecting a first-order M-NM transition.

Figure 1

Selected structure factor $S(Q)$ in the M-NM transition region of fluid Hg at densities from 11.4 to $5.7\mathrm{g}{\mathrm{cm}}^{-3}$ at 1930 bar. The successive spectra are displaced by 0.5 for clarity. The inset shows Ornstein-Zernike plots of $S(Q)$ at 6.3 and $9.1\mathrm{g}{\mathrm{cm}}^{-3}$.

Figure 2

The optimized fluctuations in a particle number $S(0)$ (upper panel) and the correlation length $\xi$ (lower panel) at 1750 bar (○), 1800 bar (□), and 1930 bar (▵).

Figure 3

The short-range correlation length $R$ as a function of reduced density, $\rho /{\rho }_c$. ○ and ▵ denote data from SAXS at 1750 and 1930 bar, respectively. Also shown are $R$ of supercritical water at constant temperature at 663 K (+) [17] and at constant pressure at 400 bar (■) [18].

Figure 4

The effective pair potential $\phi (r)$ at densities indicated in the figure deduced from $S(Q)$ obtained experimentally. Each $\phi (r)$ is shifted by 0.25 eV for clarity.

Figure 5

The normalized second and fourth frequency moments, ${\omega }_0(Q)$ and ${\omega }_L(Q)$, calculated from $S(Q)$ and the $\phi (r)$ at $9\mathrm{g}{\mathrm{cm}}^{-3}$, respectively. Also shown are the excitation energy obtained from IXS and the adiabatic sound velocity by squares and a broken line, respectively.