X-ray diffraction studies of expanded fluid mercury using synchrotron radiation

Energy-dispersive x-ray diffraction measurements using synchrotron radiation at SPring-8 for expanded fluid tig were carried out over the wide density range from the liquid to the dense-vapour region including the metal-non-metal (M-NM) transition region. The density ranges from 13.6 to 1.9 g cm(-3). We developed a high-pressure vessel and a sapphire cell for the x-ray diffraction measurements under high temperature and pressure up to 1520 degrees C and 1765 bar. We obtained the structure factor S(k) and the pair distribution functions g(r). The density variations of the interatomic distance r(1) and coordination number N-1 obtained were discussed in relation to the M-NM transition in fluid 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 is mainly caused by a decrease of NI. 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 and concluded that it is essential for explaining the M-NM transition in liquid Hg to take the fluctuation of N-1 into account. When the density decreases down to the dense-vapour region, the first maxima of g(r) are located at around 3.3-3.4 Angstrom, which is close to the interatomic distance of tig dimers.