Structural changes in supercritical fluids at high pressures

The structure of an archetypal model simple fluid system as argon has been investigated by x-ray diffraction at high pressures and room and high temperatures. Despite the markedly supercritical conditions (T=2--4Tc, P>10²Pc), the structure factor $S(Q)$ is very similar, close to the melting line, to that observed in the liquid phase, thereby assessing a liquidlike structure with high atomic correlation, as proposed in a recent inelastic experiment. On the other hand, the $S(Q)$ continuously changes upon approaching the extrapolation of the liquid-gas coexistence line in the (P/Pc, T/Tc) plane, ultimately exhibiting low atomic correlation, which reasonably indicates intermediate character between liquid and gas. The analysis of the $S(Q)$s based on the hard-sphere model shows that the changes are driven by the decrease in the packing fraction and the increase in the nearest-neighbor distance with decreasing pressure.