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 $\left(T=2–4T_c,P>{10}^2{P}_c\right)$, the structure factor $S\left(Q\right)$ 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 $S\left(Q\right)$ continuously changes upon approaching the extrapolation of the liquid-gas coexistence line in the $\left(P/P_c,T/T_c\right)$ plane, ultimately exhibiting low atomic correlation, which reasonably indicates intermediate character between liquid and gas. The analysis of the $S\left(Q\right)$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.

• Received 21 May 2008

DOI:https://doi.org/10.1103/PhysRevB.77.212103