Partial pair correlation functions of low-density supercritical water determined by neutron diffraction with the H/D isotopic substitution method

Neutron diffraction measurements were carried out on H/ D isotopically substituted water in the low-density supercritical condition (T = 673 K, P = 26.3 MPa, and ρ = 0.0068 molecules-Å^-3) in order to obtain direct information on the intermolecular partial structure functions, gHH ^inter(r), gOH^inter(r), and gOO^inter(r). In correspondence to the high-density supercritical water previously reported, the intermolecular nearest neighbor peaks in gHH^inter(r), gOH ^inter(r), and gOO^inter(r) are diffuse compared with the ambient ones. The nearest neighbor O⋯O distance (3.3 Å) and its coordination number (2.6) were determined from the observed gOO ^inter(r). These results indicate that the orientational correlation between neighboring water molecules is considerably lost in low-density supercritical water. Small clusters involving a few water molecules are preferentially formed in low-density supercritical water, which is consistent with results obtained by previous IR and NMR studies.