STRUCTURES OF WATER AND AQUEOUS LiCl SOLUTIONS IN SUB- AND SUPER-CRITICAL STATE BY X-RAY AND NEUTRON DIFFRACTION

The structure of water in the sub- and super-critical state has been investigated with a newly developed rapid X-ray diffractometer using an imaging plate area detector. The temperature and pressure covered in the measurements were 300∼649 K and 0.1∼98.1 MPa, respectively, with nearly constant densities 0.7∼1.0 g cm^-3 (T_c=647.3 K, P_c=22.12 MPa, ρ_c=0.322 g cm^-3.) The chloride ion hydration in highly concentrated LiCl solutions in D₂O has also been investigated in the temperature range 94∼648 K by pulsed neutron diffraction measurements with ³⁵Cl/³⁷Cl isotopic substitution. The X-ray radial distribution functions have revealed that the tetrahedral icelike structure has vanished in water above 416 K and 0.95 g cm^-3 and that the short-range O-O interactions at ∼2.9 Å still exist at ∼40% for the coordination number of 3.9, though, with a large fluctuation and a distorted hydrogen bonding at 649 K and 0.7 g cm^-3. The neutron data have revealed that the orientational correlation of water molecules around Cl^- is greatly affected by temperature and almost lost above the critical point of water.