X-ray diffraction studies on the structure of water at high temperatures and pressures

Abstract

X-Ray diffraction measurements of liquid water at high temperatures and pressures (up to 200 °C and 200 MPa) are reported. A diffractometer with a transmission geometry and a cylindrical sample holder made of beryllium were used, with a specially designed high pressure unit. The data reduction procedures are described in detail. The analysis of the structure functions and radial distribution functions show that the O-O distances shorten due to pressure but they are elongated with increasing temperature. The average contraction rate is 0·02 pm MPa^-1. The average coordination number of the first neighbour molecules does not change significantly with increasing temperature but slightly increases with increasing pressure. The density dependence of the distances and coordination are discussed. The expansion of water is proportional to the expansion of first neighbour distances in the medium density region only (0·95-1·01 g cm^-3), while the coordination number produces a minimum in the medium density region. This is an indication of coexistence of unchanged H bonds with deformed or disrupted H bonds in the pressure and temperature region studied.