Neutron diffraction of heavy water to 15·6 kbar

Abstract

The neutron diffraction pattern of liquid D₂0 has been investigated at pressures up to 9·1 kbar at 25°C and 15·6 kbar at 85°C using an aluminium alloy pressure vessel of novel design. The corrected neutron counts were scaled by a constant factor so that our low-pressure measurements agreed with the best absolute measurements of the structure factor at low pressure. The structure factors were Fourier transformed to give the neutron-weighted pair distribution functions G(r). The principal conclusions drawn from the results are as follows. (1) The amplitudes of the peaks in the radial distribution function greatly increase under pressure, showing that there is an increase in the correlations of molecular positions and orientations. A graph of 4πr ² G(r) against pressure shows no evidence that the first-neighbour coordination number increases by much more than the density. The number of near neighbours at 15·6 kbar and 85°C appears to be about 6·4. The volume of melting of ice VII near the triple point of 81·6^o and 22 kbar suggests that the coordination number of the liquid at the triple point is about 6·9. (2) The close intermolecular distances, particularly the hydrogen-bonded D–O distance between first-neighbour molecules, change with pressure much more slowly than the cube root of the molar volume, and the further distances decrease much faster, particularly up to ∼10 kbar. This shows that the compression is due mostly to the approach of further neighbours to a central molecule. The first-neighbour distances change only slowly or not at all. (3) The amplitude of the 3·5 Å peak, which is due to both first and second neighbour molecules, increases rapidly up to ∼10 kbar, and increases much more slowly at higher pressures. This is interpreted as suggesting that at high pressures, the additional near neighbours are largely orientationally uncorrelated with the central molecule. This is as expected because at 22 kbar and 85°C water becomes the melt of ice VII, in which four of the eight neighbours of a molecule are not connected to the molecule by hydrogen bonding.