Local bonding in heavy water from 20 to 220 deg. C

Recent results in the use of time-of-flight neutron diffraction for studying the microscopic properties of water are presented. It is believed that the mean distance between water molecules increases with a decrease in water temperature due to the increase in 'hydrogen bonds'. This implies an increase in the local fourfold coordination. The behaviour of the mean distance between neighbouring molecules is observed by neutron diffraction. In order to study how far the effect of increasing or decreasing in 'hydrogen bonds' extends over a wide range of temperature we have made constant-volume measurements for temperature extending from 20-220 deg. C. The molecule structure factor of the water SM(Q) is obtained from diffraction measurements in a wave-vector range of Q = (0.1 - 10) A-1. It is concluded that SM (Q) can be expressed as a sum of the molecular form factor representing an intramolecular convolution and a term representing the arrangement of the molecular units that is the intermolecular correlation. New data about temperature dependence of this structure factors are also presented. The neutron-weighted pair correlation function as a function of T is derived from SM(Q). (authors)