Recent Structural Studies of Liquid D₂O by Neutron Diffraction

Abstract

We present our recent structural studies of liquid D₂O performed by neutron diffraction. The results cover a large temperature range extending from 95°C down to −31.5°C i.e. close to the temperature of homogeneous nucleation and particularly in the low temperature range where liquid water exhibits an abnormal behaviour. The structural changes versus temperature are discussed using a difference function technique. We show that the isochoric temperature derivative function Δd_L (r,ΔT)/ΔT where d_L (r,T) is the pair correlation function at T, has a universal shape for distances larger than 2 Å, indicating a systematic change in spatial correlations over the density maximum. The structural investigation of deeply supercooled water has been achieved by using emulsions with water droplets of some µm diameter. At −31.5°C the pair distribution function of water shows an increase of the spatial correlations which confirms the fact that the structure of deeply supercooled water tends towards that of low density amorphous ice.

Finally, the method of H/D substitution is described and it is demonstrated how this method can improve the determination of the pair partial functions, when applied to D₂O/H₂O mixtures containing relatively small amounts of H₂O (α_H = 0.0, 0.03, 0.05, 0.10, 0.20). We make a comparison of the results obtained at room temperature with earlier experimental results and with computer simulation studies.