Neutron diffraction studies of an argon/amorphous ice co-deposit using isotopic substitution

Neutron diffraction measurements have been made for vapour co-deposited samples of amorphous ice with 8% argon. Two equivalent samples were prepared using Ar-nat and Ar-36 gas. The b-value for Ar-36 is very large and the isotopic substitution technique allows a difference function method to be used to extract a composite function, which determines the local environment around the argon atoms. In real space, the function is found to have an oscillatory structure with a strong peak at 3.6 Angstrom, which represents the distance from the argon to the surrounding water molecules. The results indicate that the argon sits at the centre of a disordered hydrogen-bonded clathrate cage and that the plane of the water molecules is approximately perpendicular to the line joining the argon and oxygen atoms. This result is in good agreement with molecular dynamics calculations of argon in liquid water and also the known structures of the crystalline clathrates. Furthermore, it is apparent that the embedding of the argon in the extended H-bonded network is equivalent to the creation of 'holes' in the continuous random network of the pure low density amorphous ice and consequently the apolar atom is behaving like a perfect network modifier in terms of glass phase terminology.