Neutron Diffraction Study on the Structure of Hydrated Li⁺ in Dilute Aqueous Solutions

Neutron diffraction measurements have been carried out for ⁶Li/⁷Li isotopically substituted aqueous 1.0 mol % (0.5 mol/kg) LiCl and 1.1 mol % (0.56 mol/kg) LiClO₄ solutions in D₂O to obtain structural insight concerning hydration structure of Li⁺ in more dilute electrolyte solutions. The first-order difference function, Δ_Li(Q), was analyzed by means of the least squares fitting procedure to obtain short-range structural parameters around the Li⁺. It was revealed that the nearest neighbor Li⁺···O(D₂O) distance, r_LiO, and the coordination number, n_LiO, for the aqueous 1.0 mol % LiCl solution are 2.01 ± 0.02 Å and 5.9 ± 0.1, respectively. The values, r_LiO = 1.97 ± 0.02 Å and n_LiO = 6.1 ± 0.1, are obtained for aqueous 1.1 mol % LiClO₄ solution. These results indicate that the hydration number of Li⁺ in a dilute solution is close to 6, which is much larger than 4, which has long been believed. A possible explanation is that the hydration number of Li⁺ varies with the solute concentration.