Neutron diffraction study of aqueous transition metal salt solutions by isomorphic substitution

Abstract

Experiments were initially carried out on 4.3 mol dm^–3 MCl₂(M = Ni or Mn) and 3.8 mol dm^–3 M(ClO₄)₂(M = Ni or Cu) heavy-water solutions to test the method of isomorphic substitution and see whether Ni, Mn, Cu are suitable isomorphs. The results obtained for chloride solutions are compared with previous results obtained by the technique of isotopic substitution. Good agreement is found between the two techniques; the cation–oxygen and cation–deuterium distances are 2.09±0.02 and 2.69±0.02 Å, respectively. The influence of the different-sized Mn²⁺ and Ni²⁺ ions on the results is discussed.

The isomorphs used in the perchlorate solutions were copper and an isotopic mixture of nickel (^ϕNi), which does not contribute to the coherent neutron scattering pattern. Again the results show good agreement in the region of the first hydration shell with those obtained by isotopic substitution on Ni(ClO₄)₂ solutions. The use of ^ϕNi as an isomorph with zero scattering length shows that the isomorphic technique gives precise information concerning the Cu²⁺–water conformation. The copper–oxygen and copper–deuterium distances are 1.97 and 2.60 Å, respectively, and the angle of tilt between the copper–oxygen axis and the plane of the water was 39±10 °. There is no evidence for two water sites around copper.

The isomorphic substitution technique was then applied to a 2.6 mol dm^–3 MSO₄ solution (M = Ni or Mn). The results were similar to those obtained for the chloride case. The angle of tilt between the cation–oxygen axis and the plane of the water molecules is calculated to be 20±15 ° for Mn²⁺.