EXAFS Study of Ni + Coordination in Nitrate Solutions

The coordination of the nickel ion in aqueous solutions of several salts, particularly of NiCl 2 , has been extensively studied using different techniques [1-13]. Very little structural information exists, however, on solutions of nickel nitrate. A 1 M solution was investigated by X-ray diffract ion [1], while the cation coordination was studied by the EXAFS technique in two extreme cases: a very dilute solution, 0.1 M [2], and a very concentrated one, about 4 M [3]. It is well known that the two mentioned techniques are able to give information on the structuring of ions in solutions. In particular, an X-ray diffraction experiment contains all the interactions existing in the system under examination. Isolation of single contributions can also be achieved, generally through the use of structural models which are fitted to the experimental data [14-15] ; the values of the structural parameters so obtained are more reliable when the peaks corresponding to different contributions are well resolved in the radial functions. In solutions of N i ( N 0 3 ) 2 , the distance values of 0 — 0 interactions within the nitrate group are very close to the expected value for Ni -nearest neighbour water molecules, according to several studies of solutions of nickel salts [1-13] . In this case, direct information on the Ni 2 + coordination may be obtained using the more selective EXAFS technique which provides distances and coordination numbers of the neighbours of the absorbing