In order to understand codeposition in Mo-Ni alloy electroplating, we studied the environmental atomic scale structure around metallic cations in aqueous solutions for electroplating using anomalous X-ray scattering (AXS) and EXAFS. The size of complex ions in aqueous solutions was also estimated by applying small-angle X-ray scattering (SAXS), and the distribution of chemical species in solutions was obtained from multivariate analysis of visible absorption spectra. The results of AXS, EXAFS and SAXS indicate that a solution containing molybdate involves a isopoly-molybdate anion consisting of seven edge-shared MoO6 octahedra and a solution added nickel sulfate quite likely involves a heteropoly-molybdate anion formed by six edge-shared MoO6 octahedra surrounding a nickel. On the other hand, linkages of octahedra in the heteropoly anion are broken when citrate is added to the Mo-Ni solution. From the multivariate analysis of visible absorption spectra, we found that addod citric ions form new complexes with Mo as well as Ni in aqueous solutions. Thus, disconnection of relatively large complex appears to be essential for the codeposition process as Mo-Ni alloy, and citric ions also play an important role in this electroplating.
