Interfacial Fe(III)-hydroxide formation during Fe–Pt alloy deposition

Fe–Pt films with an Fe/Pt ratio close to one can be electrodeposited from an FeSO4–H2PtCl6–Na2SO4electrolyte. At the deposition potential, the hydrogen evolution and the reduction of the Pt complex are diffusion limited, and Fe overpotential deposition has not yet set in. The sources of the Fe incorporation are iron hydroxide formation together with Fe underpotential deposition due to Fe–Pt alloy formation. Mössbauer measurements show that the iron in the iron hydroxide is predominantly Fe(III). For stoichiometry reasons, a Pt-rich Fe–Pt phase must be present in addition to the Fe(III)-hydroxide. The Fe3+ that takes part in the hydroxide formation is produced in the electrolyte by the oxidation of Fe2+ by the complexed Pt ion. This exchange reaction results in a significantly higher Fe3+ content in the FeSO4–H2PtCl6–Na2SO4 electrolyte in comparison to the same electrolyte without H2PtCl6. Fe(III)-hydroxide formation can be depressed by adding citric acid, that acts as buffering and complexing agent. This leads to a lower iron content of the deposits. The Fe/Pt ratio close to one that is needed for hard magnetic properties can, however, only be achieved with a significant incorporation of iron hydroxide.