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 FeSO 4–H 2PtCl 6–Na 2SO 4electrolyte. 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 Fe 3+ that takes part in the hydroxide formation is produced in the electrolyte by the oxidation of Fe 2+ by the complexed Pt ion. This exchange reaction results in a significantly higher Fe 3+ content in the FeSO 4–H 2PtCl 6–Na 2SO 4 electrolyte in comparison to the same electrolyte without H 2PtCl 6. 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.