Understanding the Stability and Recrystallization Behavior of Amorphous Zinc Phosphate
2021
Maslyk, Marcel | Bach, Sven | Li, Wenyu | Shylin, Sergii I. | Panthöfer, Martin | Barton, Bastian | Ksenofontov, Vadim | Xu, Ke | Meermann, Björn | Kolb, Ute | Schmedt auf der Günne, Jörn | Tremel, Wolfgang
Zinc phosphate, an important pigment in phosphate conversion coatings, forms protective films on rubbing surfaces. We have simulated the underlying reactions under shear by ball-milling zinc phosphate and monitored the reaction of hopeite (Zn₃(PO₄)₂·4H₂O) and the retarded recrystallization of the amorphous reaction product by powder X-ray diffraction (PXRD) and quantitative infrared (IR) spectroscopy. Abrasion of stainless steel was simulated by addition of pure ⁵⁷Fe. The results provide insight into the chemistry of phosphate conversion coatings or during battery cycling of metal phosphates and give theoretical guidance for the preparation of amorphous phosphates. Thermal analysis revealed that the release of structural water is a key step during the reaction of hopeite under shear to ball-milled amorphous zinc phosphate. The back-reaction and associated recrystallization kinetics of amorphous zinc phosphate show a classical Langmuir behavior. Fe impurities inhibit the recrystallization of ball-milled amorphous zinc phosphate strongly. ⁵⁷Fe Mössbauer spectroscopy and PXRD revealed that Fe is oxidized to Fe²⁺ and Fe³⁺ during ball-milling and incorporated locally at the tetrahedral and octahedral sites of the structure. Ball-milled amorphous zinc phosphate is metastable as γ-Zn₃–ₓFeₓ(PO₄)₂. EPR studies showed the incorporation of Fe³⁺ to be coupled with the formation of Zn²⁺ vacancies. The Fe³⁺ defect sites bind water because of their higher Pearson hardness (compared to Fe²⁺ and Zn²⁺), thereby reducing water mobility and inhibiting further reactions like the recrystallization to hopeite. Our findings reveal the amorphization mechanism of Zn₃(PO₄)₂·4H₂O in stainless steel ball mills at the atomic scale and highlight how the reactivity of amorphous products is affected by impurities associated with the processing method.
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