Nucleation and Crystallization of Ferrous Phosphate Hydrate via an Amorphous Intermediate

The fundamental processes of nucleation and crystallizationarewidely observed in systems relevant to material synthesis and biomineralization;yet most often, their mechanism remains unclear. In this study, weunravel the discrete stages of nucleation and crystallization of Fe-3(PO4)(2)& BULL;8H(2)O (vivianite).We experimentally monitored the formation and transformation fromions to solid products by employing correlated, time-resolved in situ and ex situ approaches. We showthat vivianite crystallization occurs in distinct stages via a transientamorphous precursor phase. The metastable amorphous ferrous phosphate(AFEP) intermediate could be isolated and stabilized. We resolvedthe differences in bonding environments, structure, and symmetricchanges of the Fe site during the transformation of AFEP to crystallinevivianite through synchrotron X-ray absorption spectroscopy at theFe K-edge. This intermediate AFEP phase has a lower water contentand less distorted local symmetry, compared to the crystalline endproduct vivianite. Our combined results indicate that a nonclassical,hydration-induced nucleation and transformation driven by the incorporationand rearrangement of water molecules and ions (Fe2+ andPO(4) (3-)) within the AFEP is the dominatingmechanism of vivianite formation at moderately high to low vivianitesupersaturations (saturation index & LE; 10.19). We offer fundamentalinsights into the aqueous, amorphous-to-crystalline transformationsin the Fe2+-PO4 system and highlightthe different attributes of the AFEP, compared to its crystallinecounterpart.

Automated summary

In this study, the nucleation and crystallization processes of vivianite were investigated. The formation of an intermediate amorphous phase and its transformation to crystalline vivianite were monitored. The results reveal that water molecules and ions play a crucial role in the nonclassical hydration-induced nucleation and transformation mechanism of vivianite.