Metastable phosphates of 3d metals - illuminating fundamental crystallization and nucleation processes
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Abstract
The present work studies the formation of metastable intermediates of 3d metal phosphate hydrates during precipitation and under influence of mechanical stress. It was the goal of this manuscript to obtain a better understanding of the underlying mechanisms including non-equilibrium phases, which are still subject of controversial discussions.
The first part of the thesis deals with the early stages of the precipitation of 3d metal phosphate hydrates. The formation of amorphous precursors followed by dissolution and recrystallization ending up with the respective crystalline modifications was observed for the iron, cobalt, nickel, copper, and zinc phosphate hydrate systems. The amorphous nanoparticles were determined to be 5–30 nm in size and revealed a lower content of structural water than their crystalline counterparts. The amorphous solids were stabilized kinetically in the absence of water. Simulations of CREDOR NMR spectra displayed a rather stiff hydrogen-bounded network to be responsible for this behavior. This fact was utilized to inhibit crystallization even in the presence of water by encapsulation the amorphous nanoparticles by a silica shell. Next, the synthesized compounds were characterized comprehensively in terms of composition, recrystallization kinetics (in situ IR spectroscopy), and short-range order (EXAFS and XANES).
Furthermore, the role of water as active reaction partner during nucleation and crystal growth was investigated by precipitation in anhydrous solvents. The anhydrous amorphous phosphates adsorb water provided by humidity according to a Langmuir mechanism, which was determined in situ. Further, application of a highly concentrated HPO42- solution resulted in the dissolution of crystalline phosphate hydrates that involved the formation of a macroscopic dense liquid. First results of SS-NMR measurements displayed a hydrogen-bonded network and suggested the formation of metal hydrogen phosphate complexes. In addition, prenucleation clusters of the copper phosphate system were detected prior to the formation of an amorphous solid in solution with a constant-pH titration setup and an ion-sensitive electrode. The clusters were formed upon undercritically concentrated solution and densified towards nanoscopic liquid-condensed phases (LCP) during titration. 3d metal phosphate hydrates represent well-suited model systems for the investigation of nucleation and crystal growth in a more complex way because the reversible hydration of the metal ions allows multistage crystallization processes with different hydrated intermediates, which can be isolated.
The second part of the present thesis addresses the mechanochemical amorphization of thermodynamically stable phosphate hydrates. This unusual solid-state reaction was studied quantitatively by IR spectroscopy and X-ray diffraction ex situ. Structural water was expelled during the reaction which results in the loss of crystallinity. The comparison of (i) the treatment of Co3(PO4)2 × 8 H2O in a planetary ball mill with (ii) an in situ high-pressure study revealed hydrostatic pressure not to be responsible for amorphization.
In addition, the influence of impurities on the stabilization of amorphized zinc phosphate hydrate was examined according to the different recrystallization behavior of solid bodies synthesized in a stainless steel jar and in a zirconia jar. Impurities of iron (after oxidation to Fe2+- and Fe3+-compounds) inhibited the crystallization in water to the thermodynamically stable crystalline phase, hopeite, effectively. The oxidation state and chemical surrounding of iron in the amorphous zinc phosphate hydrate and its annealing products were determined by 57Fe Mössbauer spectroscopy. Incorporation of Fe in the structure was accompanied by the generation of zinc vacancies. Water which was needed for crystallization of hopeite was bound to iron species at particles’ surface due to its higher Pearson hardness. As a result, an effective inhibition of the crystallization of hopeite was achieved. Using the amorphous product, crystalline α- and γ-Zn3(PO4)2 were selectively synthesized by controlling heating conditions.