Analytische Chemie
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- Phosphates (2) (entfernen)
Organisationseinheit der BAM
Here, we elucidate nonclassical multistep crystallization pathways of transition metal phosphates from aqueous solutions. We followed precipitation processes of M-struvites, NH4MPO4·6H2O, and M-phosphate octahydrates, M3(PO4)2·8H2O, where M = Ni, Co, or NixCo1–x, by using in situ scattering and spectroscopy-based techniques, supported by elemental mass spectrometry analyses and advanced electron microscopy. Ni and Co phosphates crystallize via intermediate colloidal amorphous nanophases, which change their complex structures while agglomerating, condensing, and densifying throughout the extended reaction times. We reconstructed the three-dimensional morphology of these precursors by employing cryo-electron tomography (cryo-ET). We found that the complex interplay between metastable amorphous colloids and protocrystalline units determines the reaction pathways. Ultimately, the same crystalline structure, such as struvite, is formed. However, the multistep process stages vary in complexity and can last from a few minutes to several hours depending on the selected transition metal(s), their concentration, and the Ni/Co ratio.
The nature of the bound water in solids with hydrogen-bonded networks depends not only on temperature and pressure but also on the nature of the constituents. The collapse and reorientation of these network structures determines the stability of hydrated solids and transitions to other crystalline or amorphous phases. Here, we study the mechanochemically induced loss of bound water in Co₃(PO₄)₂·8H₂O and compare this process to the behavior under hydrostatic pressure.
The associated phase transition and its kinetics were monitored by X-ray powder diffraction with Synchrotron radiation and quantitative IR spectroscopy. High shearing forces are responsible for the degradation of the hydrogen-bonded network and the concomitant crystalline–amorphous transformation. UV/Vis spectroscopy, extended X-ray absorption spectroscopy (EXAFS), and X-ray absorption near-edge spectroscopy (XANES) provided information about the short-range order in the amorphous solid, and thermal analysis revealed its composition and showed that the moderate charge densities of the Co²⁺ and PO₄³⁻ ions, which make the hydration enthalpy comparable to the binding energy of the counteranions, and the Formation of hydrogen-bonded networks favor multistage crystallization processes associated with the release and uptake of coordinated water. The changes of the Co²⁺ coordination induce a color change from pink to blue; therefore, Co₃(PO₄)₂·8H₂O can be used as an overheat temperature indicator.