Non-classical crystallization pathway of transition metal phosphate compounds

Here, we elucidate non-classical 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, 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 subsequently 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 proto-crystalline 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.