Accordingly, as indicated by the found pH trends (ESI: Figure ) and the color changes, for investigations of the intermediate amorphous nanophases, cryo-TEM measurements were performed at selected reaction times for the pure Ni-system (Figure ) and the pure Co-system (Figure ). Importantly, Ni and Co exhibit clearly different reaction kinetics, where Ni fully develops within several minutes, while Co evolves over tens of minutes (≈10x factor difference). By the observation of the color change (Figure and Figure ) and the pH trends (ESI: Figure ), moments of interest in the crystallization pathway were selected and quenched using the cryo-TEM approach. Ideally, the interesting stages should comprise the formation of the smallest to the largest nucleating entities until their final transformation to a crystal. Indeed, we observed complex amorphous particulate morphologies before any crystals were formed. For both Ni-PO4 (Figure ) and Co-PO4 (Figure ), the as-observed intermediate solids are amorphous as indicated by SAED (see the insets in each micrograph). Interestingly, the precursors have a three-dimensional (3D) morphology evidenced by the electron density contrast and branched agglomeration topology. Thus, even after the immediate pH drop due to mixing phosphate and metal solutions (ESI: Figure ) associated with color change (Figure ), the initial moments of crystallization do not yield crystalline but amorphous solids. We visualized these 3D structural features in the complex amorphous framework of the nanophases by performing cryo-electron tomography (cryo-ET) . In Figure andFehler! Verweisquelle konnte nicht gefunden werden.C a gallery of XY slices of the amorphous Niand Co-phosphate nanophases at t = 5 min (Ni) and t = 10 min (Co) are presented. Using segmentation and subsequent surface rendering of the particles in the tomographic reconstruction, the three-dimensional internal structure of the compounds can be revealed. At the early stages (t = 5 s) of the reaction, the Ni-system forms regular spherical amorphous nanoparticles of around 50 nm diameter agglomerated to µm-large frameworks (Figure ). With progressive reaction times (t = 90 s, 300 s and 900 s), the extent of agglomeration increases while particles also develop dense outer shells and hollow interiors (Figure ). From cryo-ET imaging, the observed structure comprises interconnected tubes with dense shells and voluminous interior. After t = 300 s, the agglomerates coalesce to a final µm-sized crystal of Ni-struvite at t = 900 s. These single crystals show a regular faceted morphology and exhibit usual diffraction spots in the SAED pattern. The actual transformation of the amorphous Niphases to a final crystal can be also correlated with the pH trends, where the pH equilibrates within five minutes regardless of the nickel concentration (ESI: Figure ). In the Co-system (Figure ) the observed trends are similar to those for Ni. Firstly (Figure , t = 5 s -900 s), near-spherical particles agglomerate into larger units, and all the involved species are amorphous until 1500 s (see SAED in Figure ). Noticeably, the particles at 5 s are highly irregular and smaller than those in the Ni-system (average size Ni: 50 nm compared to average size Co: 35 nm). Again, these agglomerating frameworks progressively condense with increasing reaction times and form larger assemblies up to 1500 s. After t = 2100 s, final µmsized single crystals appeared, showing a highly regular faceted structure with clear diffraction spots in the SAED pattern. Videos of the tomographic reconstruction of the Ni-and Co-PO4 precursors based on the cryo-TEM micrographs in different orientations are attached in the electronic supplementary information. Based on the pH curves and the cryo-TEM monitoring, the Co-system demonstrates a significantly longer time (~10x factor) to reach reaction equilibrium compared to the pure Ni-system, which is in addition dependent on the initial Co concentration in solution. Looking at the pH measurements of the Co-system, the time to reach an equilibrium pH and its value itself change starkly after increasing the metal concentration over > 0.04 mol/l (MP > 0.4) [ t = 500 s for (c(Co 2+ ) = 0.02 mol/l, MP = 0.2, pHeq = 5.6 to t = 3500 s for c(Co 2+ ) = 0.04 mol/l, MP = 0.4, pHeq = 4.5]. Here, the kinetics and the decrease in the pH behavior can be explained by a change in the final crystalline product. Specifically, with increasing Co concentration, the crystallization pathway leads to either Costruvite (c(Co 2+ ) < 0.04 mol/l, MP = 0.4, eq. 1) or to Co-phosphate octahydrate (c(Co 2+ )> 0.04 mol/l, MP = 0.4, eq. 2) . Consequently, the increasing Co 2+ concentration induces a shift towards lower values in the pH curves at around MP ≈ 0.4 4, 28, 54 (ESI: Figure ). Since the Ni-system always leads to Ni-struvite regardless of the metal concentration, the pH trends for this system exhibit practically constant final values (ESI: Figure ). Based on the multistep patterns observed in the pH curves and the transformation of the three-dimensional framework of the intermediate phases, we assume a complex interplay among amorphous and crystalline solids, and possibly smaller species in solution in the form of (aqueous) clusters.