For any analyses requiring dry crystalline powders, the solid phases precipitated in solutions were collected onto a cellulose filter (pore size 1 µm, LABSOLUTE) by using a vacuum filtration kit (i.e. Büchner funnel). Afterwards, the powders were washed with distilled water and left to dry at room temperature in air for 2 hours. To elucidate the crystal formation pathways, we used various in-situ and ex-situ characterization methods (Table ), with which we analyzed solids in solution, dry solids, and supernatants, etc. at different stages. Detailed technical descriptions of the used characterization methods, including X-ray diffraction (XRD) for identification of the phase composition, pH measurements to monitor the precipitation reaction, infrared spectroscopy (IR) for tracking the phosphate and water/ammonium environment in the solids, scanning electron microscopy (SEM), conventional-and cryogenic transmission electron microscopy (-CTEM, cryo-TEM) for imaging, diffraction and elemental mappings/line scans, in-situ smalland wide-angle X-ray scattering (SAXS/WAXS) for tracking amorphous phaser associated with the phase composition, and inductively coupled plasma mass spectrometry (ICP-MS) analysis for determining the changing chemical composition of liquid and solid as a function of time can be found in the ESI (ESI: Methods). The rationale behind the use of each method is described in Table .