Based on our SAXS/WAXS results we observed a significant change in the pore size during heating (Figure ), which is counterintuitive considering just simple evaporation of ammonia and water. This finding suggests a potential condensation or change of the amorphous structure with extended heating times resulting in a decrease of the pore sizes. Therefore, to characterize the amorphous structure, and explain its correlation with the occurrence of the mesoporous framework, X-ray absorption spectroscopy (XAS) measurements were performed. The obtained spectra of the timeseries of heated Ni-struvite are displayed in Figure . They correlate the evolution of mesoporosity with the change in coordination environment. In addition, the selected NixCo1-x-struvites and their heated products at T = 90°C were measured for the Ni-and Co-edge to compare their distortion of the M 2+ -octahedron in a solid solution. Especially the intensity of the pre-peak (in the XANES region) originating from the 1s-3d transitions expresses the degree of centrosymmetry or distortion in the vicinity of the considered ions (inset of Figure , ESI: Supplementary Note 2). As only 3d-4p hybridization enable parity allowed transitions the extent of those is strongly correlated with the coordination geometry. Near-ideal centrosymmetric coordinations show very low prepeak intensities while more distorted geometries exhibit higher intensities. As X-ray spectroscopy considers primarily the core shell of the absorber atom, being element specific, an analysis of crystalline and amorphous samples is possible. In general, highly crystalline phases demonstrate narrower and more intense peaks in the R-space compared to amorphous phases because more ions have similar bond distances to the metal cation due to a regular periodic structure (towards a delta-function-like distribution). Conversely, as amorphous phases do not display an established long-range order, they show broader, less intense, and less-pronounced peaks in the R-space mostly in the low R-region due to a irregular arrangement of the lattice ions. The XANES region (Figure ) clearly prove the exclusive presence of bivalent Ni 2+ and/or Co 2+ species in the materials. No indications for oxidation to Ni 3+ or Co 3+ were observed either in the pure endmember nor in the bicationic solid solutions. Quantitative pre-peak integration results App (area of pre-peak) were calculated for comparison of the distortion in the first coordination sphere (ESI: Figure , Table -S9). Ni exhibits a slightly higher degree of distortion in the amorphous phase than in the precursor Ni-struvite, which is visible through higher pre-peak intensities (Ni-struvite: Area of pre-peak App (t = 0 min) = 0.0250(1), App (t=150 min) = 0.0298(1), ESI: Figure , Table ).