In contrast, Ni-struvite decomposes much slower than Mg-struvite to an amorphous phase and preserves a significant fraction of isolated micropores. Therefore, we assume that Nistruvite forms only a limited amount of interconnected pore frameworks which would explain the significant lower BET surface area compared to a heated Mg-struvite (Mg: 240 m²/g vs. Ni: 90 m²/g) and the higher amount of micropores compared to mesopores (approximately for Ni: φmicropores ≈ 60% to φmesopores ≈ 40%). The ratio of the absolute surface areas, pore volumes of heated Mg-(240 m²/g, 0.31 cm 3 g -1 ) and Ni-struvite (90 m²/g, 0.13 cm 3 g -1 ) and the average pore sizes obtained from BET N2 gas sorption are mostly in the same order of magnitude. Small discrepancies of a few nanometers in the calculated BJH pore sizes and the SAXS pore sizes could be originated in the BJH method as it always includes desorption artefacts. Within the NixCo1-x-struvite mixtures, a higher Co 2+ content leads to a stronger polydispersity and higher average pore size (ESI: Figure , Table ). Coincidentally, the absolute BET surface area and pore volume decreases significantly at Ni# <0.5. After a thermal treatment, only a limited amount of Co 2+ can be probably incorporated into an amorphous phase, since most of Co tends to form its own crystalline phase, namely Co-dittmarite (ESI: Figure ). This results in a binary phase mixture: amorphous and crystalline. Based on the EXAFS analysis, the structure of the amorphous phases resembles the local structure of crystalline Ni-dittmarite NH4NiPO4•H2O with a less pronounced long-range order. An indicator for this assumption is the linking of the phosphate units with the metal octahedron associated with the characteristic appearance of R-distances of around 2.7 Å (Figure ). Intrestingly, a high Ni content in the precursor struvite and heated products leads to a significant distortion of Co 2+ in its octahedral coordination. Ni 2+ shows barely any correlation with increasing Co content. Co 2+ with a d 7 electron configuration demonstrates a strong Jahn-Teller effect in a high-spin octahedral coordination (H2O weak field ligand) due to more unpaired electrons compared to Ni 2+ as a d 8 -ion. Based on the pre-peak integration results (ESI: Figure , Table -S9), a mutual influence of both 3d metals in the shared solid solution on the coordination sphere is assumed, whether in the precursor M-struvite, in the amorphous phase or M-dittmarite structure. The significant correlation of the distortion degree of Co 2+ in NixCo1-x-mixtures is in agreement with electron spin resonance spectroscopy . As stated also in our previous study , we assume a strong dependency between the stabilities of the different transition metal struvites and their electronical d-configuration.