To further explore the tunability of B via targeted changes of Ap, we have synthesised the solid solutions [PEP]1-x[DPP]xNi(C2N3)3 with x = 0, 0.45, 0.69, 0.85, 0.96 and 1. For their synthesis, the stoichiometry of precursor salts [PEP]Br and [DPP]Br has been varied and mixed with Na(C2N3) and Ni(NO3)26H2O in water as a solvent, see SI (S-6). Independent of the applied precursor stoichiometry, we only obtained members of the solid solution with x ≥ 0.45. In previous studies, a full solid solution, i.e. 0 ≤ x ≤ 1, has only been found in one example where chemically similar A + were used. Therefore, we rationalise the here observed x range by the difference in crystallisation kinetics due to the chemical difference of A + . H NMR spectroscopy on aciddigested samples has been used to determine the chemical composition x (Table and Fig. ). We confirmed the formation of a solid solution using laboratory PXRD measurements of a mixture of the parent materials [PEP]Ni(C2N3)3 and [DPP]Ni(C2N3)3 (Fig. ). We determined the lattice parameters using Pawley refinements, using P21/c for x = 0 -0.96 (Fig. and S51-S54) and C2/c for x = 1 (Fig. ). We observe a monotonic expansion of the volume from 2029.7(4) Å 3 (x = 0) to 2076.9(4) Å 3 (x = 1) with increasing x at ambient pressure (Table ).