*2.1. Synthesis*

Ca–Sr oxalates were crystallized by precipitation from aqueous solutions (0.5 L volume) containing calcium chloride (CaCl2, 99% purity, Vekton), strontium nitrate (Sr(NO3)2, 99% purity, Vekton), sodium oxalate (1.5 mmol, Na2C2O4, 98% purity, Vekton) and citric acid (6.5 mmol, C6H8O7·H2O, 99% purity, Vekton). The atomic ratios of Sr/(Sr + Ca) cations ranged from 0 to 100%. The total amount of Ca2<sup>+</sup> and Sr2<sup>+</sup> ions was 5 times higher than the content of oxalate ions in each synthesis.

A non-stoichiometric composition of the solution and the presence of citric acid were used to stabilize weddellite formation [25]. The pH range of the solutions was 4.6–6.1 due to the small additions of solutions of NaOH (NaOH, 99%, Vekton) or HCl (HCl, 35–38 wt.% aqueous solution, 99.9% purity, Vekton). The syntheses were carried out at room temperature (22–25 ◦C), with the exposure of the solution for five days until complete precipitation. The resulting precipitate was filtered off, washed with distilled water and dried in air at room temperature. The precipitate consisted of fine-grained white crystalline powder, each 0.5 L solution volume provided around 200 mg of it.
