Abstract
The hydrothermal synthesis of the purely inorganic open‐framework indium selenide (NH~4~)~4~In~12~Se~20~ (1) is reported. Compound 1 exhibits a unique three‐dimensional open‐framework structure. The framework of 1 shows an unusual, for a chalcogenide compound, rigidity arising from the unprecedented connection mode of its building blocks. Compound 1 possesses ion exchange capacity for Cs^+^, Rb^+^, NH~4~^+^, but it has selectivity against Na^+^ and Li^+^. It also showed exceptional stability in relatively concentrated hydrochloric acid. Ion exchange of 1 with hydrochloric water solutions can produce its solid acid analogue H~2~(NH~4~)~2~In~12~Se~20~. The maximum cation‐exchange capacity of 1 was found equal to two equivalents per mol, which is consistent with an exchange mechanism taking place in the 1D‐channels formed by the largest cavities. In addition, 1 can do ion‐exchange with heavy‐metal ions like Hg^2+^, Pb^2+^, and Ag^+^. The capacity of 1 to clean water solutions from heavy‐metal ions was preliminarily investigated and found very high. Specifically, 1 can remove 99.9 % of Hg^2+^, 99.8 % of Ag^+^, and 94.9 % of Pb^2+^ from aqueous solutions of each of these ions. Using different synthetic conditions, we isolated compound (NH~4~)~2~In~12~Se~19~ (2), which also has as good an acid stability as 1, but no ion‐exchange properties. Overall, this work provides new hydrothermal synthetic approaches for isolation of all‐inorganic open‐framework chalcogenides.