Targeting Vacancies in Nitridosilicates: Aliovalent Substitution of M²⁺ (M=Ca, Sr) by Sc³⁺ and U³

Based on the known linking options of their fundamental building unit, that is the SiN₄ tetrahedron, nitridosilicates belong to the inorganic compound classes with the greatest structural variability. Although facilitating the discovery of novel Si-N networks, this variability represents a challenge when targeting non-stoichometric compounds. Meeting this challenge, a strategy for targeted creation of vacancies in highly condensed nitridosilicates by exchanging divalent M²⁺ for trivalent M³⁺ using the ion exchange approach is reported. As proof of concept, the first Sc and U nitridosilicates were prepared from α-Ca₂ Si₅ N₈ and Sr₂ Si₅ N₈ . Powder X-ray diffraction (XRD) and synchrotron single-crystal XRD showed random vacancy distribution in Sc_0.2 Ca_1.7 Si₅ N₈ , and partial vacancy ordering in U_0.5x Sr_2-0.75x Si₅ N₈ with x≈1.05. The high chemical stability of U nitridosilicates makes them interesting candidates for immobilization of actinides.