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 Sc0.2 Ca1.7 Si5 N8 , and partial vacancy ordering in U0.5x Sr2-0.75x Si5 N8 with x≈1.05. The high chemical stability of U nitridosilicates makes them interesting candidates for immobilization of actinides.