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

Abstract

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‐Ca2Si5N8 and Sr2Si5N8. Powder X‐ray diffraction (XRD) and synchrotron single‐crystal XRD showed random vacancy distribution in Sc0.2Ca1.7Si5N8, and partial vacancy ordering in U0.5xSr2−0.75xSi5N8 with x≈1.05. The high chemical stability of U nitridosilicates makes them interesting candidates for immobilization of actinides.

Post‐Synthetic Modification: Systematic Study on a Simple Access to Nitridophosphates

“…Considering possible driving forces of the presented approach, the targeted formation of the more stable halides and Coulomb's law certainly have a major influence. Similar explanations are given for metathesis reactions or a corresponding ion exchange in nitridosilicates [21–24, 37] . Furthermore, a possible reduction in volume can be considered as driving force, as MP and HP/HT conditions were applied.…”

“…Such topotactic ion-exchange reactions on nitridophosphates have so far only been reported for the synthesis of sodalite-like Zn 7 [6,20] Moreover, post-synthetic modification by ion exchange has generally been investigated very sparsely on nitride substance classes, although the huge potential of this approach has been demonstrated for related nitridosilicates. [21][22][23][24] In this contribution we present the first systematic study on the post-synthetic modification of nitridophosphates by ion exchange reactions. But, as preliminary tests have shown, that an analogous approach as used for (nitrido-)silicates cannot be easily transferred to nitridophosphates ( Figure S1-S3), we combine ion exchange reactions/salt metathesis with HP/HT and MP methods within this work.…”

“…Similar explanations are given for metathesis reactions or a corresponding ion exchange in nitridosilicates. [21][22][23][24]37] Furthermore, a possible reduction in volume can be considered as driving force, as MP and HP/HT conditions were applied. However, more precise conclusions about this assumption cannot be drawn, since structural information on all starting materials and products under the experimental reaction conditions would be necessary.…”

Post‐Synthetic Modification: Systematic Study on a Simple Access to Nitridophosphates

“…Such topotactic ion‐exchange reactions on nitridophosphates have so far only been reported for the synthesis of sodalite‐like Zn 7 [P 12 N 24 ]Cl 2 by deprotonating Zn 5 H 4 [P 12 N 24 ]Cl 2 with ZnCl 2 [6, 20] . Moreover, post‐synthetic modification by ion exchange has generally been investigated very sparsely on nitride substance classes, although the huge potential of this approach has been demonstrated for related nitridosilicates [21–24] …”

“…First experiments were carried out on BaP 2 N 4 , as it can be classified as a highly condensed framework like the AE 2 Si 5 N 8 structure, for which the ion exchange reaction has been established. [21][22][23][24] Using the multianvil technique (5 GPa, 1000 8C) and aiming for BaP 2 N 4 -type b-SrP 2 N 4 , a partial replacement of 2/3 of the Ba atoms could be achieved by reaction with SrCl 2 , yielding homeotypic, literature-known BaSr 2 P 6 N 12 [Eq. 1, Figures 1 and S5, Table S4].…”