Ammonothermal Synthesis and Solid-State NMR Study of the Imidonitridosilicate Rb₃Si₆N₅(NH)₆

The imidonitridosilicate Rb₃Si₆N₅(NH)₆, being only the second representative of this compound class, was synthesized ammonothermally at 870 K and 230 MPa. Its crystal structure was solved from single-crystal X-ray diffraction data. The imidonitridosilicate crystallizes isotypically with the respective potassium compound in space group P4₁32 with the lattice parameter a=10.9422(4) Å forming a three-dimensional imidonitridosilicate tetrahedra network with voids for the rubidium ions. The structure model and the presence of the imide groups were verified by Fourier-Transform infrared (FTIR) and magic-angle spinning (MAS) NMR spectroscopy, using cross polarization ¹⁵N{¹H} and ²⁹Si{¹H} MAS NMR experiments. Rb₃Si₆N₅(NH)₆ represents a possible intermediate during the ammonothermal synthesis of nitridosilicates. The characterization of such intermediates improves the understanding of the reaction pathway from ammonothermal solutions to nitrides. Thus, the ammonothermal synthesis is an alternative approach to the well-established high-temperature synthesis leading to the compound class of nitridosilicates.