Synthesis, crystal structure and solid-state NMR spectroscopic investigation of the oxonitridosilicate BaSi6N8O

The phase-pure oxonitridosilicate BaSi6N8O has been synthesized starting from BaCO3 and silicon diimide Si(NH)(2) in a radiofrequency furnace at temperatures below 1630 degrees C as a coarsely crystalline colorless material. The Structure has been determined by single-crystal X-ray diffraction analysis (BaSi6N8O, space group Imm2 (no. 44), a = 810.5(2) b = 967.8(2), c = 483.7(1) pm, V = 379.4(2) (.) 10(6) pm(3), Z = 2, R1 = 0.014, 618 independent reflections, 44 parameters). The oxonitridosilicate comprises a three-dimensional network structure of corner sharing SiN4 and SiON3 tetrahedra with Ba2+ located in the resulting voids. BaSi6N8O is isostructural with the oxonitridoalumosilicate (sialon) Sr2AlxSi16-xN2+xO2+x (x approximate to 2) that previously has been described in the literature. Furthermore, the anionic network of BaSi6N8O derives from that of the homeotypic reduced nitridosilicate SrSi6N8 by a topotactic insertion of oxygen into the Si-Si single bonds. In the Si-29 MAS-NMR spectrum two sharp isotropic signals have been observed at -54.0 and -56.3 ppm, respectively. With respect to their observed intensity ratio of 1:2.1(1) these two signals have to be attributed to the central atoms of SiON3 and SiN4 tetrahedra, respectively, which is in accordance with the X-ray crystal structure determination (Si at Wyckoff positions 4d (SiON3) and 8e (SiN4)).