La_(16)[Si_8N_(22)][SiON_3]_2 - A nitridosilicate with isolated, corner-sharing and edge-sharing tetrahedra

摘要

The mixed anion oxonitridosilicate La_(16)[Si_8N_(22)][SiON_3]_2 has been synthesized by the reaction of LaN, finely ground SiO_2 and silicon diimide (Si(NH)2) at 1600 deg C in a radiofrequency furnace. The structure has been determined by single-crystal X-ray diffraction (La_(16)[Si_8N_(22)][SiON_3)_2, P1-bar, a = 5.718(2), b = 11.391(2), c=13.435(3)A, alpha = 112.02(3), beta = 90.19(3), gamma = 90.58(3) deg, V=811.1(3) A~3, Z = 2, R_1= 0.027, 4460 independent reflections, 258 refined parameters). Besides La~(3+) and non-condensed [SiON_3]~(7-) tetrahedra, there are chains of both corner-sharing and edge-sharing SiN_4 tetrahedra with additional Q~1-type SiN_4 groups attached to all non-edge-sharing SiN_4 units of the chain. The resulting infinite chains are alternately built up of pairs of edge-sharing [Si_2N_6]~(10-) units that are linked by a common nitrogen atom to two corner-sharing single branched Q~3-type SiN_4 tetrahedra. According to its low degree of condensation, La_(16)[Si_8N_(22)][SiON_3]_2 is moisture sensitive and undergoes rapid hydrolysis on exposure to air. Rietveld refinement of X-ray powder diffraction data is in accordance with the results of the single-crystal X-ray structure determination.