NITRIDO-SILICATES .2. HIGH-TEMPERATURE SYNTHESES AND CRYSTAL-STRUCTURES OF SR2SI5N8 AND BA2SI5N8

Pure Sr2Si5N8 and Ba2Si5N8 were obtained by reaction of silicon diimide with metallic strontium and barium, respectively. The reactions have been carried out under nitrogen atmosphere in a specially developed high-frequency furnace at temperatures between 1550 and 1650 degrees C. Sr2Si5N8 (Pmn2(1), a = 571.0(2), b = 682.2(2), c = 934.1(2) pm, Z = 2, R = 0.037, wR = 0.021) and Ba5Si5N8 (Pmn2(1), a = 578.3(2), b = 695.9(2), c = 939.1(2) pm, Z = 2, R = 0.022, wR = 0.018) are isotypic and contain M(2+) ions as well as a three-dimensional covalent network structure of corner-sharing SiN4 tetrahedra. Two sorts of N occur with molar ratio 1:1 which are bonded to two and three Si, respectively. Predominantly, the N which are bonded to two Si belong to the coordination spheres of the M(2+) ions.