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

Pure Sr2Si5N8 and Ba2Si5N8 were obtained by reaction of silicon diimid e with metallic strontium and barium, respectively. The reactions have been carried out under nitrogen atmosphere in a specially developed h igh-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 str ucture of corner-sharing SiN4 tetrahedra. Two sorts of N occur with mo lar ratio 1:1 which are bonded to two and three Si, respectively. Pred ominantly, the N which are bonded to two Si belong to the coordination spheres of the M(2+) ions.