The crystal structure ofa-Si3N4 was reinvestigated by an X-ray method using a single-crystal specimen prepared by chemical vapor deposition. The oxygen content of the specimen (0.05?0.03%) was determined by 14 MeV neutron activation analysis. The a form crystallizes in the space group P31c with uniteell dimensions a=7.818?0.003 di and c=5.591~0.004 A. The structure was refined by the full-matrix least-squares method using 1143 observed structure factors. The Si-N distances determind ranged from 1.715 to 1.759 di, and the fmal value of R was 0.041. Structural evidences indicated that the a form is essentially pure Si,N,.
Abstract. Monoclinic, P21/n, a = 13.159 (12), b = 7.161 (3), c=7.499 (6) A, /?=93.76 (5) °. Z=8. Om=7"17, Dx= 7"26 g cm -3. Crystals were synthesized under high pressure at high temperature. The structure has no direct resemblance either to the scheelite-or to the wolframitetype. It consists of densely packed layers of WO6 octahedra, which are connected by edge-and corner-sharing; barium atoms are located between them. The coordination number of the barium atoms has increased in comparison with the structure under ordinary conditions.Introduction. This phase exists above the pressure P(kbar) = 26.7 + 0.0265 T (°C), (T= 600-1000 °C), and is quenchable to normal pressure (Fujita, Yamaoka & Fukunaga, 1974).The intensity data were collected on a Rigaku fourcircle diffractometer with Mo Kez (2=0.70926 A,) radiation monochromatized by graphite. In the range 20 < 90°, 6099 independent reflexions were measured, of which 3791 were considered not observed. The systematic absences were h0/, h + l= 2n + 1, and 0k0, k =
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