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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,.

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