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,.

The size dependence of the band gap for barium titanate nanoparticles prepared by RF-plasma chemical vapor deposition was investigated by measuring the diffuse reflectance spectra. The band gap of the indirect transition gradually increased, as the particle sizes decreased to less than 11.5 nm. Consequently, the band gap for particles of 6.7 nm in size was measured to be 3.47 eV, which was approximately 0.25 eV larger than that of BaTiO3 bulk. This phenomenon was discussed on the basis of quantum confinement model.

Self-diffusion coefficients of nitrogen in polycrystalline alpha- and beta-silicon nitride were measured by a gas–solid isotope exchange technique using 15N as a tracer at temperatures in the range 1200–1410°C. The diffusion coefficients for single-crystal grains can be expressed as D (cm2/sec) =1.2×10−12 exp(−55 700/RT) for α-Si3N4 and D (cm2/sec) =5.8×10+6 exp(−185 700/RT) for β-Si3N4. The diffusion of nitrogen along grain boundaries in the reaction-sintered materials was considerably faster than the diffusion through the bulk.

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