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Optical absorption and emission spectra have been measured for Nd-doped Y3Al5O12 ceramics obtained by a urea precipitation method. The optical properties of the ceramics are almost the same as those of single crystals grown by the Czochralski method and floating-zone method, except for a higher background absorption of 2.5–3 cm−1. The energy-level structure for Nd3+ in the Y3Al5O12 ceramics is determined for a 1-at. % Nd concentration. The induced emission cross section for the 1-at. %-Nd-doped ceramic is calculated to be 4.9×10−19 cm2, which falls in the range reported for the Nd:Y3Al5O12 single crystals. The Nd concentration dependence of the emission decay time at the 1064-nm laser wavelength was also measured, and typical concentration quenching was observed. The decay time of the 1-at. %-Nd-doped ceramic is 219 μs, which is slightly smaller than the reported values for single crystals.

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.

Undoped and rare-earth- (Pr, Nd, Eu, and Er) doped transparent Y3Al5O12 ceramics have been prepared, and their optical spectra have been measured. It is found that absorption coefficient of the undoped ceramic Y3Al5O12 is almost independent of wavelength with 0.258 cm−1, which gives transmittance of the undoped ceramic Y3Al5O12 to be, for example, 95% for a 2-mm height. Peaks in the absorption and emission spectra of Pr:Y3Al5O12 are assigned to the transitions in 4f2 configuration. The transitions from the multiplets with total angular momentum J=0 to multiplets with even J momentum are strong as predicted by Judd–Ofelt theory. The overall and detail structure of the absorption and emission spectra of Nd:Y3Al5O12 is the same as those in a previous publication. However, the absorption coefficient at nonabsorbing wavelengths by Nd3+ is reduced from more than 1.7 to 0.25 cm−1. A simple estimation of the population inversion threshold of the Nd:Y3Al5O12 ceramic reveals that the threshold is still about 25 times larger than that of single-crystal Nd:Y3Al5O12. Peaks in the absorption and emission spectra of Er:Y3Al5O12 ceramic are assigned to the transitions in 4f11 configuration. The transition energies agree well with those reported for single-crystal Er:Y3Al5O12 and the other hosts within 30 cm−1. A Stark splitting scheme for some multiplets has been constructed. Peaks in the emission, absorption, and excitation spectra of Eu:Y3Al5O12 ceramic are assigned to the transitions in 4f6 configuration. The transition energies are in very good agreement with those of single-crystal Eu:Y3Al5O12 within 8 cm−1 for the emission peaks and 3 cm−1 for the absorption and excitation peaks. Spectral characteristics and derivation of a broad peak in the absorption spectrum at around 280 nm are discussed in some detail.