Laser-induced Raman spectra were obtained for powders and multidomain single crystals of PbZrO, at room temperature. Factor-group analysis was used to interpret the spectra on the basis of the space group CZos-Pba2. Raman spectra were also observed for single crystals of PbZrO, at several other temperatures between 76" and 520°K and related to crystal structural changes. No first-order Raman bands were noted above the Curie temperature, indicating a phase transformation to a cubic zirconate structure. The interpretation of Raman spectra for thermally decomposed PbZrO, is discussed.

Iron is shown to increase the tendency for Si3N4 to decompose by formation of stable Fe-Si alloys. The decomposition reaction is temperature-and pressuresensitive, with increasing temperatures causing greater reaction. The liquid Fe-Si alloy produced at elevated temperatures solidifies on cooling and results in metallic inclusions that act as flaws and thus mechanical failure origins.

The effect of surface oxygen concentration of silicon nitride powders on the properties of resulting ceramics was studied. A high-purity silicon nitride powder was treated physically and chemically to modify its surface oxygen content. The resulting powders were hot-pressed into dense ceramics using 6 wt% yttria as a sintering aid. Strength and oxidation resistance of these ceramics were measured and correlated with the powder and ceramic compositions as well as the resulting intergranular phases. Results show that the phases developed in yttria-containing silicon nitride ceramics vary with slight changes in the initial powder oxygen content, as predicted, and that strength can be correlated to initial oxygen concentration. The mechanical strength vs oxygen content curve has a definite maximum; i.e., there is a small oxygen concentration range at which optimum ceramic strength is realized. Best results are obtained when the oxygen content is increased by thermal oxidation; other techniques such as chemical oxidation or addition of silica are not as effective, particularly in attaining high strength at elevated temperatures.