Phase relationships in the system Si3N4-SiO2-La2O3

Phase relationships in the system Si₃N₄-SiO₂-La₂O₃ have been investigated after cooling from 1700° C. Two phases, 2Si₃N₄·La₂O₃ (monoclinic) and La₅ (SiO₄)₃N (hexagonal), were identified; the other two phases in the system, LaSiO₂N (monoclinic) and La₄Si₂O₇N₂ (monoclinic), were found to dissociate to La₅ (SiO₄)₃N and a glass after cooling from temperatures above 1650° C. The unit cells of 2Si₃N₄·La₂O₃, LaSiO₂N and La₄Si₂O₇N₂ have been determined and compared with those of preceding works. The results are discussed in relation to the intergranular phases observed when Si₃N₄ is sintered with La₂O₃ additions.