Reinvestigation of the Phase Equilibria in the La_2O_3-P_2O_5 System

Abstract

The phase equilibria between the solid phases in the La_2O_3-P_2O_5 system were reinvestigated with solid state reaction method. The existence of La_3PO_7, La_7P_3O_{18}, LaPO_4, and LaP_3O_9 was confirmed. La_7P_3O_{18} can form only at ~1200℃ or higher possibly due to kinetic reasons, but it is stable at lower temperatures. On the other hand, three intermediate compounds proposed to exist, La_5PO_10, La_4(P_2O_7)_3, and La_2P_4O_{13} were not obtained in this study. Based on the literature, La_4(P_2O_7)_3 and La_2P_4O_{13} are considered to be metastable phases, although thermodynamic evidence is still required.