Phase Diagram and Metastable Phases in the LaPO<sub>4</sub>–YPO<sub>4</sub>–(H<sub>2</sub>O) System

Phase formation in the LaPO4-YPO4-(H2O) system was studied under hydrothermal conditions at T≈230°C and after thermal treatment in the temperature range 1000–1400°C. The phase equilibrium diagram was constructed for the LaPO4-YPO4 system. The regions of metastable binodal and spinodal phase transition monazite-structured with a critical point Tcr = 931°C have been calculated. The experimentally determined eutectic temperature of 1850±35°C is in good agreement with the calculated value Te=1820°C. The maximum solubility of YPO4 in LaPO4 at eutectic temperature obtained from the thermodynamic optimized phase diagram is 50.5 mol.%.