Phase Diagram and Metastable Phases in the LaPO₄–YPO₄–(H₂O) System

Phase formation in the LaPO₄–YPO₄–(H₂O) system have been studied under hydrothermal conditions at T ~ 230°C and after heat treatment in air in the temperature range 1000–1400°C. The phase equilibria diagram in the system has been constructed for the LaPO₄–YPO₄ system. The regions of metastable binodal and spinodal decomposition of a phase with a monazite structure with a critical point T_cr = 931°C have been calculated. The experimentally determined eutectic temperature of 1850 ± 35°C is in good agreement with the calculated value T_e = 1820°C. The maximum solubility of YPO₄ in LaPO₄ at eutectic temperature obtained from the results of thermodynamic optimization of the phase diagram is 50.5 mol %.