Phase formation under hydrothermal conditions and thermodynamics properties in the GdPO4-YPO4 system

Phase formation in the GdPO4-YPO4 system has been studied. The samples were synthesized under hydrothermal conditions at a temperature of 503 K and a pressure of approximately 10 MPa for 28 days of isothermal holding. The results of thermal treatment of the obtained samples at 1473 and 1673 K and the results of thermal treatment of GdPO4 at 1873 K are presented. It is shown that in the system, solid solutions GdYPO4 are formed based on phases with monoclinic structure GdPO4 and xenotime structure YPO4. The range of existence of two solid solutions at temperatures 503–1473 K lies in the range of compositions from x≈0.20 to x≈0.40. At a temperature of 1673 K, the formation of a phase with anhydrite structure is observed at x≈0.26, and the beginning of transformation of GdPO4 from monoclinic to xenotime structure. Using Knudsen effusion mass spectrometry (KEMS), the qualitative and quantitative composition of the vapor over the (1-x)GdPO4-xYPO4 system has been determined, which consists of a mixture of PO, PO2, and O2. The enthalpy value of the vaporization reaction and the standard formation enthalpies for the GdPO4 and YPO4, as well as the activity of P4O10 in the condensed phase of the (1-x)GdPO4-xYPO4 system, were determined in the temperature range 1650–1850 K.