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 Gd_1-xY_xPO₄ 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.