Synthesis under hydrothermal conditions and structural transformations of nanocrystals in the LaPO$_4$–YPO$_4$–(H$_2$O) system

Structural transformations of nanocrystals in the LaPO$_4$–YPO$_4$–(H$_2$O) oxide system were investigated under hydrothermal conditions at 230$^\circ$C, depending on isothermal holding times (2 hours, 7 days and 28 days). It was shown that before hydrothermal processing, phases crystallize in the system with rhabdophane structures La$_{1-x}$Y$_x$PO$_4$ $\cdot$ $n$H$_2$O (0 $\le x\le$ 0.80) and xenotime YPO$_4$. It has been determined that increasing the duration of isothermal treatment under hydrothermal conditions leads to the transformation of the rhabdophane phase into phases with monazite and equilibrium xenotime structures, with an intermediate crystallization of the metastable monazite phase. It is noted that only after 28 days of hydrothermal treatment at 230$^\circ$C, the system approaches the equilibrium composition of the phases with monazite structures La$_{0.97}$Y$_{0.03}$PO$_4$ (with crystal sizes of 18–50 nm) and xenotime YPO$_4$ (with crystal sizes of 45–90 nm). In a single-phase sample with a monazite structure La$_{0.75}$Y$_{0.25}$PO$_4$, the average crystal size remains unchanged at around 20 nm after 2 hours, 7 days and 28 days of hydrothermal treatment at 230$^\circ$C.