Phase formation and thermal analysis in the LaPO$_4$–GdPO$_4$–H$_2$O system

Structural transformations of nanocrystals in the LaPO$_4$–GdPO$_4$–(H$_2$O) system under hydrothermal conditions at 230$^\circ$C were studied depending on the duration of isothermal holding (2 hours, 3 days, and 5 days). It has been shown that a phase with the rhabdophane-like structure, La$_{1-x}$Gd$_x$PO$_4$ $\cdot$ $n$ H$_2$O (0.00 $\le x\le$ 1.00), exhibits a weighted average crystallite size of 4–7 nm and crystallizes in the system prior to hydrothermal treatment. As a result of hydrothermal treatment, samples in the LaPO$_4$–GdPO$_4$–(H$_2$O) system are completely transformed into a phase with a monazite structure within five days, with the slowest transformation observed for gadolinium orthophosphate. It was found that nanoparticles with a rhabdophane structure, GdPO$_4$ $\cdot$ $n$H$_2$O, possess a single-crystal structure. The thermal analysis data indicated that the samples obtained via the precipitation method contain an X-ray amorphous phase and impurity compounds. The onset temperature of the structural transformation from rhabdophane to monazite, as well as the number of water molecules in the rhabdophane-like structure, depends on the chemical composition of the compound, particularly with regard to the isomorphic substitution of lanthanum cations with gadolinium cations.