Formation of nanocrystals based on equimolar mixture of lanthanum and yttrium orthophosphates under microwave-assisted hydrothermal synthesis

The effect of hydrothermal-microwave treatment time at 180$^{\circ}$C on the phase composition, dimensional parameters of crystallites and nanoparticles of solid solutions of lanthanum and yttrium orthophosphates in the system 0.53LaPO$_{4}$-0.47YPO$_{4}$-($n$H$_{2}$O) has been determined. It has been proposed the mechanism for structural transition of lanthanum-yttrium orthophosphate solid solution with rhabdophane structure into monazite structure, which consists in the degeneration of nanocrystals having rhabdophane structure along certain edges into monazite structure. It is shown that phase nanoparticles of monazite structure having average crystallite size of 15-17 nm begin to form after 30 minutes of hydrothermalmicrowave treatment at 180$^{\circ}$C immediately after complete crystallization of amorphous phase in the system. The nanoparticle size increase (length of nanorods) with monazite structure after the stage of their formation occurs, mainly due to matter transfer from nanoparticles having rhabdophane structure to nanoparticles having monazite structure. In this case, the system considered conditions of hydrothermal treatment (temperature - 180$^{\circ}$C, pressure $\sim$1–1.5 MPa, duration - up to 120 min) remains two-phase.