Phase formation in the BiPO4–YPO4–(H2O) system

This work is devoted to the study of phase equilibria in the BiPO 4 –YPO 4 –(nH 2 O) system under mild conditions. It was shown that using the precipitation method leads to crystallization of the samples into the rhabdophane phase YPO 4 · nH 2 O and the ximengite phase BiPO 4 . Hydrothermal treatment of the samples at 160 ◦ C results in the gradual transformation of hexagonal yttrium phosphate with a rhabdophane-type structure into tetragonal xenotime YPO 4 , and hexagonal bismuth phosphate with a ximengite-type structure into monoclinic bismuth phosphate (space group P 2 1 /n). The transformation into the stable phases of xenotime and monoclinic bismuth phosphate is almost complete after 28 days of isothermal holding under hydrothermal conditions at 160 ◦ C. Moreover, the lower the content of the second component in samples containing both Bi and Y, the faster the structural transformation into the stable phase proceeds. A solid solution based on monoclinic bismuth phosphate with the composition Bi0.94Y0.06PO4 is formed in the system. Before disappearing, the rhabdophane-type phase represents a solid solution with the composition Y 0.8 Bi 0.2 PO 4 · nH 2 O. The crystallite sizes of all phases increase with an increase in the bismuth content in the system.