Abstract:
This work is devoted to the study of phase equilibria in the BiPO$_4$-YPO$_4$-($n$H$_2$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$$\cdot$$n$H$_2$O and the ximengite phase BiPO$_4$. Hydrothermal treatment of the samples at 160$^\circ$C results in the gradual transformation of hexagonal yttrium phosphate with a rhabdophane-type structure into tetragonal xenotime YPO$_4$, and hexagonal bismuth phosphate with a ximengite-type structure into monoclinic bismuth phosphate (space group $P2_1/n$). 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$^\circ$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 Bi$_{0.94}$Y$_{0.06}$PO$_4$ 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$$\cdot$$n$H$_2$O. The crystallite sizes of all phases increase with an increase in the bismuth content in the system.
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