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The phase equilibria occurring in the YPO 4 -Rb 3 PO 4 system were investigated by thermoanalytical methods, X-ray powder diffraction, and ICP-OES.

The system Rb 3 PO 4-Ba 3 (PO 4) 2 was investigated by thermoanalytical methods, X-ray powder diffraction, ICP, and FT-IR. On the basis of the obtained results its phase diagram was proposed. For this system with one intermediate compound, BaRbPO 4 , we found that this compound melts congruently at 1700°C, exhibits a polymorphic transition at 1195°C and is high-temperature unstable. Also, the intermediate compound was subject to gradual decomposes to Ba 3 (PO 4) 2 (the solid phase) and vaporization (with conversion of phosphorus and rubidium oxides into vapor phase). We also found that Rb 3 PO 4 melts congruently at 1450°C and shows a polymorphic transition at 1040°C. Regarding Ba 3 (PO 4) 2 , we have confirmed that it melts congruently at 1605°C and exhibits a polymorphic transition at 1360°C.

A revised version of the Ca 3 (PO 4 ) 2 -YPO 4 phase diagram has been proposed on the basis of results obtained by thermal analysis (DTA/DSC/TG) and X-ray diffraction methods. A limited solid solution with the structure of b-Ca 3 (PO 4 ) 2 exists in the system. At 1,100°C the maximal concentration of YPO 4 in the solid solution is *15 mass%. The solid-solution phase field exists in the temperature range upto *1,380°C. Two high-temperature solid solutions with the structure of a 0 and a-Ca 3 (PO 4 ) 2 form in system as well, however only the a phase can be obtained by quenching from high temperatures. The Ca 3 Y(PO 4 ) 3 compound with the structure of eulytite forms in the Ca 3 (PO 4 ) 2 -YPO 4 system at temperatures exceeding 1,255°C and does not show any polymorphic transition.

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