Stable and metastable phase equilibria in the Bi2O3-BiPO4 system

The stable and metastable phase equilibria in the Bi2O3-P2O5 system were studied in the range 0-50 mol % P2O5 by differential thermal analysis and x-ray diffraction. The results were used to construct the corresponding phase diagrams. In the equilibrium state, the system contains one sillenite phase with Bi2O3 P2O5 = 12 : 1 and four other bismuth phosphates (2 : 1, 3 : 1, 5 : 1, and 1 : 1). In a metastable state, resulting from solidification of metastable melts, there exist delta* solid solutions (0-25 mol % P2O5) based on the high-temperature form of Bi2O3. At lower temperatures, the delta* phase transforms eutectoidally into the metastable 8 phase, which has the same structural basis as the high-temperature solid solutions. At room temperature, the 6 phase exists in a narrow composition range around 15 mol % P2O5. At lower P2O5 contents, the delta* phase decomposes at 868 K by a monotectoid reaction to form a mixture of the metastable alpha. and delta phases. The phases 3Bi(2)O(3) . P2O5 (beta) and 2Bi(2)O(3) . P2O5 (sigma), melting incongruently, at 1193 and 1223 K, respectively, appear in both the equilibrium and metastable phase diagrams.