Phase equilibria in the ternary system MgO–Na2O–P2O5. The partial system MgO–Mg3(PO4)2–Mg4Na(PO4)3–Na4P2O7

The MgO–Mg3(PO4)2–Mg4Na(PO4)3–Na4P2O7 subsystem of the ternary system MgO–Na2O–P2O5 was investigated using thermal analysis (heating and cooling), X-ray diffraction techniques and microscopy in reflected light. Three intermediate compounds: Mg4Na(PO4)3, MgNaPO4 and MgNa4(PO4)2 occur in the partial system. Mg4Na(PO4)3 melts incongruently at 1155 °C, MgNa4(PO4)2 melts congruently at 1655 °C, whereas MgNaPO4 exists only in the solid phase and decomposes at 950–960 °C. A phase diagram for the partial system is proposed. It has been established that the MgNa4(PO4)2 phosphate forms a binary system with Na4P2O7. A phase diagram of this system has been constructed. It has been found that a ternary peritectic (tP=1145±3.0 °C) and two ternary eutectics (tE1=984±3.0 °C, tE2=732±1.5 °C) occur in the MgO–Mg3(PO4)2–Mg4Na(PO4)3–Na4P2O7 system.