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In the ternary system La2Oa-Na20-P2Os, the binary system La(PO3)a-NaPO3 was examined by means of thermal (heating and cooling), dilatometric, X-ray powder diffraction and microscopic analyses. The occurrence of the phosphate NaLaP,Ot2 was confirmed and its temperature of congruent melting was determined to be 1174 ~ The powder data indicate a monoclinic system with the lattice constants a=12.36(6); b=13.45(0); c=6.57(4) A; y= 109.48~ ; v= 1030.97/~a. The investigations were carried out on monocrystals.

In the ternary system La203-P2Os-Na20 the partial system La203-Na4LazP4015-LaPO4 has been examined by the thermal, dilatometrie X-ray and microscopic analyses and its phase diagram provided. The lanthanum oxyphosphate La3PO7 melts incongruently at the 1590~ temperature and crystallizes in a monoclinie system a = 11.20/~., b = 11.94/~, c=7.01 .~, ?= 93.79 and V = 936.97 A s.The purpose of present paper is to determine the phase diagram of the partial system La203-Na4La2P4015-LaPO4. As yet this system has not been known. It is limited from three sides by the following binary system: La203-LaPO4-; La203-Na4La2P4015; LaPO4-Na4La2P4015. The oxyphosphate with a formula La3PO7 for the first time has been obtained by Serra J. J. [1], in a solid state reaction from NH4H2PO4 and La203 at 1180 ~ and by chemical decomposition of the orthophosphate LaPO4 at a high temperature in an atmospheric pressure conditions [2]. Serra et al. [1] reported that the structures of La3PO7 and La7P3018 are monoclinic and LaTP3018 exists in lowand high -temperature forms, transforming reversibley at 1650 ~ Kizilyalli and Welch made La3PO7 by solid state reaction of LaPO4 with Na2CO3 at 700 ~ to 900 ~ [3]. Lanthanum oxyphosphate La3PO7 was found by quenching studies to have a slogish inversion at 935 ~ [4]. The oxyphosphate La3PO7 melts incongruently at 1590 ~ temperature [5,6].

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