Abstract
The synthesis of various rare-earth cyclo-tetraphosphates, La4(P4O12)3·14H2O (LaP4m), Ce4(P4O12)3·13H2O(CeP4m), Pr4(P4O12)3·13H2O(PrP4m), and Nd4(P4O12)3·13H2O(NdP4m), were prepared and their thermal behaviors were investigated using TG-DTA, HPLC, and X-ray diffraction analysis.
1) LaP4m, PrP4m, and NdP4m gradually decomposed to relatively short-chain phosphates, ortho-(P1), pyro-(P2), tri-(P3), tetra-(P4), and oligophosphates(Poligo, n=5-14), with the elimination of water of crystallization. With an increase of temperature, their phosphates condensed to insoluble phosphates(Pinsoluble). The decomposition of P4m and the condensation of short-chain phosphates formed proceeded faster under humid air than dry air. In the range of temperature of 600~700°C, amorphous Pinsoluble changed to cyclo-triphosphate(MP3m, M=La, Pr, and Nd). The crystallinity of their cyclo-triphosphates was better one heated in humid air than in dry air. MP3m changed to orthophosphate, LaPO4, PrPO4, and NdPO4, with the evolution of P2O5 above 1000°C.
2) Ce4P4m also decomposed to short-chain phosphates such as ortho-(P1), pyro-(P2), tri-(P3), and tetraphosphate (P4) with the elimination of water of crystallization, followed by the dehydration-condensation reaction of short-chain phosphates and the oxidation reaction of Ce3+ to Ce4+, resulting in accelerating the hydrolysis reaction and in depressing the dehydration-condensation reaction because of formation of stable CeP2O7. CeP2O7 crystallized by 600°C. At the temperature above 1000°C, CePO4 was formed through cerium cyclo-triphosphate (CeP3m).
All of the rare-earth phosphates prepared didn't melt up to 1050°C.
