THERMAL AND THERMODYNAMIC STUDY ON LITHIU M CYCLOHEXAPHOSPHATE

We report in this paper the results of our thermal and thermodynamic i nvestigation on lithium cyclohexaphosphate, Li6P6O18. 5H(2)O between 2 98 and 1007 K. The different transitions with re spect to temperature (successive dehydrations, solid-solid transition and melting) were stu died with the help of differential thermal analysis and thermogravimet ry. The different phases were characterized by X-ray diffraction and b y infrared absorption. Finally, the enthalpy of these phases vs. tempe rature was measured by isothermal drop calorimetry Their heat capaciti es as well as the enthalpies of dehydration, of solid-solid transition and of melting were deduced. We pointed out that the lithium cyclohex aphosphate loses a molecule of water at 333 K (54.3 kJ . mol(-1)), thr ee molecules of water at 413 K (151 kJ . mol(-1)) and the last one at 488 K (50.6 kJ . mol(-1)). The anhydrous lithium cyclohexaphosphate, L i6P6O18, give the polyphosphate,LiPO3, at 708 K (second order transiti on) and melt at 933 K (24.6 kJ . mol(-1)).