NaLa(SO4)2,H2O thermal conversion and Na3La(SO4)3 crystal growth

NaLa(SO4)2,H2O crystalline powder was obtained under hydrothermal conditions at 220 degrees C. A coupled TGA/DTA experiment of NaLa(SO4)2,H2O exhibits a weight loss at 260 degrees C corresponding to the dehydration and an endo-thermal peak at 774 degrees C. To elucidate the transformation mechanism as a function of temperature, single crystals have been grown at 80 degrees C, 300 and 800 degrees C. For each phase, single crystals have been isolated and structure determination was performed. As already published, NaLa(SO4)2,H2O crystallizes in a P3121 space group. However, the dehydration at 260 degrees C is not a simple loss of the water molecule but a radical change in the structure. The removal of the water molecules inside the tunnels formed by the framework leads to a change in the coor-dination of the LaO9 Lanthanum-based polyhedrons. The compound obtained after dehydration is a new triple sulfate of the formula Na3La(SO4)3 crystallizing in the R-3 space group (a = 14.0976(1) angstrom; c = 8.1267(1) angstrom) with LaO12 icosahedrons. Millimeter size single crystals of this new phase have been grown under hydrothermal conditions (300 degrees C, 157 bars). After the endothermal peak at 774 degrees C, Na3La(SO4)3 decomposes by forming the anhydrous double sulfate NaLa(SO4)2 crystallizing in the P-1 space group with LaO10 polyhedrons. The structure of the three (NaLa)-compounds at RT, 300 degrees C and 800 degrees C is compatible with the expected Raman signatures. Finally, a complete transformation of NaLa(SO4)2,H2O up to 800 degrees C is proposed. After 1000 degrees C, the compound decomposes chemically with a large weight loss.