Synthesis, Crystal Structure, and Thermal Properties of CaSO4·2H2O Single Crystals

Abstract

Single crystals were grown at room temperature from the precursors CaCO 3 and NH 2 SO 3 H by a gel method using agar-agar gel as the medium of growth. Differential scanning calorimetry, thermogravimetric–differential thermal analysis, and X-ray diffraction measurements were performed on the single crystals. The crystal structure of the grown crystals was confirmed to be very similar to that of calcium sulfate dihydrate (CaSO 4 ·2H 2 O, gypsum). The temperature at which dehydration and evaporation of intercalated water molecules in the single crystal occurred was found to be 375.9 K, which was very close to that in CaSO 4 ·2H 2 O. From all these results, we identified the grown single crystals as CaSO 4 ·2H 2 O. The weight loss owing to the thermal decomposition of anhydrous CaSO 4 formed by the dehydration of the single gypsum crystals was found to occur at ~1530 K. This weight loss was probably caused by the evolution of SO 3 gas by sublimation, and the chalky white substance left in the open vessel after decomposition was calcium oxide (CaO).