Thermal Properties, Crystal Structure, and Phase Transition of Racemic CaC4H4O6•4H2O Single Crystals

Abstract

Single crystals of racemic calcium tartrate tetrahydrate, CaC4H4O6·4H2O, were grown at 308 K by a gel method using silica 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 space group symmetry (triclinic, P1(__)) and structural parameters were determined at room temperature. The O–H–O hydrogen-bonding networks formed between adjacent C4H4O6 molecules, and between C4H4O6 and H2O molecules, were found to extend along the c-axis and [01(__)1] directions, respectively. A phase transition, driven by intramolecular proton transfer between three possible positions in the vicinity of O atoms, was found to occur at around 310 K. Weight losses due to thermal decomposition of racemic CaC4H4O6·4H2O occurred in the temperature range of 300–1200 K. The weight losses during decomposition were suggested to be caused by the evaporation of bound water molecules and the evolution of 2H2CO, (1/2)O2, and 2CO gases.