Phase transformations during de- and rehydration of scholzite CaZn2(PO4)2·2H2O

Abstract

The reversibility of the formation of CaZn2(PO4)2·H2O after dehydration of CaZn2(PO4)2·2H2O is proven by combining methods of thermal analysis and X-ray diffraction. The dehydration enthalpy is determined from differential calorimetric analysis. Scanning electron microscopy (SEM) is utilized to investigate effects of respective reactions on the particle morphology. The dehydration of scholzite to the monohydrate is also probed by temperature dependent in-situ X-ray diffraction. The measured diffraction patterns reveal a structural change in the crystal lattice upon dehydration, which is yet unknown. Lattice parameters of scholzite are analyzed as a function of temperature to trace structural changes. Expansion coefficients of the lattice constant c and of the unit cell volume of scholzite are determined. In combination with results from SEM, an understanding of the corresponding de- and rehydration mechanism is emphasized with regard to a possible application of the material in thermochemical energy storage and -conversion.