Synthesis, reversibility and mechanism of the dehydration of monoclinic CaZn2(PO4)2•2H2O

The reversibility of the dehydration of monoclinic CaZn2(PO4)(2)center dot 2H(2)O to the corresponding monohydrate is proven and the dehydration enthalpy determined by combination of thermal analysis and X-ray diffraction. The mechanism of the monohydrate formation is investigated by scanning electron microscopy (SEM) and temperature dependent in situ X-ray powder diffraction. Mechanistic impact on the powder bulk properties are deduced. As for the orthorhombic dimorph, a crystalline monohydrate species is identified and firstly indexed. Thermal induced changes of the lattice parameters up to the formation of the monohydrate are analyzed. A linear elongation of the lattice parallel to the directions [100] and [001] is found. The corresponding elongation coefficients and the volumetric expansion of the unit cell are determined. With regard to the option of synthetic production of the compound, a possible application of the material for thermochemical energy storage and conversion is conceivable. The relevance of fundamental mechanistic features on the bulk behaviour upon cyclic de- and rehydration is discussed.