Crystal Structure, Polymorphism, and Anisotropic Thermal Expansion of α-Ca(CH₃COO)₂

Abstract

Dehydration of calcium acetate monohydrate (Ca­(CH3COO)2·H2O) by heating to 300 °C leads to the formation of anhydrous α-Ca­(CH3COO)2. During heating and cooling cycles, high- and low-temperature forms of α-Ca­(CH3COO)2 were discovered. The reversible first-order phase transformation between the two forms occurs in a temperature range between 150 and 170 °C. The crystal structures were solved from laboratory powder X-ray diffraction (PXRD) data. The low temperature form of α-calcium acetate (LT-α-Ca­(CH3COO)2) crystallizes at room temperature in a primitive triclinic unit cell with space group P with lattice parameters of a = 8.7168(3) Å, b = 12.6408(3) Å, c = 12.3084(3) Å, α = 117.4363(17)°, β = 77.827(2)°, γ = 115.053(2)°, and a unit cell volume of 1090.23(6) Å3. High-temperature α-calcium acetate (HT-α-Ca­(CH3COO)2) crystallizes at 300 °C in a rhombohedral unit cell with space group R , lattice parameters of a = 21.1030(5) Å, c = 8.7965(2) Å, and a unit cell volume of 3392.58(17) Å3. In both crystal structures, edge sharing polyhedra of calcium cations and acetate anions that coordinate in both a mono- and bidentate way build up channel-like motifs. During the phase transition, the coordination mode of a bridging acetate anion changes from monodentate to bidentate, and the elliptical channels of the low temperature form become circular. This leads to both negative and positive thermal expansion along different principal axes in the crystal structure of LT-α-Ca­(CH3COO)2 and to an overall considerably big volumetric thermal expansion.

Simple and Rapid Synthesis of Calcium Acetate from Scallop Shells to Reduce Environmental Issues

“…This indicates that phase of calcium acetate monohydrate of CA70 was different from other three samples. All patterns of CA samples were in good agreement with the reference data given by Sebastian et al [27,49,50] and agreed with observations from IR and TGA. Some peak intensities differ, which is ascribed to the fact that the reference data relating to flat plate (Bragg-Brentano Geometry) measurements and therefore, this may be affected by preferred orientation effects.…”

“…Figure 3 shows X-ray diffraction patterns of SSP and CA samples prepared from four different acetic acid concentrations. The XRD results were compared with those of calcium carbonate and calcium acetate reported in other literatures [3,21,23,27,47,48]. The XRD pattern of SSP clearly showed that the phase present was calcite, which was found to match with the JCPDS data file of CaCO 3 (File no.…”

“…Its hemihydrate has also been found, and the crystal structure has been studied [25,26]. CA can be prepared by soaking calcium carbonate (found in eggshells or in common carbonate rocks, such as limestone or marble or hydrated lime) in vinegar [27]:…”

“…Its hemihydrate has also been found, and the crystal structure has been studied [25, 26]. CA can be prepared by soaking calcium carbonate (found in eggshells or in common carbonate rocks, such as limestone or marble or hydrated lime) in vinegar [27]: (1) CaCO 3 s + 2 C H 3 COOH aq ⟶ Ca CH 3 COO 2 · nH 2 O s + C O 2 g , (2) Ca OH 2 s + 2 C H 3 COOH aq ⟶ Ca CH 3 OO 2 · 2 H 2 O s . …”

Chemical composition, coordination, and stability of Ca–organic associations in the presence of dissolving calcite

“…43,48,49,51 In addition, both apatite and aragonite feature bidentate coordination of Ca, which is not observed in calcite. Bidentate Ca carbonate pairs are more commonly observed in solutions and in amorphous carbonates, 60,61 for Ca salts of organic acids, 61,62 and when Ca is coordinated within protein structures. 63 Determining the type of coordination bonds of Ca ions requires an extensive molecular simulation approach, 14 which was not within the scope of this study.…”