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

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 P1 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) ų. High-temperature α-calcium acetate (HT-α-Ca­(CH₃COO)₂) crystallizes at 300 °C in a rhombohedral unit cell with space group R3, lattice parameters of a = 21.1030(5) Å, c = 8.7965(2) Å, and a unit cell volume of 3392.58(17) ų. 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.