Polymorphism and Crystallization of Sodium Acetate

Sodium acetate (NaOAc), a simple and widely used compound, has been studied structurally in several works; still, uncertainties concerning its polymorphism remain. In the present study, the crystallization conditions of the known polymorphs of anhydrous NaOAc were clarified, whereas their crystal structures were corrected. Two new polymorphs were discovered, one of which (NaOAc II) crystallizes as a minor phase from solution, while the other (α-NaOAc) is the low-temperature form of the polymorph obtained from the melt. The corresponding phase transition occurs at 259 K and was studied by variable-temperature powder X-ray diffraction and differential scanning calorimetry. In addition, the crystal structures of two polymorphs of rubidium acetate (RbOAc) were determined for the first time to complete the alkali metal acetate series. A comparison of sodium, potassium, and rubidium acetates showed the dependence of the structural parameters, hygroscopicity, and solid-state phase transition temperatures on the atomic radius.