New insight on the investigation of the role of water in the solid-state structures of potassium croconate, K(2)C(5)O(5).2H(2)O, and its anhydrate

This work presents a comparative study of dihydrated and anhydrous forms of potassium croconate crystals by vibrational spectroscopy, X-ray powder diffraction, and thermogravimetry. These compounds have different colors (dihydrated is orange, and dehydrated is yellow) due to the presence of coordinated water molecules. X-ray diffraction patterns show that the unit cell of the yellow compound is smaller than that of the orange analogue, suggesting that the croconate ion layers are more closely bonded in this salt. The loss of water is reversible due to the potassium cation size which is intermediate between small (Li+ and Na+) and large (Rb+ and Cs+) alkaline metal ions. However, the hydrated compound (orange) is more stable, and with a small quantity of water the yellow compound is quickly converted to the orange compound. A diagnostic feature of the Raman spectrum for the orange (hydrated) and yellow (anhydrous) analogues is the singlet at 1240 cm(-1) in the former, assigned to a nu(CC) + delta(CCC) + nu(CO) + beta(CO) mode of E'2 symmetry, which splits in the yellow form to a doublet at 1256 and 1232 cm(-1).