A study of phase transitions in Cd(ClO4)2⋅6H2O by adiabatic calorimetry and infrared spectroscopy

A calorimetric study of Cd(ClO4)2 ⋅ 6H2O from 20 to 345 K shows three phase transformations at 124.8, 238.45, and 271.62 K. Infrared spectra indicate that the low-temperature structure (below 124.8 K) contains one type of [Cd(OH2)6]2+, of symmetry C3 or C3v, and two types of ClO−4, both strongly distorted from tetrahedral symmetry. There appears to be some disorder in this phase. The sharp transition at 124.8 K leads to major spectral changes, which indicate that hydrogen bonding is weakened, distortion of the perchlorate ions is diminished, and [Cd(OH2)6]2+ symmetry is increased to D3d. The more gradual phase transition at 238.45 K involves no observable infrared spectroscopic changes and may correspond to an increase in ClO−4 disorder. The very sharp transition at 271.62 K involves only very minor changes in the infrared spectra and may involve an increased disorder of the [Cd(OH2)6]2+ units.