Calorimetric and Dielectric Studies on an Order−Disorder Transition in the Hydrogen-Bond System Formed by Water Molecules in the [H31O14][CdCu2(CN)7] Crystal

The heat capacity of [H 31 O 14 ][CdCu 2 (CN) 7 ] has been measured in the temperature range between 14 and 340 K with an adiabatic calorimeter. A first-order phase transition appeared at 217.5 ± 0.2 K involving an entropy change of 37.5 ± 1.5 J K -1 mol -1 corresponding to 2.68 ± 0.11 J K -1 (mol of H 2 O) -1 (= R ln 1.38). Dielectric permittivity has been measured at 10 kHz at temperatures between 130 and 280 K. An abrupt increase of the real part (Δε‘ ≈ 100) appeared at around 215 K. The most likely mechanism of the phase transition is the orientational ordering of the water molecules forming a hydrogen-bonded network in the crystal. The orientation of the water molecules is constrained by the ice rules as in the various ice polymorphs and clathrate hydrates.