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  • Title: Glass transition on the development of a hydrogen-bond network in nano-channel ice, and subsequent phase transitions of the ordering of hydrogen atom positions within the network in [Co(H(2)bim)(3)](TMA)·20H(2)O.
    Author: Watanabe K, Oguni M, Tadokoro M, Oohata Y, Nakamura R.
    Journal: J Phys Condens Matter; 2006 Sep 20; 18(37):8427-36. PubMed ID: 21690898.
    Abstract:
    Low-temperature thermal properties of crystalline [Co(H(2)bim)(3)](TMA)·20H(2)O were studied by adiabatic calorimetry, where H(2)bim is 2,2'-biimidazole, TMA is 1,3,5-benzene tricarboxylic acid, and 20H(2)O represents the water forming nano-channel in the crystal. A glass transition was observed at T(g) = 107 K. It was discussed as a freezing-in phenomenon of a small number of water molecules remaining partially disordered in their positional arrangement. The possibility that some defects really remain in the hydrogen-bond network of channel water was mentioned. Two subsequent phase transitions were observed at 54.8 and 59 K. These were interpreted as being of a (super-structural commensurate)-incommensurate-(normal commensurate) type in the heating direction with respect to the hydrogen-atom positions as referred to the periodicity of the hydrogen-bond network. The transition entropy was evaluated to be 0.65 J K(-1)(H(2)O-mol)(-1) as a total of the two, indicating that the disorder of the hydrogen atoms is present only in part of the water molecules of the channel. Based on the fact that the excess heat capacity due to the equilibrium phase transition is observed down to 35-40 K, the relaxation time for the rearrangement of the hydrogen-atom positions was assumed at the longest to be 1 ks at 35 K. This indicates that the activation energy of the rearrangement amounts to at most 13 kJ mol(-1) and that the transfer of Bjerrum defects is attributed to the rearrangement.
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