H-1 and Al-27 NMR study of the ferroelectric transition in dimethylammonium aluminum sulphate hexahydrate (CH3)(2)NH2Al(SO4)(2)center dot 6H2O

摘要

Dimethylammonium aluminum sulphate hexahydrate (CH3)(2)NH2Al(SO4)(2). 6H(2)O (DMAAS) is a representative of a family of inorganic hydrogen-bonded insulators with a complicated structure of the H-bond network. The microscopic nature of the ferroelectric phase transition at T-c = 152 K was studied via the H-1 and Al-27 NMR spectrum, spin-lattice relaxation, and relaxation in the dipolar frame. Two kinds of molecular motions were detected in the paraphase with frequencies differing for about five orders of magnitude. The slow motion corresponds to the dimethylammonium (DMA) reorientational dynamics that freezes out at the ferroelectric transition whereas the fast motion reflects the dynamics of the H-bond network, which shows no anomaly at T-c. The results demonstrate that the DMA reorientation freeze-out is the prime reason for the ferroelectric transition in DMAAS. The DMA slowing-down dynamics has a profound effect on the other two sublattices of the DMAAS structure, the SO4 and the Al(H2O)(6), via the hydrogen bonding. The effect of the relatively slow DMA reorientations is a gradual lowering of the time-average local crystal symmetry which biases the local potentials of water molecules in the Al(H2O)(6) complexes as well as the potentials of the H bonds. The gradual freeze-out of the water "jump-over" motion seems to be responsible for the appearance of four minima in the Al-27 spin-lattice relaxation rate in the paraphase which appear in addition to the global minimum at the ferroelectric transition. The splitting of the Al-27 spectral lines much below the ferroelectric transition temperature indicates that proton ordering in the H bonds begins to take place below 90 K. [S0163-1829(99)11105-6]. [References: 20]