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

Dimethylammonium aluminum sulphate hexahydrate (CH3)(2)NH2Al(SO4)(2). 6H(2) O (DMAAS) is a representative of a family of inorganic hydrogen-bonded insu lators 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 par aphase with frequencies differing for about five orders of magnitude. The s low motion corresponds to the dimethylammonium (DMA) reorientational dynami cs 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 prim e reason for the ferroelectric transition in DMAAS. The DMA slowing-down dy namics has a profound effect on the other two sublattices of the DMAAS stru cture, 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-av erage local crystal symmetry which biases the local potentials of water mol ecules 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 respon sible for the appearance of four minima in the Al-27 spin-lattice relaxatio n rate in the paraphase which appear in addition to the global minimum at t he ferroelectric transition. The splitting of the Al-27 spectral lines much below the ferroelectric transition temperature indicates that proton order ing in the H bonds begins to take place below 90 K. [S0163-1829(99)11105-6] .