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Using a torsion pendulum, the peculiarities of low-frequency shear elastic and anelastic properties of the improper ferroelastic crystal K2Ba(NO2)4 have been investigated for various crystallographic oriented samples at a temperature range in the vicinity of the phase transition point (TC = 420 K). The work gives insights into the nature of the internal friction and spontaneous torque arising in the samples at the Curie point.

Dimethylammonium aluminum sulphate hexahydrate ͑CH 3 ͒ 2 NH 2 Al͑SO 4 ͒ 2 •6H 2 O ͑DMAAS͒ 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 ϭ152 K was studied via the 1 H and 27 Al 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 ͑DMA͒ 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 SO 4 and the Al͑H 2 O͒ 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 Al͑H 2 O͒ 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 27 Al spin-lattice relaxation rate in the paraphase which appear in addition to the global minimum at the ferroelectric transition. The splitting of the 27 Al spectral lines much below the ferroelectric transition temperature indicates that proton ordering in the H bonds begins to take place below 90 K.