GLASSY STATE IN THE FERROICS WITH HYDROGEN-BONDS

It has been recently found that crystals of [NH2(CH3)(2)](2)MeCl(4) (M e = Zn, Co, abbreviated DMAZC and DMACC) and NH2(CH3)(2)Al(SO4)(2) . 6 H2O (DMAAS) undergo a complicated sequence of phase transitions inclu ding ferroelectric and ferroelastic ones. A study of the above compoun ds shows that their dielectric properties strongly depend on the previ ous history of the sample. Keeping the crystal in a low temperature co mmensurate phase, in particular, is followed by an anomalous behaviour of dielectric permittivity and losses at higher temperatures. The ano malies manifesting considerable dispersion in the low frequency region (1 to 100 kHz) disappear after annealing of the sample. The most inte resting case is observed in DMAAS crystals, where the jump-like anomal y of Eb is gradually shifted toward high temperatures with increasing measuring field frequency, The above effects are obviously connected w ith freezing of the reorientational motion of structural groups (such as a DMA cation) in the crystals under the influence of the remaining strains. The latter are concentrated along the domain walls when keepi ng a sample in the ferroelectric or ferroelastic phases. At comparativ ely high heating rate, these strains have no time to be diffused and a bove the phase transition cause the appearance of clusters with frozen structural groups, characterized by slowing-down of the relaxational processes. Such a situation takes place in the crystals with the so-ca lled glassy state of rotational type. Reorientation of structural grou ps at the phase transition with simultaneous existence of frozen regio ns is followed by curving and breaking of hydrogen bonds linking catio ns and anions. As a result one can observe anomalously high protonic c onductivity in the investigated temperature region. Such an effect is most clearly seen in DMACC. It must be noted that features characteris tic of the glassy state are found also in other compounds with H bonds , for example, in trimethylammonium tetrachlormetallates and dimethyla mmonium trichlormetallates.