Glassy state in the ferroics with hydrogen bonds

It has been recently found that crystals of [NH2(CH3)(2)](2)MeCl(4) (Me = Zn, Co, abbreviated DMAZC and DMACC) and NH2(CH3)(2)Al(SO4)(2) . 6 H2O (DMAAS) undergo a complicated sequence of phase transitions including ferroelectric and ferroelastic ones. A study of the above compounds shows that their dielectric properties strongly depend on the previous history of the sample. Keeping the crystal in a low temperature commensurate phase, in particular, is followed by an anomalous behaviour of dielectric permittivity and losses at higher temperatures. The anomalies manifesting considerable dispersion in the low frequency region (1 to 100 kHz) disappear after annealing of the sample. The most interesting case is observed in DMAAS crystals, where the jump-like anomaly of Eb is gradually shifted toward high temperatures with increasing measuring field frequency, The above effects are obviously connected with 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 keeping a sample in the ferroelectric or ferroelastic phases. At comparatively high heating rate, these strains have no time to be diffused and above 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-called glassy state of rotational type. Reorientation of structural groups at the phase transition with simultaneous existence of frozen regions is followed by curving and breaking of hydrogen bonds linking cations and anions. As a result one can observe anomalously high protonic conductivity in the investigated temperature region. Such an effect is most clearly seen in DMACC. It must be noted that features characteristic of the glassy state are found also in other compounds with H bonds, for example, in trimethylammonium tetrachlormetallates and dimethylammonium trichlormetallates.