On the Domain Structure of Rochelle Salt Crystal

It is well known that rochelle salt crystal splits into ferroelectric domains in ferroelectric temperature region. Observations using a polarization microscope give informations on domain structure. In the present experiment effect of the condition of crystallization on the domain pattern was studied; effect of temperature change on the domain pattern and its relation to pyroelectric effect and permittivity was also studied. Factors affecting the domain structure were searched. The findings are summarised in the following. (1) Crystallization at the temperature below the upper Curie point gives domains parallel to b-axis (b-axis domains). (2) Crystallization at the temperature above the upper Curie point gives c-axis domains. The b-axis domains change into c-axis domains after being kept at a temperature above the upper Curie point for a long time. (3) On raising temperature through the upper Curie point, monoclinicity decreases gradually until the domain pattern disappears, without any change of domain width. This process corresponds to pyroelectric effect. The temperature where the domain pattern disappears is 24.25±0.2℃, the peak value of permittivity being attained at the same temperature. The pyroelectric effect, however, continues to exist up to some higher temperature. (4) By applying an electric field of the order of coercive field, very long ielaxation phenomenon is observed, the change of polarization being almost completed after several hours. (5) The change of polarization is caused largely by the formation of new domains, as well, as by the domain boundary movements. Consequently, Barkhausen effect presumably corresponds not only to the discontinuities in domain boundary movements but also to the formation of new domains and oombination of domains. (6) The effect of annealing crystals short-circuited is similar to that of annealing free crystals in the air. This fact, together with the fact that the crystallization of the sample immersed in a conducting solution in ferroelectric temperature region also causes splitting into domains, may show that the domain structure is mainly affected by stress distribution. (7) Suppose the existence of screw dislocation lines in c-direction, then some portions close to them should be largely polarized. Therefore, on cooling through the upper Curie point, nuclei of domains long in c-direction, thin in b-direction, and taper in a-direction may be formed. These nuclei may grow into c-axis domains, giving a possible explanation of the phenomenon, described above in (2).