It has been verified by means of the polarization microscope that rochelle salt in the ferroelectric state consists of many domains. The domain structure in an annealed crystal is caused by the electrostatic self energy. When the electric field along the $X$ direction is increased and then decreased gradually, successive positions of the domain wall produce a hysteresis loop, which proves the existence of a restoring force on the wall. This restoring force causes lag of charging, and its variation with time produces a fatigue effect. The propagation velocity of a domain wall is about 0.2 cm/sec for 100 v/cm. A group of domains parallel to the $b$ axis has been created artificially. When a stress $Y_z$ is applied, a set of domains inverts its polarization direction. $Z$-cut specimens of K${\mathrm{H}}_2$P${\mathrm{O}}_4$ cooled below the Curie temperature divide into many regions which appear to be domains. The phase transition in K${\mathrm{H}}_2$P${\mathrm{O}}_4$ propagates from only one nucleus.

Theoretically it has been concluded that the domain wall energy of rochelle salt is $1.4\times {10}^{-10\mathrm{}}{P_0}^3$ erg/${\mathrm{cm}}^2$ and the wall width $\sim 2\times {10}^{-4\mathrm{}}/P_0$ cm, where $P_0$ is the saturation polarization.

• Received 14 January 1952

DOI:https://doi.org/10.1103/PhysRev.90.193