Crystalline phases of thiourea. II. Phase diagram in an electric field

The successive phases of completely deuterated thiourea [SC${\left(\mathrm{N}{\mathrm{D}}_2\right)}_2$] in the presence of an external electric field are described in the framework of a one-dimensional model. The wave-vector modulation of the incommensurate phase diminishes in increasing electric field. Phase diagrams in the ($E,T$) plane are sketched. Lock-in phases with wave-vector modulations of $\frac{1}{7},\frac{1}{8}$, and $\frac{1}{9}$ have been found. It has been shown that only the lowest-temperature phase and the lock-in phase $\frac{1}{8}$ are ferroelectric.