Polarization modulation instability in liquid crystals with spontaneous chiral symmetry breaking

We present a theoretical model which describes the polarization-modulated and layer-undulated structure of the B7 phase and gives the phase transition from the synclinic ferroelectric smectic- C_SP_F phase to the B7 phase as observed experimentally. The system is driven into the modulated phase due to the coupling between the polarization splay and the tilt of the molecules with respect to the smectic layer normal. The modulation wavelength and the width of the wall between two domains of opposite chirality are estimated.