  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 B2 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.
