  Intense, few-cycle pulses in the terahertz frequency range have strong
potential for schemes of control over vibrational modes in solid-state
materials in the electronic ground-state. Here we report an experiment using
single cycle terahertz pulses to directly excite lattice vibrations in the
ferroelectric material $\mathrm{Sn_2P_2S_6}$ and ultrafast x-ray diffraction to
quantify the resulting structural dynamics. A model of a damped harmonic
oscillator driven by the transient electric field of the terahertz pulses
describes well the movement of the Sn$^{2+}$ ion along the ferroelectric soft
mode. Finally, we describe an anharmonic extension of this model which predicts
coherent switching of domains at peak THz-frequency fields of 790 kV/cm.
