  We explore the effect of charge carrier doping on ferroelectricity using
density functional calculations and phenomenological modeling. By considering a
prototypical ferroelectric material, BaTiO3, we demonstrate that ferroelectric
displacements are sustained up to the critical concentration of 0.11 electron
per unit cell volume. This result is consistent with experimental observations
and reveals that the ferroelectric phase and conductivity can coexist. Our
investigations show that the ferroelectric instability requires only a
short-range portion of the Coulomb force with an interaction range of the order
of the lattice constant. These results provide a new insight into the origin of
ferroelectricity in displacive ferroelectrics and open opportunities for using
doped ferroelectrics in novel electronic devices.
