  The possibility that metals may support ferroelectricity is an open issue.
Anderson and Blount showed that certain martensitic transitions involve
inversion symmetry breaking and the formal existence of a polar axis, so
'metallic ferroelectric' behavior has been claimed for metals undergoing a
centrosymmetric (CS) to non-CS structural transformation (Cd2ReO7, LiOsO3) or
natively non-CS (SrCaRu2O), or for ferroelectric insulators whose polar
distortion survives moderate metallicity induced by doping or proximity.
However, none of these systems, nor any other to our knowledge, embodies a
truly ferroelectric metal with native switchable polarization and native
metallicity coexisting in a single phase. Here we report the first-ever
theoretical prediction of such a material. By first-principles calculations, we
show that the layered perovskite Bi5Ti5O17 has a non-zero density of states at
the Fermi level and metal-like conductivity, as well as a spontaneous
polarization in zero field. Further, we predict that the polarization of
Bi5Ti5O17 is switchable both in principle, as the material complies with the
sufficient symmetry requirements, and in practice, as Bi5Ti5O17 can sustain a
sizable potential drop along the polar direction, as needed to revert its
polarization by application of an electric bias.
