  Ferroelectric materials are characterized by a spontaneous polar distortion.
The behavior of such distortions in the presence of free charge is the key to
the physics of metallized ferroelectrics in particular, and of
structurally-polar metals more generally. Using first-principles simulations,
here we show that a polar distortion resists metallization and the attendant
suppression of long-range dipolar interactions in the vast majority of a sample
of 11 representative ferroelectrics. We identify a meta-screening effect,
occurring in the doped compounds as a consequence of the charge rearrangements
associated to electrostatic screening, as the main factor determining the
survival of a non-centrosymmetric phase. Our findings advance greatly our
understanding of the essentials of structurally-polar metals, and offer
guidelines on the behavior of ferroelectrics upon field-effect charge injection
or proximity to conductive device elements.
