  Using density-functional calculations we study the structure and polarization
response of tetragonal PbTiO3, BaTiO3 and SrTiO3 in a strain regime that is
previously overlooked. Different from common expectations, we find that the
polarizations in all three substances saturate at large strains, demonstrating
a universal phenomenon. The saturation is shown to originate from an unusual
and strong electron-ion correlation that leads to cancellation between
electronic and ionic polarizations. Our results shed new insight on the
polarization properties, and reveal the existence of a fundamental limit to the
strain-induced polarization enhancement.
