Coexistence of Polar Distortion and Metallicity in PbTi_1-xNb_xO₃

Ferroelectricity has been believed unable to coexist with metallicity since the free carriers can screen the internal coulomb interactions of dipoles. Very recently, one kind of materials called as ferroelectric metal was reexamined. Here, we report the coexistence of metallicity and polar distortion in a new candidate for ferroelectric metal PbTi_1-xNb_xO₃ via doping engineering. The ferroelectric-like polar distortion in all the doped PbTi_1-xNb_xO₃, with x ranging from 0.04 to 0.12, was observed by the piezoresponse force microscopy and the scanning transmission electron microscopy measurements. PbTi_1-xNb_xO₃ films become more conductive with more doping density, and emerge a metallic behavior when x reaches 0.12. Our first principles calculations further revealed that the doped Nb ions in the films can only provide free electrons, but not be able to damage the dipoles in unite cells even with the heaviest doping density of 0.12. We believe that these results confirm a feasibility of realizing the coexistence of metallicity and polar distortion for other ferroelectrics in a common way, and motivate the synthesis of some new materials with artificially designed properties even incompatible in nature.