Realization of a strong ferroelectric metal by Nb-doping in strained EuTiO3

Ferroelectric (FE) metals have been attracting attention as they possess both metallicity and ferroelectricity, the two seemingly incompatible physical properties. An important problem for both fundamental research and potential applications is how to realize a strong FE metal. One strategy is to dope a strong FE insulator and turn it into an FE metal, provided that the induced free electrons do not destroy FE distortion. Strained EuTiO3 is a strong FE ferromagnet and a promising candidate for such a strategy. Here, we calculate the structural, electronic, and polarization properties of Nb-doped strained EuTiO3 using the hybrid density-functional theory. The results show that the strained EuTi0.875Nb0.125O3 is a strong FE metal as the Nb doping induces metallicity without weakening the FE distortion. The underlying atomistic mechanism of the coexistence of metallicity and strong ferroelectricity is discussed. The findings show that combining doping and strain engineering is a promising way to realize new EuTiO3-based strong FE metals and may be used in other materials as well.