Persistence of Ferroelectricity in BaTiO3 through the Insulator-Metal Transition

The ferroelectric BaTiO3 is a band-gap insulator. Itinerant electrons can be introduced in this material by doping, for example, with oxygen vacancies. Above a critical electron concentration of n(c) approximate to 1 x 10(20) cm(-3). BaTiO3-delta becomes metallic. This immediately raises a question: Does metallic BaTiO3-delta still retain ferroelectricity? One may expect itinerant electrons to destroy ferroelectricity as they screen the long-range Coulomb interactions. We followed the phase transitions in BaTiO3-delta as a function of n far into metallic phase. Although their stability range decreases with n, the low-symmetry phases in metallic BaTiO3-delta are still retained up to an estimated concentration of n* approximate to 1.9 x 10(21) cm(-3). Moreover, it appears that the itinerant electrons partially stabilize the ferroelectric phases in metallic BaTiO3-delta by screening strong crystal field perturbations caused by oxygen vacancies.