Doping A Dipole into an Incipient Ferroelectric: Route to Relaxor Ferroelectrics

TiO2 in the rutile phase is known to be an incipient ferroelectric. Considering Nb-Cr codoping we examine if ferroelectricity can be induced at the low doping limit in Ti(1-x)(Nb0.5Cr0.5)(x)O-2 (x = 0.05%, 1%, 5%, and 10%). A relaxor behavior is found in the temperature range 20-120 K which obeys the Vogel-Fulcher relation while pyrocurrent measurements confirm switching of the electric polarization. The spontaneous net polarization is doping dependent with a maximum at 1% and for doping concentrations above 5% is found to be paraelectric. Ab initio density functional theory based calculations suggest that the Nb-Cr pair behaves like a dipole and polarizes the neighboring TiO6 octahedra, stabilizing a ferroelectric ground state akin to magnetic impurities in dilute magnetic semiconductors. At larger doping concentrations one finds that Nb-Cr clusters result in a vanishing polarization.