Carrier-mediated control over the soft mode and ferroelectricity in BaTiO 3

We calculate the effects of conduction band electrons (CBEs), introduced by doping or photoexcitation, on the ferroelectricity and phonon dynamics of ${\\mathrm{BaTiO}}_{3}$ (BTO). We show that CBEs destabilize ferroelectricity, which would lower the Curie temperature and coercive field, and might help to improve the speed or efficiency with which polarization domains can be switched in ferroelectric devices. We show that CBEs lower the frequencies of the ${A}_{1}$ soft and ferroelectric modes in BTO's ferroelectric phases, and raise the soft-mode frequency in its paraelectric phase. We also show that femtosecond laser pulses could be used to selectively excite a coherent ${A}_{1}$ ferroelectric-mode phonon. This would allow this much-studied excitation to be monitored by pump-probe spectroscopy as it decays into other modes. We show that many of the properties of doped and undoped BTO have simple and intuitive explanations, within an ionic picture of BTO's bonding, if it is assumed that ferroelectricity is not driven by long-range interactions, but by the attraction between Ti and O neighbors. Most of the effects of CBEs are consequences of them reducing ions' charges and increasing their polarizabilities, thereby weakening the Ti-O attraction. As the CBE density increases, so does the density of delocalized interstitial electrons, which would increase conductivity. We argue that a polar metallic phase exists if the threshold for metallic conductivity can be reached before the CBEs make polar distortions energetically unfavorable by critically weakening the Ti-O attraction.