‘Ferroelectric’ metals reexamined: fundamental mechanisms and design considerations for new materials

Abstract

The recent observation of a ferroelectric-like structural transition in metallic LiOsO3 has generated a flurry of interest in the properties of polar metals. Such materials are thought to be rare because free electrons screen out the long-range electrostatic forces that favor a polar structure with a dipole moment in every unit cell. In this work, we question whether long-range electrostatic forces are always the most important ingredient in driving polar distortions. We use crystal chemical models, in combination with first-principles Density Functional Theory calculations, to explore the mechanisms of inversion-symmetry breaking in LiOsO3 and both insulating and electron-doped ATiO3 perovskites, A = Ba, Sr, Ca. Although electrostatic forces do play a significant role in driving the polar instability of BaTiO3 (which is suppressed under electron doping), the polar phases of CaTiO3 and LiOsO3 emerge through a mechanism driven by local bonding preferences and this mechanism is 'resistant' to the presence of charge carriers. Hence, our results suggest that there is no fundamental incompatibility between metallicity and polar distortions. We use the insights gained from our calculations to suggest design principles for new polar metals and promising avenues for further research.

Pressure-induced insulator–metal transition in EuMnO3

“…We also found that the non-centrosymetric E * -AFM state is metallic in EuMnO 3 and tends to be nearly degenerate with the FM ground state at high pressures. Thus, EuMnO 3 hosts a potential realization of a new type of (magnetically-induced) ferroelectric metal that can add an extra dimension to the thought-provoking question of ferroelectricity emerging in metals [31][32][33][34][35] . These findings are expected to motivate further experimental and theoretical work.…”

Effect of intrinsic point defects on ferroelectric polarization behavior of SrTiO3

“…O defect complexes may also contribute to the resistive switching in Ti-rich SrTiO 3 ; however, the interplay between polarization and metallic conductivity being in the focus of many recent studies of perovskite oxides [38,39] deserves a separate detailed investigation.…”

Phase stability and large in-plane resistivity anisotropy in the 112-type iron-based superconductor Ca1−xLaxFeAs2

“…This is a very interesting observation given the fact that Ca is by far the most electropositive element in this compound and has a closed shell configuration, and as a result, is not chemically active and should not be the driving force of a polar distortion. It is neither in a particular coordination geometry that is known to give rise to some type of geometric-ferroelectricity, which would be robust even though the compound is conducting [46,47].…”