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The recent observation of a ferroelectric-like structural transition inmetallic LiOsO$_3$ has generated a flurry of interest in the properties ofpolar metals. Such materials are thought to be rare because free electronsscreen out the long-range electrostatic forces that favor a polar structurewith a dipole moment in every unit cell. In this work, we question whetherlong-range electrostatic forces are always the most important ingredient indriving polar distortions. We use crystal chemical models, in combination withfirst-principles Density Functional Theory calculations, to explore themechanisms of inversion-symmetry breaking in LiOsO$_3$ and both insulating andelectron-doped ATiO$_3$ perovskites, A = Ba, Sr, Ca. Although electrostaticforces do play a significant role in driving the polar instability of BaTiO$_3$(which is suppressed under electron doping), the polar phases of CaTiO$_3$ andLiOsO$_3$ emerge through a mechanism driven by local bonding preferences andthis mechanism is `resistant' to the presence of charge carriers. Hence, ourresults suggest that there is no fundamental incompatibility betweenmetallicity and polar distortions. We use the insights gained from ourcalculations to suggest design principles for new polar metals and promisingavenues for further research.

`Ferroelectric' Metals Reexamined: Fundamental Mechanisms and Design Considerations for New Materials