Pressure-induced reconstructive phase transitions, polarization with metallicity, and enhanced hardness in antiperovskite MgCNi3.

In general, hydrostatic pressure can suppress electrical polarization, instead of creating and/or enhancing polarization like strain engineering. Here, a combination of first-principles calculations and CALYPSO crystal structures prediction is used to point out that hydrostatic pressure applied on antiperovskite MgCNi3 can stabilize polarization with metallicity, and thus a polar metal can exist under high pressure. Strikingly, the metallic polar phase of MgCNi3 exhibits an original linear-cubic coupling between polar and nonpolar modes, resulting in an asymmetrical double-well when the polarization is switched. Moreover, another novel phase of MgCNi3 under high pressure possesses an enhanced hardness stemming from a robust s-s electrons interaction of an unexpected C-C bond, rather than typical sp3 orbital hybridization. These discoveries open new routes to design superhard materials and polar metals.