A family of ionic supersalts with covalent-like directionality and unconventional multiferroicity

Ionic crystals composed of elemental ions such as NaCl are non-polar due to directionless ionic bonding interactions. Here, we show that these can develop polarity by changing their building blocks from elemental ions to superalkali and superhalogen cluster-ions, which mimic the chemistry of alkali and halogen atoms, respectively. Due to the non-spherical geometries of these cluster ions, corresponding supersalts form anisotropic polar structures with ionic bonding, yet covalent-like directionality, akin to sp(3) hybridized systems. Using density functional theory and extensive structure searches, we predict a series of stable ferroelectric/ferroelastic supersalts, PnH(4)MX(4) (Pn = N, P; M = B, Al, Fe; X = Cl, Br) composed of superalkali PnH(4) and superhalogen MX(4) ions. Unlike traditional ferroelectric/ferroelastic materials, the cluster-ion based supersalts possess ultra-low switching barrier and can endure large ion displacements and reversible strain. In particular, PH(4)FeBr(4) exhibits triferroic coupling of ferroelectricity, ferroelasticity, and antiferromagnetism with controllable spin directions via either ferroelastic or 90-degree ferroelectric switching.