SbCl: An Exceptional Superhalogen as the Building Block of a Mixed Valence Supercrystal with Unconventional Ferroelectricity.

Superhalogens are nanoclusters with high electron affinities, exhibiting behavior similar to that of halogens. Their dimerization yields nonpolar symmetrical clusters, akin to diatomic halogen molecules, and they are unstable in the condensed phase in the absence of charge-compensating cations. Herein, we provide evidence that SbCl superhalogen is an exception: its dimerization yields a polar cluster that can be viewed as a quasi-bonded [SbCl] and [SbCl] Lewis acid-base cluster. The symmetry breaking arises from the valence stratification of Sb into Sb and Sb as well as their lone pair electrons. When assembled, SbCl clusters form a supercrystal that is thermodynamically stable up to 600 K, with the unique bonding feature of SbCl prevailing in the bulk phase. Combination of mixed valence and lone pair electrons leads to electric polarizations along all directions, generating a type of unconventional multimode ferroelectricity in which three different modes of ferroelectricity with distinct magnitudes and Curie temperature are revealed.