SbCl₄: An Exceptional Superhalogen as the Building Block of a Mixed Valence Supercrystal with Unconventional Ferroelectricity

Abstract

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 ab initio evidence that SbCl 4 superhalogen is an exception: its dimerization yields a polar cluster that can be viewed as a quasi-bonded [SbCl 5 ] δ− and [SbCl 3 ] δ+ Lewis acid−base cluster. The symmetry breaking arises from the valence stratification of Sb into Sb 5+ and Sb 3+ as well as their lone pair electrons. When assembled, SbCl 4 clusters form a supercrystal that is thermodynamically stable up to 600 K, with the unique bonding feature of Sb 2 Cl 8 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.

Theoretical study on molecular properties of SbXn (X = F and Cl, n = 1–5) and SbXn− (X = F and Cl, n = 1–6) including spin–orbit coupling

“…Therefore, a direct comparison between MRCI and CCSD(T) in this study is not feasible. A recent DFT calculation with the triple‐ζ basis set (PBE/def2‐TZVP) 8 of SbCl 2 also overestimated the bond length of SbCl 2 compared with those obtained by CCSD(T)/AVQZ and M06‐2X/QZVPP. SbX 2 − (X = F and Cl) is generated by occupying one more electron in the SOMO of SbX 2 (X = F and Cl), as shown in Figure 2c.…”

“…In general, the overestimation of the bond lengths obtained by M06‐2X/QZVPP in comparison to those obtained by CCSD(T)/AVQZ occurs for all bond lengths of SbX n (X = F and Cl, n = 1–5) and SbX n − (X = F and Cl, n = 1–6). In addition, PBE/def2‐TZVP calculations in a recent study 8 overestimated all bond lengths of SbCl n ( n = 1–5) and SbCl n − ( n = 1–6). Different from SbX 3 (X = F and Cl), SbX 3 − (X = F and Cl) has a point group of C 2V , and its ground state is the 2 B 1 state.…”

“…Thus, the SbX 1 bonds of SbX 4 − showed larger bond elongations than the SbX 2 bonds of SbX 4 − . In a recent study that used PBE/def2‐TZVP, both SbCl 4 and SbCl 4 − possess the T d structure because they depicted the molecular structures of SbCl 4 and SbCl 4 − with only one bond length 8 . Therefore, we cannot identify their actual molecular structures.…”

“…Different from SbX 5 (X = F and Cl), SbX 5 − (X = F and Cl) has a C 4V molecular structure. A recent study that used PBE/def2‐TZVP also predicted that SbCl 5 possessed a D 3h structure 8 . This incorrect prediction was also attributed to the absence of vibrational frequency calculations.…”

“…In addition, SbCl 3 has been used as a precursor to synthesize graphene quantum dots functionalized Sb 2 WO 6 7 . Recent theoretical calculations have predicted SbCl 4 to be a superhalogen, whose electron affinity (EA) is larger than that of halogen atoms, thus forming a highly stable anion 8 . Considering the crucial role of SbF n and SbCl n in many applications, it is surprising that systematic quantum chemical calculations involving SbX n (X = F and Cl, n = 1–5) and SbX n − (X = F and Cl, n = 1–6) are scarce; only part of among SbX n (X = F and Cl, n = 1–5) and SbX n − (X = F and Cl, n = 1–6) has been calculated 9,10 .…”