The motley family of polar compounds (MV)[M(X5−xX′x)] based on anionic chains of trans-connected M(III)(X,X′)6 octahedra (M=Bi, Sb; X, X′=Cl, Br, I) and methylviologen (MV) dications

The search for hybrid organic-inorganic materials remains a great challenge in the field of ferroelectrics. Following the discovery of the room temperature ferroelectric material (MV)[BiI3Cl2] (MV2+: methylviologen) exhibiting the highest polarization value in the field of hybrid ferroelectrics, we report here nine new hybrids with the general formulation (MV)[M(III)X5−xX′x] (M=Bi, Sb; X, X′=Cl, Br, I): (MV)[BiCl3.3Br1.7] (1), (MV)[BiCl1.3Br3.7] (2), (MV)[BiBr3.2I1.8] (3), (MV)[SbCl5] (4), (MV)[SbBr5] (5), (MV)[SbCl3.8Br1.2] (6), (MV)[SbCl2.4Br2.6] (7), (MV)[SbI3Cl2] (8) and (MV)[SbBr3.8I1.2] (9). Depending on the presence of polar chains or not, and on the coupling of polar chains, two types of centrosymmetrical structures [C1] and [C2] and two types of polar structures [P1] and [P2] are defined. (2) undergoes a paraelectric-to-relaxor ferroelectric transition around 100–150 K depending of the frequency showing that the Curie temperature, TC, of (MV)[BiBr5] (243 K) can be modulated by the substitution of Br by Cl. The most interesting family is the [P2] type because the syn coupling of polar chains is in favor of high polarization values, as in (MV)[BiI3Cl2]. Five of the nine new hybrids, (4), (6–9), which have the [P2] type structure are potential ferroelectrics. - Graphical abstract: The methylviologen haloantimonate (MV)[SbX5−xX′x] families (X, X′=Cl, Br, I) – [P1] and [P2] are the two kinds of polar structures – and view of the (MV)[SbBr3.8I1.2] hybrid based on chiral polar chains which are in syn coupling. Highlights: ► Nine hybrids based on methylviologen and halometalate chains have been discovered. ► The polar nature of chains is due to the ns2 stereoactivity of Sb(III) or Bi(III). ► A syn or anti coupling of polar chains leads to two types of polar structures. ► Five of the nine hybrids (syn coupling) are potential interesting ferroelectrics. ► The para-to-ferroelectric TC depends on the bromide content x of (MV)[BiCl5−xBrx].