Progressive Structural Complexity in Ferroelectric 1,2,4-Triazolium Hexabromoantimonate(III): Interplay of “Order–Disorder” and “Displacive” Contributions to the Structural Phase Transitions

[Image: see text] Halobismuthates(III) and haloantimonates(III) with the R(3)MX(6) chemical composition create a new and broadly unexplored class of ferroelectric compounds. In this paper, we report the haloantimonate(III) ferroelectric comprising an aromatic (1,2,4-triazolium) cation, i.e., (C(2)N(3)H(4))(3)[SbBr(6)] (TBA). Temperature-resolved structural and spectroscopic studies indicate that TBA undergoes two solid–solid phase transitions between tetragonal [P4(2)/m (I)] and monoclinic [P2(1)/n (II) and P2(1) (III)] phases. TBA experiences a paraelectric–ferroelectric phase transition at 271/268 K (II–III) driven by “order–disorder” and “displacive” molecular mechanisms. The ferroelectric properties of phase III have been confirmed by hysteresis loop measurement, and additionally, the acentric order has been further supported by second-harmonic generation measurements. Insight into the molecular origins of the ferroelectric polarization was provided by periodic ab initio calculations using the Berry phase approach at the density functional theory (DFT-D3) method level employed for calculations of spontaneous polarization.