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

Halobismuthates(III) and haloantimonates(III) with the R3MX6 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., (C2N3H4)3[SbBr6] (TBA). Temperature-resolved structural and spectroscopic studies indicate that TBA undergoes two solid-solid phase transitions between tetragonal [P42/m (I)] and monoclinic [P21/n (II) and P21 (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.

Automated summary

This paper reports a new class of ferroelectric compounds, halobismuthates(III) and haloantimonates(III) with the R3MX6 chemical composition. A specific haloantimonate(III) ferroelectric, (C2N3H4)3[SbBr6] (TBA), is studied and found to undergo two solid-solid phase transitions and one paraelectric-ferroelectric phase transition. The ferroelectric properties of TBA and its molecular origins of polarization are confirmed by experimental and computational methods.