Unprecedented solid-state chemical reaction—from image. From centrosymmetric to non-centrosymmetric crystal structure

Tris(imidazolium) hexabromoantimonate(III) hydrate, (C3N2H5)3SbBr6·H2O(C3N2H5)3SbBr6·H2O, (abbreviated as TIBA) has been synthesized and characterized by X-ray (at 295, 225, 160, and 110 K), differential scanning calorimetry, dilatometry, and dielectric spectroscopy. At room temperature (phase I), the structure consists of discrete SbB6− anions, disordered imidazolium cations, and water molecules forming a 3D array of hydrogen bonds. Below room temperature, TIBA was found to undergo isostructural discontinuous phase transition at 212/221 K (cooling–heating) (P21/c↔P21/c)(P21/c↔P21/c). The phase transition mechanism is characterized by two contributions: an order–disorder (cationic substructure) and a displacive (water molecules) one. At high temperatures, the TIBA crystal was found to undergo an unprecedented in situ solid-state chemical reaction: equation(1) 2(C3N2H5)3SbBr6·H2O(S)→(C3N2H5)5Sb2Br11(S)+(C3N2H5)Br(S)+2H2O2(C3N2H5)3SbBr6·H2O(S)→(C3N2H5)5Sb2Br11(S)+(C3N2H5)Br(S)+2H2O Turn MathJax on This chemical transformation leads to multiphase crystallites dominated by an amorphous phase of (C3N2H5)5Sb2Br11(C3N2H5)5Sb2Br11. The creation of ferroelectric crystallites – (C3N2H5)5Sb2Br11(C3N2H5)5Sb2Br11 – in an “annealed” sample of (C3N2H5)3SbBr6·H2O(C3N2H5)3SbBr6·H2O was confirmed by X-ray diffraction phase analysis, dielectric spectroscopy, and pyroelectric measurements. The dielectric response of the electric permittivity and the critical slowing down of the process observed near 140 K in the “annealed” sample of TIBA are treated as a “fingerprint” of a neat (C3N2H5)5Sb2Br11(C3N2H5)5Sb2Br11 ferroelectric.