Temperature symmetry breaking and properties of lead-free organic-inorganic hybrids: bismuth(III) iodide and antimony(III) iodide: (S(CH 3 ) 3 ) 3 [Bi 2 I 9 ] and (S(CH 3 ) 3 ) 3 [Sb 2 I 9 ].

We have synthesized and characterized two novel lead-free organic-inorganic hybrid crystals: (S(CH 3 ) 3 ) 3 [Bi 2 I 9 ] (TBI) and (S(CH 3 ) 3 ) 3 [Sb 2 I 9 ] (TSI). Thermal DSC, TG, and DTA analyses indicate structural phase transitions (PTs) in both compounds; TBI undergoes two structural phase transitions at 314.2/314.8 K (cooling/heating) and at 181.5 K of first (I ↔ II) and second order (II ↔ III), respectively. The crystal structures of TBI are refined for phases I (325 K), II (200 K) and III (100 K). TBI exhibits ferroelastic properties since both PTs are accompanied by a change in the symmetry of crystals: P 6 3 / mmc → C 2/ c (I → II) and C 2/ c → P 1̄ (II → III). The presence of a ferroelastic domain structure has been confirmed by optical observations. In turn, TSI also reveals two PTs: I ↔ II (at 303.9/304.1 K) and II ↔ III (212.9/221.4 K). To compare and obtain insight into the mechanism of the PTs of TBI, we have carried out temperature dependent single crystal X-ray diffraction studies. Additionally, to confirm the change in the dynamical states of molecules in PTs, dielectric measurements have been carried out between 100 K and 400 K in the frequency range of 200 Hz to 2 MHz. Moreover, the measurements of the 1 H NMR spin-lattice relaxation time, T 1 , and a second moment, M 2 , of the 1 H NMR line have been undertaken in the temperature range between 100 and 300 K.