(C3N2H5)3Sb2I9 and (C3N2H5)3Bi2I9: ferroelastic lead-free hybrid perovskite-like materials as potential semiconducting absorbers.

We have synthesised and characterised novel organic-inorganic hybrid crystals: (C3N2H5)3Sb2I9 and (C3N2H5)3Bi2I9 (PSI and PBI). The thermal DSC and TG analyses indicate four structural phase transitions (PTs) at 366.2/366.8, 274.6/275.4, 233.3/233.3 and 142.8/143.1 K (on cooling/heating) for PSI and two reversible PTs at 365.2/370.8 and 252.6/257.9 K for PBI. Both analogues crystallize at room temperature in the orthorhombic Cmcm structure, which transforms, in the case of PBI, to monoclinic P21/n at low temperature. According to the X-ray diffraction results, the anionic component is discrete and built of face-sharing bioctahedra, [M2I9]3-, in both compounds, whereas cations exhibit distinct dynamical disorder over high temperature phases. Dielectric spectroscopy and 1H NMR spectroscopy have been used to characterise the dynamical state of the C3N2H5+ cations. The ferroelastic domain structure has been characterised by observations under a polarized optical microscope. Both compounds are semiconductors with narrow bandgaps of 1.97 eV (PBI) and 2.10 eV (PSI).