(C₃H₇NH₃)₄Bi_1-SbI₉: 0D hybrid halide perovskite-like compounds with isolated triiodide units

Antimony/bismuth-based organic-inorganic hybrid halide perovskite-like compounds have generated enormous research interest due to their excellent optical properties. Exploration of new compounds and understanding of their structural stability and optoelectronic properties is of utmost importance for practical applications of these materials. We report two new 0D perovskite-like compounds and their solid solution, (C₃H₇NH₃)₄Bi_1-SbI₉, having propyl amine as the spacer cation and iodine as the halide ion. All compounds crystallized in the space group C2/m at room temperature and undergo a phase transition from C2/m to P2₁/c at low temperature (90 K) as observed from the single-crystal study. A low-temperature (250 K, 180 K, 150 K and 90 K) single-crystal study shows that the (PA)₄BiI₉ compound retains the monoclinic space group C2/m until 150 K and undergoes a phase transition to the P2₁/c space group at 90 K. Further, it is observed that ordering, rearrangement and relaxation of the long-chain propyl amine group are primarily responsible for the structural transition. The structure contains [(Bi/Sb)I₆]^3- polyhedra along with linear I₃^- units, giving rise to the formula of (PA)₃(Bi/Sb)I₆·(PA)I₃. The I₃^- units interact poorly while the [MI₆]^3- (M = Bi, Sb) octahedral units interact significantly with spacer cations via the H-bond, resulting in more distortion in these octahedral units. Theoretical calculations revealed that iodide ions have dual roles and contribute largely to both the valence band maxima and conduction band minima in these compounds. From both experimental and theoretical calculations, it is observed that the pristine compounds are of the indirect band gap-type and Sb substitution in (PA)₄Bi_1-SbI₉ led to a gradual decrease in the band gap.