Heterovalent BiIII/PbIIIonic Substitution in One-Dimensional Trimethylsulfoxonium Halide Pseudo-Perovskites (X = I, Br)

We report on the synthesis and characterization of novel lead and bismuth hybrid (organic-inorganic) iodide and bromide pseudo-perovskites (ABX3) containing the trimethylsulfoxonium cation (CH3)3SO+ (TMSO) in the A site, Pb/Bi in the B site, and Br or I as X anions. All of these compounds are isomorphic and crystallize in the orthorhombic Pnma space group. Lead-based pseudo-perovskites consist of one-dimensional (1D) chains of face-sharing [PbX6] octahedra, while in the bismuth-based ones, the chains of [BiX6] are interrupted, with one vacancy every third site, leading to a zero-dimensional (0D) local structure based on separated [Bi2I9]3- dimers. Five solid solutions for the iodide with different Pb2+/Bi3+ ratios between (TMSO)PbI3 and (TMSO)3Bi2I9, and two for the bromide counterparts, were synthetized. Due to the charge compensation mechanism, these systems are best described by the (TMSO)3Pb3xBi2(1-x)I9 (x = 0.98, 0.92, 0.89, 0.56, and 0.33) and (TMSO)3Pb3xBi2(1-x)Br9 (x = 0.83 and 0.37) formulae. X-ray powder diffraction (XRPD) measurements were employed to determine the crystal structure of all studied species and further used to test the metal cation miscibility within monophasic samples not showing cation segregation. These systems can be described through an ionic defectivity on the pseudo-perovskite B site, where the Pb2+/Bi3+ replacement is compensated by one Pb2+ vacancy for every Bi3+ pair. This leads to a wide range of possible different (numerical and geometrical) chain configurations, leading to the unique features observed in XRPD patterns. The optical band gap of the iodide samples falls in the 2.11-2.74 eV range and decreases upon increasing the Bi3+ content. Interestingly, even a very low loading of Bi3+ (1%) is sufficient to reduce the band gap substantially from 2.74 to 2.25 eV. Periodic density functional theory (DFT) calculations were used to simulate the atomic and electronic structures of our samples, with predicted band gap trends in good agreement with the experimental ones. This work highlights the structural flexibility of such systems and accurately interprets the ionic defectivity of the different pseudo-perovskite structures.