Heterovalent Bi^III/Pb^II Ionic Substitution in One-Dimensional Trimethylsulfoxonium Halide Pseudo-Perovskites (X = I, Br)

Abstract

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)3Pb3x Bi2(1–x)I9 (x = 0.98, 0.92, 0.89, 0.56, and 0.33) and (TMSO)3Pb3x Bi2(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.

Bi3+ doping in 1D ((CH3)3SO)PbI3: a model for defect interactions in halide perovskites

“…(TMSO)3Pb0.99Bi1.34I9 was prepared as described in ref. 19. The extended X-ray absorption fine structure (EXAFS) spectra were acquired at beamline LISA of the ESRF and analyzed with Viper 27 .…”

“…In particular, the mechanism (i) was previously assumed -from stereochemical considerations -to be the primary charge compensation mechanism. 19 To corroborate this hypothesis and evaluate the possibility of coexisting charge compensation mechanisms, a computational study on the energy formation and the geometrical parameters for these defects complexes is carried out in the following.…”

“…TMSOPbI3 and (TMSO)3Bi2I9 19 . In this respect, the supercells described below are realistic descriptions of the structure and composition of the doped compounds at various Bi levels between 12% and 67%.…”

“…While the insertion of Bi 3+ cations in the perovskite B-site gives rise to a single-phase roughly following the Vegard law, very peculiar X-ray powder diffraction peak shapes are observed at higher Bi content hinting at significant short-range order, driven by first neighbor interactions between B-sites 19 . Atomistic simulations were therefore carried out to study the energetics of different defect configurations in highly defective supercells.…”

“…18 We recently described the crystal structure and phase diagram of the (TMSO)PbX3/(TMSO)3Bi2X9 system with X=(I, Br), reporting complete miscibility of Pb 2+ /Bi 3+ in the B-site throughout the composition range. 19 This work explores the defect formation mechanisms needed to compensate for the difference in charge between lead and bismuth. Our simulations show that the formation of Pb 2+ vacancies for each two Bi 3+ dopant atoms is the most probable mechanism, confirming the previous assumption from XRD data.…”