STRUCTURAL PROPERTIES DETERMINING THE NEAR-EDGE X-RAY ABSORPTION SPECTRA OF LEAD HALIDE PEROVSKITES

X-ray absorption spectroscopy (XAS) is an excellent complement to diffraction techniques for studying the structure of materials. Despite the extensive research on lead halide perovskites for optoelectronic applications, the application of XAS to these materials has been relatively limited and yielded varying degrees of success. In order to develop generalizable approaches for analyzing XAS spectra of halide perovskites with different compositions, we conducted an experimental and computational study on a hybrid Pb/Bi iodide solid solution, serving as a model system. The monodimensional lead halide “perovskite” (TMSO)3Pb3xBi2(1-x)I9 [1] exhibits correlated disorder at the metal cation site, resulting in various possible arrangements of Pb, Bi, and metal vacancies (Figure 1). Through simulations, we observed that the X-ray absorption near-edge structure spectra (at the Pb, Bi, and I X-ray absorption edges) show some sensitive to these alternations. Surprisingly, we discovered that the cation spectra can be explained by simple distortions of independent PbI6/BiI6 octahedral units, without considering long-range multiple scattering contributions that typically dominate the near-edge region.[2] This finding enables the prediction and modeling of X-ray absorption near-edge spectra using simple structural units, suggesting that similar approaches can be successfully extended to other halide perovskites in the future

Heterovalent BiIII/PbII ionic 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 12inorganic) iodide and bromide pseudo-perovskites (ABX3) containing the trimethylsulfoxonium cation (CH3)3SO+ (TMSO) in the A site, Pb/Bi in the Bsite, 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 facesharing [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 12 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 12x)I9 (x = 0.98, 0.92, 0.89, 0.56, and 0.33) and (TMSO)3Pb3xBi2(1 12x)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 2.11 122.74 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

The structural versatility of proton sponge bismuth halides

Hybrid halometalates containing lead, tin, bismuth and antimony and organic cations have recently shown a bevy of interesting photophysical properties. Aiming at finding chemically stable and thermally inert species, three halobismutate species of this class, crystallized with proton sponge-derived cations (PRSH), have been isolated as microcrystalline powders by mixing 1,8-bis(dimethylamino)-naphthalene (proton sponge, or PRS) and bismuth oxide in concentrated HX acids (X = Cl, Br and I). The two isomorphous (PRSH)3Bi2X9 (X = Br, I) species, containing isolated [Bi2X9]3- anions, are triclinic at room temperature and convert upon heating into a monoclinic structure through a displacive phase transformation, fully reversible for X = I and only partially for X = Br. At variance, (PRSH)BiCl4 is polymeric, and contains extended zigzagging 1D chains formed by edge-sharing BiCl6 octahedra. These species were extensively studied by synchrotron and laboratory X-ray powder diffraction measurements, which enabled to detect the evolution toward the two high-temperature beta-phases (X = Br, I), to derive the structure of five different (significantly complex) species and to assess the thermal strain tensors in the different regimes. Additional thermal, spectroscopic and computational analyses completed the characterization of these materials

Modeling bismuth insertion in 1D hybrid lead halide TMSO(PbxBiy)I3 pseudo-perovskites

The structures of the disordered 1D (pseudo-)perovskites of general TMSO(PbxBiy)I3formulation [TMSO = (CH3)3SO+], obtained by doping the TMSOPbI3species with Bi3+ions, are investigated through the formulation of a statistical model of correlated disorder, which addresses the sequences of differently occupied BI6face-sharing octahedra (B = Pb, Bi or vacant site) within ideally infinite [(BI3)-]nchains. The x-ray diffraction patterns simulated on the basis of the model are matched to the experimental traces, which show many broad peaks with awkward (nearly trapezoidal) shapes, under the assumption that the charge balance is fully accomplished within each chain. The analysis allowed to establish a definite tendency of the metal species to cluster as pure Pb and Bi sequences. The application of the model is discussed critically, in particular as what concerns the possibility that further B-site neighbors beyond the second may influence the overall B-site occupancies

Molecular Design and Crystal Chemistry of Polyfluorinated Naphthalene-bis-phenylhydrazimides with Superior Thermal and Polymorphic Stability and High Solution Processability

Naphthalene tetracarboxylic diimides (NDIs) are highly promising air-stable n-type molecular semiconductor candidates for flexible and cost-effective organic solar cells and thermoelectrics. Nonetheless, thermal and polymorphic stabilities of environmentally stable NDIs in the low-to-medium temperature regime (<300°C) remain challenging properties. Structural, thermal, spectroscopic and computational features of polyfluorinated NDI-based molecular solids (with up to 14 F atoms per NDI molecule) are discussed upon increasing the fluorination level. Slip-stacked arrangement of the NDI cores with suitable p-p stacking and systematically short interplanar distances (< 3.2 Å) are found. All these materials exhibit superior thermal stability (up to 260 °C or above) and thermal expansion coefficients indicating a response compatible with flexible polymeric substrates, Optical band-gaps increase from 2.78 to 2.93 eV with fluorination, while LUMO energy levels decrease down to -4.37 eV, as per DFT calculations. The compounds exhibit excellent solubility of 30 mg mL-1 in 1,4-dioxane and DMF