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Mixed halide perovskites are emerging as promising candidates for wide‐bandgap components in tandem solar cells and color‐tunable light‐emitting diodes. Yet, halide mixing poses a fundamental question of whether the inhomogeneous halide distribution impacts the intrinsic electronic disorder in these materials. To address this point, density functional theory (DFT)‐based molecular dynamics (MD) simulations are performed for pure and mixed halide perovskites, accounting for disorder stemming from inhomogeneous chemical composition associated with the halide component and from finite temperature effects. For pure halide perovskites, finite‐temperature band gap fluctuations from the MD simulations are in good agreement with the broadening measured using photoluminescence. Furthermore, these calculations confirm the natively modest inhomogeneous disorder in the electronic structure of these materials. Most notably, such a low degree of electronic disorder is preserved in models mimicking finely intermixed Br/I solid‐state solutions. In contrast, models featuring halide segregation show comparably wider band gap fluctuations, with a sizable contribution from inhomogeneous (static) broadening, which is associated, at least in part, with structural distortions stemming from lattice mismatch. [ABSTRACT FROM AUTHOR]

Halide mixing is a key strategy to tune the emission wavelength in lead halide perovskites but is also effective in improving the performance of halide perovskite solar cells. Yet, a clear and global picture of how halide alloying and related spatial in/homogeneous distribution influence the electronic and optical properties of halide perovskites is currently lacking. Considering the preeminent mixed iodine/bromine perovskite as a case study, state‐of‐the‐art hybrid density functional theory calculations are performed, exploring the full space of chemical composition and accounting for phase segregation effects. It is shown that, at low doping regime, halide impurities do not act as trap states and that a quasi‐linear opening of the bandgap occurs with increasing the bromine content. Phase segregation at the nanoscale, in turn, drastically affects the electronic structure of mixed halide systems, with namely bromine rich domains acting as barrier to hole diffusion. The energetic disorder probed by optical absorption in these mixed systems is surprisingly insensitive to configurational disorder, but mostly dominated by the coexistence of phases with various degrees of halide segregation. [ABSTRACT FROM AUTHOR]

Solid state NMR spectroscopy is swiftly emerging as useful tool to characterize the structure, composition and dynamic properties of lead halide perovskites. On the other hand, interpretation of solid state NMR signatures is often challenging, because of the potential presence of many overlapping signals in small range of chemical shifts, hence complicating the extraction of detailed structural features. Here, we demonstrate the reliability of periodic Density Functional Theory in providing theoretical support for the NMR characterization of halide perovskite compounds, considering nuclei with spin I=1/2. For light 1H and 13C nuclei, we predict NMR chemical shifts in good agreement with experiment, further highlighting the effects of motional narrowing. Accurate prediction of the NMR response of 207Pb nuclei is comparably more challenging, but we successfully reproduce the downshift in frequency when changing the halide composition from pure iodine to pure bromine. Furthermore, we confirm NMR as ideal tool to study mixed halide perovskite compounds, currently at the limelight for tandem solar cells and color‐tunable light emission. [ABSTRACT FROM AUTHOR]

Density functional theory calculations with periodic boundary conditions have been carried out for the 3D crystal cells of α and γ polymorphs of even polyamides (ranging from nylon-4 up to nylon-12) in order to investigate the relative stability of the two forms. The infinite 1D polymer chains possessing α and γ conformations have been also simulated to shed light on the intramolecular effects taking place in these systems and to discriminate the intermolecular effects in terms of hydrogen bonding and packing of CH 2 units. The results of this analysis allowed to give a detailed interpretation of the polymorphism properties observed in even nylons, predicting a larger stability for the α form in nylon-4 and nylon-6 and a stabilization of the γ phase from nylon-8 up to nylons having a larger number of CH 2 unit in the chain. [ABSTRACT FROM AUTHOR]

Abstract: The IR spectrum of Nylon 6,6 in the crystalline α phase is computed by means of dispersion-corrected density functional theory calculations carried out with periodic boundary conditions on the crystal. The results are carefully compared to experimental spectra through a detailed analysis of different frequency regions and focusing on the spectroscopic markers of crystallinity or regularity (i.e. of the regular conformation of the polymer chain). The previous assignments, based exclusively on experimental or semi-empirical investigations, are critically revised, demonstrating that state of the art computational methods in solid state chemistry can provide tools to obtain an unambiguous description of the vibrational properties of the crystalline phases of macromolecular materials. In particular, the ambiguities related to the assignment of some debated bands of crystallinity/regularity are solved. The structural and vibrational properties are interpreted on the basis of the peculiar intra and intermolecular interactions occurring in polyamides. [Copyright &y& Elsevier]