![]() (36) To further confirm and better elucidate this preliminary evidence, it would be desirable to cover a more extended compositional range in terms of organic cations and halides and to broaden the temperature range by including low-temperature data to analyze a detailed T evolution of local and average structures. Such a trend has been correlated to the inherent instability of high-symmetry coordination for ns 2p 0 cations. (36) In addition, an enhancement of these displacements has been observed upon replacement of FA for MA or Br for I (which has been however determined in ref (36) on only lead-based perovskites). (36) For these two compositions, Laurita and co-workers investigated the short-range order by X-ray PDF analysis from room temperature (RT) to ∼480 K, highlighting dynamic, temperature-activated B-site cation off-centering displacements at and above RT as a consequence of the lone pair stereochemical activity. On the contrary, the local structure landscape of lead-free MHPs is still poorly characterized, being limited to MASnI 3 and FASnI 3, despite the huge interest in replacing highly toxic lead with other elements like tin. (36,39−41) Because of these sets of local structure data, obtained through the use of total scattering methods coupled to pair distribution function (PDF) analysis, it was possible, for example, to unveil a common pattern of orthorhombic distortion at the local scale (1–10 Å) for the MAPbX 3 compositions. (27,38) Similar studies have also been carried out on other lead-based MHPs containing the MA or FA cation, namely, MAPbBr 3, MAPbCl 3, FAPbI 3, and FAPbBr 3. (35−37) Most of the efforts to date to understand the local structure of MHPs have been devoted to MAPbI 3, for which the role of MA cation disorder was investigated in great detail both at the average and at the local structure level. (28,33,34) For example, it has been shown that the broad energy distribution of the photoluminescence maximum in polycrystalline samples of FASnI 3 might be due to the activity of the lone pair of the Sn cation, which causes an off-centering of the cation, lowering the symmetry at the local level. (11,28−32) Another interesting aspect influencing the local structure of MHPs is the possible activity of the lone pair, because group 14 elements have an s 2p 0 electronic configuration that can be potentially involved in symmetry-lowering distortions associated with the pseudo- or second-order Jahn–Teller effects. (25−27) At the same time, the growing consensus about the presence of local octahedral distortions in MHPs, which may have an impact on the charge-carrier dynamics, makes a proper short-range order correlation between structure and properties key to fully addressing their microscopic description. However, the presence of weak chemical interactions when organic cations are present, namely, MA and FA, made the definition of the crystal structure by diffraction quite elusive, due to the presence of an order–disorder transition as a function of temperature. (10−24) Such a rich set of experimental and computational data led to a good understanding of the average structure of MHPs and provided the basis for structure–property correlations. In recent years, together with the investigation of their functional properties, significant efforts have been directed toward a thorough comprehension of their crystal structure, the nature and type of possible phase transitions, and their phase stability under extreme conditions. (1−9) These compounds have a general formula of ABX 3, where A is a small organic or inorganic cation, such as methylammonium (CH 3NH 3 +, MA), formamidinium, or cesium (Cs), B is usually Pb, Sn, or Ge, and X is a halide, notably I, Br, or Cl. Metal halide perovskites (MHPs) are appealing materials because of their manifold applications, from photovoltaics to photocatalysis, and have attracted considerable attention in the past several years. The averaged structure obtained from molecular dynamics simulations was consistent with experimental local structures determined via X-ray PDF, thus highlighting the robustness of computational modeling and strengthening the correlation between experimental and computational results. Electronic structure calculations provided good agreement with experimental band gaps for the four perovskites when local dynamical distortions were included in the calculations. These studies revealed that that none of the four perovskites has a cubic symmetry at the local scale and that a degree of increasing distortion is always present, in particular when the cation size is increased, i.e., from MA to FA, and the hardness of the anion is increased, i.e., from Br – to I –. Tin-based metal halide perovskites with a composition of ASnX 3 (where A= MA or FA and X = I or Br) have been investigated by means of X-ray total scattering techniques coupled to pair distribution function (PDF) analysis.
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