# Structural Features Governing the Near-Edge X-ray Absorption Spectra of Lead Halide Perovskites

Simone Virga, Alessandro Longo, Candida Pipitone, Francesco Giannici

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Simone Virga, Alessandro Longo, Candida Pipitone, Francesco Giannici. Structural Features Governing the Near-Edge X-ray Absorption Spectra of Lead Halide Perovskites. Journal of Physical Chemistry C, 2023, 127 (36), pp.18058–18066. 10.1021/acs.jpcc.3c03604. [hal-05095270]

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# Structural Features Governing the Near-Edge X-ray Absorption Spectra of Lead Halide Perovskites

Simone Virga, Alessandro Longo, Candida Pipitone, and Francesco Giannici*

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Cite This: J. Phys. Chem. C 2023, 127, 18058–18066

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Supporting Information

**ABSTRACT:** X-ray absorption spectroscopy (XAS) represents the ideal complement to diffraction techniques to investigate the structure of materials. Despite its strong points and the abundance of studies devoted to lead halide perovskites for application in optoelectronics, XAS has been applied to lead halide perovskites rather infrequently to date, and with varying degrees of success. In the search of generalizable approaches to be applied to data analysis of XAS spectra of halide perovskites with different compositions, we present here an experimental and computational study on a hybrid Pb/Bi iodide solid solution as a model for halide perovskites with different compositions.

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## 1. INTRODUCTION

Since their first use in 2009 as photosensitizers in solar cells,¹ hybrid lead halide perovskites have been at the forefront of materials chemistry research for almost two decades. Every aspect of composition, morphology, and processing of methylammonium lead iodide has been the subject of scrutiny and optimization, leading now to almost 30% efficiency in tandem solar cells.² However appealing, lead iodide perovskites suffer from two fundamental issues, namely, lead toxicity and limited stability to ambient conditions, stimulating in turn many lines of investigation in materials chemistry. In search of other p-block cations to substitute lead, and improved ambient stability, many different organic–inorganic hybrid metal halide compositions have been proposed recently.³,⁴ They may often crystallize layered, monodimensional, or zero-dimensional structures, collectively referred to as perovskites, although they can display various kinds of octahedral connectivity often far from the perovskite arystotype.⁵⁻⁷ Although materials research has mostly focused on low-bandgap compositions, wide-bandgap (>1.7 eV) halide perovskites have progressively emerged as possible candidates for several uses as well.⁸,⁹

(TMSO)PbI₃ and (TMSO)₃Bi₂I₉ (with TMSO = trimethylsulfoxonium, (CH₃)₃SO⁺) form a solid solution with a monodimensional pseudo-perovskite structure with edge-sharing octahedra containing either Pb²⁺, Bi³⁺, or a metal cation vacancy, the latter being required to keep charge neutrality.¹⁰ Their rich defect chemistry and ordering in metal cation sites have been recently described in detail.¹¹,¹²

X-ray absorption near-edge structure (XANES), possibly with high-energy resolved fluorescence detection (HERFD-XANES), has been proven to be a powerful tool to assess the oxidation state, coordination geometry, and metal–ligand interactions in a wide range of natural and artificial materials,

complementing conventional diffraction and electron microscopy techniques.¹³⁻¹⁹ In general, near-edge spectra do not suffer from loss of resolution due to thermal or static disorder as much as the extended X-ray absorption fine structure (EXAFS), and their applicability is therefore much broader.

While XANES and HERFD-XANES are now well-established tools in the study of chalcogenides, in general, and almost routinely employed for, e.g., heterogeneous catalysis or metallobiology, their application to the booming field of halide perovskites has been very sparse to date. On the other hand, the effects of even subtle structural features, or the choice of calculation details such as spin-orbit coupling, on the predicted frontier electronic states and optical transitions have been extensively investigated in several detailed papers.²⁰,²¹ Only in the last few years, some studies have appeared, devoted specifically to the analysis of XANES spectra of halide perovskites and lower-dimensional pseudo-perovskites. In particular, Gardner and co-workers reported the HERFD-XANES and resonant inelastic X-ray scattering (RIXS) spectra of MAPI, Cs₃Bi₂I₉, APbI₄, APb₂I₆: a 3D perovskite, two 2D edge-sharing layered pseudo-perovskites, and a 1D face-sharing pseudo-perovskite, respectively.²²⁻²⁴

Smith et al. used in situ XANES to follow the oxidation of Sn(II) to Sn(IV) in formamidinium-based thin films.²⁵

Received: May 29, 2023

Revised: August 22, 2023

Published: September 1, 2023

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Vorwerk et al. used many-body perturbation theory to assess the excitonic binding energy of the core-holes at the low-energy I L$_{3}$-edge and Pb M$_{5}$-edge of MAPI, finding that the main features of these spectra are essentially determined by the inorganic Pb–I framework by comparison with PbI$_{2}$.$^{26}$

Dridell et al. strived instead for an ab initio modeling of the metal XANES spectra, constructing a statistical number of snapshots of the methylammonium lead iodide/bromide (MAPI/MAPBr) structure with ab initio molecular dynamics. In this thorough investigation, they concluded that I/Br are randomly distributed in MAPI/MAPBr solid solutions. In addition, they found that the spectra are highly sensitive to Pb-halide distances and octahedral tilting and especially that thermal motions dominate any possible structural conformation that could be preferred energetically.$^{27}$

In the following, we present a comprehensive theoretical assessment of near-edge spectra of metal and iodide atoms alike in a 1D halide pseudo-perovskite. In search of generally applicable criteria, the ab initio simulations of the spectra and pDOS are carried out with a bottom-up approach, starting from the $[PbI_{6}]^{4-}$ and $[BiI_{6}]^{3-}$ octahedral units common to any (pseudo-)perovskite, up to the complete (TMSO)$_{3}$(Pb,Bi)I$_{9-x}$ structure. All possible patterns of Pb/Bi substitution in a supercell with eight metal sites to account for multiple scattering effects between neighboring metals have been also included. We then compare the simulations with experimental XANES spectra, highlighting that these can be effectively modeled with just $[PbI_{6}]^{4-}$ and $[BiI_{6}]^{3-}$ units but also carry indirect information about the long-range structure.

Core-level spectroscopy methods like XANES give fundamentally different chemical insights from valence-level techniques like photoelectron spectroscopy methods (UPS or XPS). The latter provides immediate information on empty states close to the band gap directly at the surface, so they can be conveniently complemented by the bulk average information only accessible to core-level spectroscopy methods.

## 2. METHODS

Samples with general formula (TMSO)$_{3}$Pb$_{3}$xBi$_{2}$(1-x)I$_{9}$ were synthetized through precipitation from aqueous solution as described in detail in our earlier work.$^{10}$ All powders were decanted for 24 h and then filtered through 0.45 µm polypropylene filters (Millipore).

X-ray diffraction patterns were acquired on all samples with a Rigaku Miniflex 600 diffractometer using Ni-filtered Cu Kα radiation and analyzed with Rietveld refinement using GSAS-II.$^{28}$ All samples were refined with an orthorhombic *Pnma* structure. Accurate stoichiometry was derived using established relations between lattice parameter *a* and compositions$^{10}$ (details reported in the SI).

X-ray absorption spectra were acquired at the Pb and Bi L$_{3}$-edge at the XAFS beamline of Elettra Sincrotrone Trieste (Trieste, Italy) and at the BM8 beamline of ESRF (Grenoble, France). Spectra were acquired in transmission mode on samples cooled at 80 K with a liquid nitrogen cryostat. Ionization chambers were filled with Ar/N$_{2}$ mixtures at different pressure to achieve 20% absorption on I$_{0}$ and 60% on I$_{t}$. A double crystal Si (111) monochromator, with Pt-coated mirrors, was used to achieve a ΔE energy resolution around 1.4 eV. The data were collected with an energy step of 5 eV in the pre-edge region, 0.4 eV in the XANES region and with a constant k-step of 0.04 Å$^{-1}$ in the postedge region. Each sample was measured on a grid of nine spots ca. 1 mm apart to

minimize beam exposure, with each scan lasting about 30 min, and all scans were averaged afterward. Data reduction was performed with Athena,$^{29}$ and the XANES spectra were simulated with FDMNES.$^{30}$

## 3. RESULTS AND DISCUSSION

The formulas of the samples are summarized in Table 1. For the sake of simplicity, considering that all samples have TMSO as an organic cation and iodine as a halogen, all samples are labeled according to their bismuth percentage.

<table><caption>**Table 1.** Names and Stoichiometry of Samples Used in This Study</caption><thead><tr><th>label</th><th>formula</th></tr></thead><tbody><tr><td>Bi0</td><td>(TMSO)PbI<sub>3</sub></td></tr><tr><td>Bi8</td><td>(TMSO)<sub>3</sub>Pb<sub>2.67</sub>Bi<sub>0.22</sub>I<sub>9</sub></td></tr><tr><td>Bi10</td><td>(TMSO)<sub>3</sub>Pb<sub>2.61</sub>Bi<sub>0.26</sub>I<sub>9</sub></td></tr><tr><td>Bi22</td><td>(TMSO)<sub>3</sub>Pb<sub>2.13</sub>Bi<sub>0.58</sub>I<sub>9</sub></td></tr><tr><td>Bi34</td><td>(TMSO)<sub>3</sub>Pb<sub>1.68</sub>Bi<sub>0.88</sub>I<sub>9</sub></td></tr><tr><td>Bi58</td><td>(TMSO)<sub>3</sub>Pb<sub>0.99</sub>Bi<sub>1.34</sub>I<sub>9</sub></td></tr><tr><td>Bi78</td><td>(TMSO)<sub>3</sub>Pb<sub>0.51</sub>Bi<sub>1.66</sub>I<sub>9</sub></td></tr><tr><td>Bi100</td><td>(TMSO)<sub>3</sub>Pb<sub>1.2</sub>I<sub>9</sub></td></tr></tbody></table>

The structure of the (TMSO)PbI$_{3}$-(TMSO)$_{3}$Bi$_{2}$I$_{9}$ solid solutions has been recently described in detail.$^{10}$ Linear face-sharing chains of PbI$_{6}$/BiI$_{6}$ octahedra lie parallel to the *a* axis (see Figure 1), which therefore contracts and expands according to the relative occupation of metal sites with Pb, Bi, or metal vacancies, according to the defect chemical equation:

$$3\text{Pb}_{\text{Pb}}^{x} + 2\text{BiI}_{3} \rightarrow 2\text{Bi}_{\bullet\text{Pb}} + \text{V}''_{\text{Pb}} + 3\text{PbI}_{2} \quad (1)$$

In particular, the Pb/Bi/vacancy alternation is both nonperiodic and nonrandom, and it can be modeled to some extent as correlated disorder governed by a second-order stochastic matrix.$^{31}$

For a (TMSO)$_{8}$Pb$_{2}$Bi$_{4}$I$_{24}$ composition, there are 16 possible dispositions of Pb/Bi/vacancy in the eight metal cation sites of a $2 \times 1 \times 1$ supercell, labeled A to R (see Figure 1b). The configurational energy is then due to the different interaction terms between cation pairs (i.e., the next-nearest-neighbor Pb–Pb, Bi–Bi, Pb–Bi, etc. correlations), as summarized in Table 2.

The following structural models were used to compute the XANES spectra at the Pb and Bi L$_{3}$ edges: (1) isolated MI$_{6}$ octahedra (M = Pb or Bi); (2) $2 \times 1 \times 1$ supercells with composition (TMSO)$_{8}$Pb$_{2}$Bi$_{4}$I$_{24}$, including all possible Pb/Bi arrangements in the eight metal cation sites, described in detail in ref. 12. XANES spectra were then calculated for all configurations (Figure S1). For iodine, XANES spectra were simulated at the L$_{1}$-edge (Figure 2a): this absorption edge corresponds to the iodine 2s orbital, therefore probing dipole allowed transitions to 5p states, which are hybridized with metal 6s in the Pb/Bi–I bonds.$^{22}$ The simulated I L$_{1}$ spectra show substantial differences in the intensity of the white line and reflect the electronic density of states of the empty 5p orbitals of iodine. Compared to literature I L$_{1}$ spectra of 3D and 2D perovskites,$^{22}$ each octahedral face is shared here between two neighboring octahedra, and the local coordination of iodine atoms gives rise to a much different edge shape. The onset of the L$_{1}$ edge also correlates well with the position of the conduction band minimum. For instance, the energy of
<figure><img src="image_11.png" /><figcaption>**Figure 1.** (a) Undoped TMSO<sub>8</sub>Pb<sub>8</sub>I<sub>24</sub> 2 × 1 × 1 supercell. (b) Doped TMSO<sub>8</sub>Pb<sub>5</sub>Bi<sub>2</sub>I<sub>24</sub> supercell, configuration A. (c) (TMSO)<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub> supercell. Iodine atoms are in violet, PbI<sub>6</sub> octahedra are in gray, BiI<sub>6</sub> octahedra in pink. Carbon, sulfur, oxygen, and hydrogen atoms are in gray, yellow, red, and white, respectively. The octahedral chains are aligned along the *a* direction.</figcaption></figure>

<table><caption>**Table 2.** Energy of All Possible Configurations in (TMSO)<sub>8</sub>Pb<sub>2</sub>Bi<sub>4</sub>I<sub>24</sub>, Relative to Lowest-Energy Configuration “A”<sup>a</sup></caption><thead><tr><th>configuration</th><th>energy (eV)</th></tr></thead><tbody><tr><td>A</td><td>0</td></tr><tr><td>B</td><td>0.58</td></tr><tr><td>C</td><td>0.58</td></tr><tr><td>D</td><td>0.56</td></tr><tr><td>E</td><td>0.48</td></tr><tr><td>F</td><td>0.52</td></tr><tr><td>G</td><td>1.11</td></tr><tr><td>H</td><td>1.04</td></tr><tr><td>I</td><td>0.97</td></tr><tr><td>L</td><td>1.03</td></tr><tr><td>M</td><td>1.48</td></tr><tr><td>N</td><td>1.39</td></tr><tr><td>O</td><td>1.39</td></tr><tr><td>P</td><td>1.47</td></tr><tr><td>Q</td><td>1.39</td></tr><tr><td>R</td><td>1.54</td></tr></tbody></table>

the I L₁ edge increases from configuration O to configurations A and B: the conduction band minimum also increases from configuration O to configurations A and B.<sup>12</sup> The absence of any pre-edge shoulder at low energy reflects the purely ionic interaction between TMSO⁺ and iodide, with no mixing with the I p states.<sup>12</sup>

The experimental Pb and Bi L<sub>3</sub>-edge spectra of the end members TMSOPbI<sub>3</sub> and (TMSO)<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub> were compared with simulations from the respective crystal structures in Figure 1a,c (see Figure 3). For mixed samples, a first comparison (Figure 4) shows that all samples are very different from PbI<sub>2</sub> and BiI<sub>3</sub>, and therefore, the decomposition to simple iodides from radiation damage can be reasonably excluded. All Pb near-edge

<figure><img src="image_12.png" /><figcaption>**Figure 2.** (a) XANES spectra of all configurations at the I L₁-edge; (b) I L₁-edge spectra (red) and the average of the p DOS (blue) of all iodine atoms.</figcaption></figure>

features of samples look similar, while Bi shows varying intensities for the edge features (three peaks up to 25 eV after

<footnote><sup>a</sup>Data taken from ref 12.</footnote>
<figure><img src="image_13.png" /><figcaption>**Figure 3.** Pb L<sub>3</sub>-edge experimental (red) and simulated (blue) XANES spectrum of (TMSO)PbI<sub>3</sub>. Inset: Bi L<sub>3</sub>-edge spectra experimental (red) and simulated (blue) XANES spectrum of (TMSO)<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub>.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 4.** (a) Pb L<sub>3</sub>-edge experimental XANES spectra compared to the precursor PbI<sub>2</sub>, and the simulated spectrum of configuration A (thick line); (b) Bi L<sub>3</sub>-edge experimental XANES spectra compared to the precursor BiI<sub>3</sub>, and the simulated spectrum of configuration A (thick line).</figcaption></figure>

the edge) depending on the Pb/Bi content. In general, configuration A is the one that resembles more the experimental data of all mixed samples for Pb and Bi L<sub>3</sub>-

edges, as seen especially in the position and intensity of maxima and minima >30 eV above the edge. This configuration features maximum segregation of Bi and vacancy defects in linear clusters [Bi-V-Bi], and having the lowest configurational energy it has the highest probability of being represented in an average ensemble in the bulk.

The different A-R configurations essentially differ for the weight of various metal-metal interoctahedral multiple scattering paths, while the nearest neighbor distances are the same in all of them. After assessing how different configurations affect the near-edge spectra, we now look into the role of each MI<sub>6</sub> octahedron. We extracted the octahedra of configuration A (five PbI<sub>6</sub> and two BiI<sub>6</sub>, depicted in Figure 5a)

<figure><img src="image_15.png" /><figcaption>**Figure 5.** (a) Configuration A. Pb atoms are depicted in gray, Bi atoms in magenta, and vacancy in black. The color code for other atoms is the same as shown in Figure 1. (b) Simulated XANES spectra of configuration A compared with its individual component octahedra at the Pb L<sub>3</sub>-edge (main panel) and Bi L<sub>3</sub>-edge (inset).</figcaption></figure>

to simulate their Pb and Bi XANES spectra (Figure 5b). With this approach, we can first evaluate whether an experimental XANES spectrum can be explained by the individual octahedra, or the whole configuration is needed. From inspection of Figure 5b, the essential features of the complete spectrum are still recognized in the spectrum of the isolated single octahedra, but important differences are evident in all portions of the spectrum, from the main edge to further peaks.

Comparing the experimental spectrum of mixed samples with the simulated spectra of configuration A and its octahedra (Figure 6), the individual octahedra (e.g., Pb<sub>3</sub> and Bi<sub>2</sub>) are almost sufficient to explain all features of the experimental
<figure><img src="image_16.png" /><figcaption>**Figure 6.** Experimental XANES spectra (thick red line) compared with the configuration A (black line) and individual octahedra (color lines) at the Pb L₃-edge (main plot) and Bi L₃-edge (inset).</figcaption></figure>

spectra in the edge region. In particular, the main edge features are much better reproduced by a single octahedron than the complete structure involving M—I—M intraoctahedral paths, a surprising observation for a well-crystallized compound. This corroborates the general observation that the crystal structure is only a snapshot that does not fully capture the reality of a “wobbly” ensemble, and it is likely to underestimate both static

and dynamic disorder.¹⁵ This observation motivates the following investigation, based on the reduction of the structure to a sum of its octahedral units.

Simulations of the XANES spectra were then carried out on a series of distorted isolated PbI₆ and BiI₆ octahedra starting from pristine regular ones. The possible applied distortions were: (i) skewing of one iodine atom; (ii) isotropic compression and expansion; (iii) anisotropic elongation/ compression along one axis; (iv) change of I—M—I bond angles. The effects of each modification on the Pb spectra are described in detail in the following: Bi spectra display mostly the same effects.

**3.1. Iodine Skewing.** Distortion of the octahedron by displacement along the *y* axis of an iodine atom, shown in the inset of Figure 7a, causes the reduction of the I—M—I angle by 6/7% ca. each step. From an angle of 90° in the pristine PbI₆ octahedron, an angle of 68.2° is obtained in PbI₆·A·4 (inset of Figure 7a). To keep the volume of the octahedron constant, the Pb—I bond must elongate, resulting in a bond of 3.45 Å in PbI₆·A·4 versus 3.2 Å in pristine PbI₆. This distortion does not cause consistent changes in the XAS spectra because there is the 7% elongation of a single bond Pb—I. As the distortion increases, there is a shift to left of the absolute maximum of 0.3 eV and the lows are shallower (Figure 7a).

**3.2. Isotropic Compression.** Reduction of the octahedron volume by about 2/3% at each step (inset of Figure 7b). From

<figure><img src="image_17.png" /><figcaption>**Figure 7.** Simulated XANES spectra at the Pb L₃-edge of reference PbI₆ octahedron and different modes of octahedral distortion: (a) skewing; (b) isotropic compression; (c) isotropic expansion; (d) symmetric change of the equatorial I—Pb—I angle.</figcaption></figure>
a volume of 43.7 Å³, a volume of 39.72 Å³ is obtained in PbI₆·B₄. The volume reduction causes in XAS spectra the shift to right of absolute maximum of ca. 1 eV each step and the lows are deeper (Figure 7b).

**3.3. Axial Compression.** Compression of the octahedron along the axial bonds causes the bond reduction of 0.05 Å each step. From a bond of 3.2 Å, a bond of 2.75 Å is obtained in PbI₆·C₉ (inset of Figure 8a). In this case, the change in the two axial Pb–I bonds is important; this also causes a change in symmetry which leads to an evident change in the XAS spectra. As the reduction in axial bonds increases, there is a shift to the

<figure><img src="image_18.png" /><figcaption>**Figure 8.** Simulated XANES spectra at the Pb L₃-edge of reference PbI₆ octahedron and further different modes of octahedral distortion: (a) axial compression; (b) axial elongation; (c) axial compression and equatorial elongation.</figcaption></figure>

right of the absolute maximum, a reduction in intensity of the same peak, and an increase in the edge. In PbI₆·C₉, in which distortion is very important, the effects of multiple scattering can be seen (Figure 8a).

**3.4. Isotropic Expansion.** The increase of the octahedron volume is by about 2.3% at each step (inset of Figure 7c). From a volume of 43.7 Å³, a volume of 47.92 Å³ is obtained in PbI₆·D₄. The volume increase causes the maximum in the spectra to shift to the left by ca. 1 eV each step (Figure 7c). In this case, the minima are also systematically shallower.

**3.5. Axial Elongation.** Elongation of the octahedron along the axial bonds causes the bonds to increase by 0.05 Å each step. From a bond of 3.2 Å, a bond of 3.65 Å is obtained in PbI₆·E₉ (inset of Figure 8b). In this case, the change in the two axial Pb–I bonds is not as important as in the case of compression, because the interaction of neighboring atoms becomes less important. As axial bonds increase, there are a slight shift to the left of the absolute maximum, a slight reduction in the intensity of the same peak, and an increase in the edge (Figure 8b).

**3.6. Equatorial Elongation and Axial Compression.** In this case, there is a compression of axial bonds and an elongation of equatorial bonds of 0.05 Å each step. From bonds of 3.2 Å, axial bonds of 2.9 Å and equatorial bonds of 3.5 Å are obtained in PbI₆·F₆ (inset of Figure 8c). In this case, there is a strong impact on symmetry that greatly changes the XANES spectra (Figure 8c).

**3.7. Rotation.** The rotation of an equatorial bond axis causes the reduction of two angles and the increase of the other two by about 5° each step. From angles of 90°, two angles of 65° and two of 115° are obtained in PbI₆·G₅ (inset of Figure 7d). In this case, there are no changes in the bond lengths, and therefore, this does not cause significant changes in the calculated spectra because multiple scattering effects are almost unaffected (Figure 7d).

The three modes of distortion depicted in Figure 8 have the biggest effects on the XANES spectra, and they all involve a reduction of symmetry from the O_h to D_4h point groups. On the contrary, the very large symmetry reduction obtained by skewing one iodine atom (Figure 7a) affects the XANES spectrum very little. It can be concluded that the spectrum is dominated by the presence and relative length of colinear I–M–I intra-octahedral paths.

For a deeper understanding of the effect of the metal-iodide distance on the XANES spectra, we analyzed the partial projection of the density of states at the absorber in all different PbI₆ octahedra. We then chose the simulated spectra that show substantial differences from pristine PbI₆ pristine, i.e., PbI₆·C₉, PbI₆·E₉, and PbI₆·F₆, were compared with the respective calculated d DOS. In an undistorted PbI₆, the d_{z2} and d_{x2-y2} states are degenerate (Figure 9a): in octahedra with different axial/equatorial distances (Figure 9b–d) the geometrical distortion breaks the symmetry and also the degeneracy, causing the edge features to be blurred. The d_{x2-y2} DOS retains a strong sharp peak in the modes of distortion that involves axial compression (PbI₆·C₉ and PbI₆·F₆), while in axial elongation (PbI₆·C₉, resembling a Jahn–Teller distortion) the two DOS of the two orbitals are only displaced, resulting in a flatter edge.

All the observations reported above suggest that the modifications on the absorption edges are eminently local in nature and they are mainly based on the relative length of colinear I–M–I intra-octahedral paths. This would allow to
<figure><img src="image_19.png" /><figcaption>**Figure 9.** Simulated XANES spectra at the Pb L<sub>3</sub>-edge and DOS of d orbitals: (a) reference PbI<sub>6</sub> octahedron; (b) axial compression; (c) axial elongation; (d) equatorial elongation and axial compression.</figcaption></figure>

model the XANES or HERFD-XANES signals in terms of relatively simple modes of distortion during static or in situ/ operando studies of halide perovskites. In this respect, many empirical attempts have been made recently to correlate XAFS spectra to synthetic parameters or post-synthetic treatments:<sup>32,33</sup> in fact, unsystematic choice of coordination shells and fitting parameters and large spread of the latter (Pb—I distances from 3.09 to 3.47 Å in a multiple-shell fit,<sup>34</sup> 3.13 Å in a single shell fit,<sup>32</sup> or even as low as 2.4–2.8 Å;<sup>33</sup> disorder factors from 0.006 to 0.019 Å<sup>2</sup>) all hindered reliable structure–property relations for practical use of core spectroscopy methods to date. In general, EXAFS data of hybrid halide perovskites contain information on just one coordination shell.<sup>32–34</sup>

On the contrary, the near-edge portion of the spectrum is much less affected by static and dynamic disorder, and its sensitivity to symmetry and overall geometry (whether or not multiple scattering contributions play a role, as seen in this case) provide an idea complement to diffraction analysis.<sup>35</sup>

The understanding of structure–property relations is of pivotal importance for the future development of halide perovskites for solar cells with improved power conversion efficiency, one of the many fields of prospected application.

4. CONCLUSIONS

The evidence accumulated to date EXAFS data, vulnerable to the substantial static disorder typical of lead halide perovskites, has proven unconclusive to date, and eventually ineffectual to draw correlations with functional properties, or even other structural features. On the other hand, near-edge spectroscopy methods (XANES and HERFD-XANES) can provide interesting information on octahedral symmetry and cation arrangement in mixed-cation halide perovskites with various dimensionality, while requiring more rigorous ab initio simulations. Using a doped Pb/Bi monodimensional perovskite as a model compound, we show that the XANES spectra on several absorption edges are very sensitive to a number of subtle structural features of the metal-halide octahedral local geometry, while remaining independent of long-range multiple scattering effects that usually dominate the near-edge region. Connections to the electronic structures close to the Fermi energy can also be drawn case-by-case, as exemplified by the iodine L<sub>1</sub>-edge. The extension of such an approach to other halide perovskites can be expected to provide simple structural correlations in the near future, usefully complementing other spectroscopic and diffraction techniques.<sup>36</sup>
■ ASSOCIATED CONTENT

● Supporting Information

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jpcc.3c03604.

Crystal structure and composition of all samples and
simulated XANES spectra for all configurations and BiI₆
octahedra (PDF)

■ AUTHOR INFORMATION

Corresponding Author

**Francesco Giannici** – Dipartimento di Fisica e Chimica
“Emilio Segrè”, Università di Palermo, Palermo I-90128,
Italy; orcid.org/0000-0003-3086-956X;
Email: francesco.giannici@unipa.it

Authors

Simone Virga – Dipartimento di Fisica e Chimica “Emilio Segrè”, Università di Palermo, Palermo I-90128, Italy; orcid.org/0000-0001-8022-5274

Alessandro Longo – European Synchrotron Radiation
Facility, Grenoble F-38043, France; Istituto per lo Studio dei
Materiali Nanostrutturati ISMN-CNR, Palermo I-90146,
Italy

Candida Pipitone – Dipartimento di Fisica e Chimica “Emilio Segrè”, Università di Palermo, Palermo I-90128, Italy

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jpcc.3c03604

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We acknowledge the European Synchrotron Radiation Facility
for provision of beamtime on beamline BM8 (proposal MA-
5455), and for access to the computing cluster, and the
CERIC-ERIC consortium for providing access to the XAFS
beamline at ELETTRA – Sincrotrone Trieste (proposal
20220258). We thank G. Das and A. Puri for assistance
during the measurements.

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