Information about the lattice of lead iodide

The single crystal and thin film of azetidinium lead iodide (azpi) were prepared. The properties of AZ cations were compared with those of Ma and FA ions commonly used in Table 1.

Single crystal XRD (scxrd) data of azpi crystal at 150k were collected, but twins and disorder were serious in the data, so it was impossible to specify a clear model for these data. Many attempts have been made to grow crystals suitable for scxrd (the crystals are grown from dichloromethane, acetonitrile and nitromethane), but all the obtained crystals show the same twins and disorder, which indicates that it is inherent to the azpi structure. Based on the following discussion, we tentatively believe that azpi has a previously unreported crystal framework, located between 3D and 2D perovskites, but more structural studies are needed to confirm our findings.

We found that nitrogen iodide could not be completely dissolved in DMF or DMSO, so we used a sequential deposition based method to prepare azpi films. Nitrogen iodide was spin coated on PbI2 film in isopropanol solution. The PbI2 thin film rapidly changed to glassy orange at room temperature (Fig. 1a), and this color did not change with annealing. Powder XRD analysis of Mapi and azpi thin films formed by two-step deposition was carried out. The main reflections in the azpi diagram are at 11.5 °, 24.9 °, 26.2 ° and 30.1 °, although there is a characteristic close to the (002) lead iodide peak at 12.7 °, this diagram is obviously different from pure PbI2 (Figure S2 (a)).

Comparing the PXRD spectra of Mapi and azpi, it was found that the obtained spectra were very different (Fig. S2D). The peaks in the Mapi map are denser and narrower, indicating that the Mapi film is more crystalline than the azpi film.

Fig. 1 (b) is the PXRD diagram of azpi film at 5 °. It is important to reiterate that the ionic radius of AZ + cation is between Ma + and FA +, so a three-dimensional perovskite can be calculated. In this case, because the ions are too large to be easily put into the 3D lattice, a two-dimensional perovskite is usually formed, which is composed of a lead iodide layer and cations sandwiched in it. The characteristic peaks of two-dimensional perovskite are between 5 ° and 10 °. 39,40 importantly, these peaks do not appear at all in Fig. 1 (b), which indicates that the two-dimensional perovskite is not formed. In order to better understand the azpi structure, the single crystal data were re studied.

We collected and partially solved 4 sets of single crystal data sets from different synthesis batches, which were consistent with each other. Therefore, we are confident in the proposed azpi overall structure outlined in Figure 1 (b) and figure S3. The possible solution only contains information about the lattice of lead iodide, and it is impossible to obtain any information about the position of cations. This structure contains a common angular group of pb-i octahedral units, located between 3D and 2D perovskites. However, it should be emphasized that the data quality is low and needs further research.

Although the quality of the solution is not high, we used DFT to improve the proposed structure, and then used it to simulate the XRD pattern of the powder, which we can compare with our experimental results. A preliminary DFT calculation was carried out using cesium instead of azetidine. These calculations retain the local position of lead iodide and replace spherical Cs ions into the voids. The near plane orientation of azide ions is obviously different from that of cesium ions. Further calculations show that the most stable orientation of azide ions is perpendicular to the Z axis. Unfortunately, the large size of the crystal unit (576 atoms) is a computational challenge, and again, this is at most an approximation of the azpi structure. The structure shown in Fig. 1 (b) is used to generate the simulated diffraction pattern shown in Fig. 1 (b). There is a strong similarity between the experimental and generated spectra - in both cases, there are no significant peaks at low angles, while most peaks at higher angles are reproduced. However, the low crystallinity of the film azpi diagram makes it impossible to qualitatively compare the simulation data with the experimental data. If this possible structure is correct, then it is very exciting, because it shows that there are "perovskite related" structures that have not been found yet, and may have interesting photovoltaic characteristics.

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