Three New Lead Iodide Chain Compounds, APbI3, Templated by Molecular Cations

The crystal structures of three new hybrid organic-inorganic lead halide compounds [IqH]PbI3, [4MiH]PbI3, and [BzH]PbI3 ([IqH+] = isoquinolinium, [4MiH+] = 4-methylimidazolium, [BzH+] = benzotriazolium) have been determined by single crystal x-ray diffraction. All three compounds have the same generic formula as perovskite, ABX3, but adopt a rare non-perovskite structure built from one dimensional (1D) edge-sharing octahedral chains. The bandgap of each compound was investigated by solid UV-Vis spectra. In comparison with previously reported hybrid compounds containing the same type of octahedral chains, [C10H7CH2NH3]Pbl3 and (C7H7N2)PbI3, all three new compounds have lower bandgaps (<2.4 ev), indicating that they may be promising for photovoltaic application.


Introduction
The study of perovskite-related materials has grown tremendously in recent years [1] due to their enormous chemical and structural diversity and excellent physical properties, for example, ferroelectricity, luminescence, or magnetism [2][3][4]. In particular, lead(II) halide perovskites show impressive performances in their electronic and photophysical properties [5], making them promising candidates for practical applications such as solar cells [6,7] and light-emitting devices (LEDs) [8,9]. α-CsPbI3, for instance, exhibits a suitable bandgap (Eg = ∼1.7 eV), and is an excellent candidate for photovoltaic applications [10][11][12]. The general formula for perovskites is ABX3, where A is a large cation, B is a smaller cation, and X is an anion. However, there are many families of "perovskite-related" materials, which contain, for example, layered structural units derived from the archetypal "cubic" perovskite structure [5]. Conversely, not all ABX3 compositions form perovskites, and several other structural architectures based on linked octahedral BX6 units are available for such a stoichiometry, incorporating either face-sharing or edge-sharing rather than perovskite-like cornersharing octahedra [13]. The simple composition CsPbI3 adopts four different polymorphic forms: α-CsPbI3 (cubic), β-CsPbI3 (tetragonal), γ-CsPbI3 (orthorhombic), and δ-CsPbI3 (orthorhombic) [10,11,14]. The first three structures are ABX3 "cubic" type perovskites (α is aristotype cubic, β, and γ can be treated as lower symmetry, distorted structures due to "tilting" of the constituent octahedral PbI6 units). Interestingly, the most stable ambient phase, δ-CsPbI3, is a non-perovskite, which adopts a one dimensional (1D) edge-sharing octahedral chain structure type [15], similar to the known compounds NH4CdCl3 [16] and RbPbI3 [17,18] (Figure 1). Both the perovskite-structure polymorphs and non-perovskite phase play a significant role in better understanding the structure-property relationships amongst these different polymorphs. In contrast to the large variety of lead halide materials reported based on the cubic perovskite structure and its layered derivatives, here, we focused on developing the much less common 1D edge-sharing octahedral chain structure type, particularly in hybrid organic-inorganic lead halide materials. The only previous examples, to the best of our knowledge, are [C10H7CH2NH3]PbI3 [19], (C7H7N2)PbI3 [20], "(ABT)2[PbBr3]" [21], and (ABT)[PbCl3] [22].
Here Single crystal x-ray diffraction experiments were performed to understand the structural variations of these three materials including structural distortions of the inorganic components and the nature of hydrogen bonding in directing the overall crystal packing. Furthermore, UV-Vis absorbance spectroscopy was carried out on powder samples of all three samples, and bandgaps were derived from Tauc-Plots [23].

Characterization
Single crystal X-ray diffraction data were collected at 173 K and 298 K on a Rigaku XtaLAB P200 diffractometer and a Rigaku SCX Mini diffractometer using Mo-Kα radiation (Rigaku, Houston, TX, USA). Data were collected using CrystalClear (Rigaku) software [24]. Structures were solved by direct methods using SHELXT [25], and full-matrix least-squares refinements on F 2 were carried out using SHELXL-2018/3 [26] incorporated in the WINGX program [27]. Absorption corrections were performed empirically from equivalent reflections on the basis of multi-scans by using CrystalClear [24]. Non-H atoms were refined anisotropically and hydrogen atoms were treated as riding atoms. CrystalMaker [28] was used in preparing Powder X-ray diffraction data were collected on a PANalytical EMPYREAN diffractometer using Cu Kα1 (λ = 1.5406 Å) radiation in the range of 3 to 70 to confirm the purity of each sample (Malvern Panalytical, Ltd, Malvern, UK). Rietveld refinements were carried out using the GSAS package [29] with the EXPGUI interface [30].
Solid UV-Vis absorbance spectra were collected on a JASCO-V550 ultraviolet-visible spectrophotometer with the wavelength range at 200 nm to 900 nm (JASCO Corporation, Essex, UK).

Results and Discussion
Crystallographic details for the three new compounds are given in Table 1. Although single crystal X-ray data were collected at both 173 K and 298 K, there were only slight changes in molecular geometry between the two temperatures, with no phase changes detected in this temperature regime. The following discussion therefore refers to the structures at 173 K only, (details at 298 K are given in the Supplementary Materials). Each crystal structure exhibits the type of [PbI3] chain found in δ-CsPbI3 ( Figure 1). This chain may be regarded as derived from the hexagonal, layered PbI2 structure by "stripping out" a double strand of condensed PbI6 octahedra from the PbI2 layer. This leads to one short unit cell axis of around 4.6 Å for each of the crystal structures, which represents the Pb-Pb distance between two adjacent edge-shared octahedra. Within each [PbI3] chain, there are three distinct types of iodide environment, designated as μ1 (terminal), μ2 (bridging two Pb centers), or μ3 (bridging three Pb centers); these were designated as I1, I2, and I3, respectively, in each of the three new structures. This intrinsic asymmetry of the environment of both the I − and Pb 2+ environments leads to considerable distortions of the PbI6 octahedra, as detailed in Figure (Table S6). It is generally the case in these structures that a larger Δd corresponds to a smaller σ 2 , but the detailed systematics and origins of the distortion behavior are not clear. There is, however, a clear trend in the amount of "underbonding" seen for the iodine sites in the [PbI3] chain, which follows the order I1 > I2 > I3 (see bond valence sums, Table 2). This lack of sufficient bonding for I1 in particular, despite the Pb-I1 bond being the shortest in each case, is compensated by I1 being a strong H-bond acceptor, a feature which presumably dictates the orientation of the organic cation relative to the inorganic chain in each case. This is especially seen in the case of [BzH]PbI3, where I1 accepts two strong H-bonds (Table 3). Similar arguments and correlations can be seen in the behavior of the I2 and I3 sites by comparing the number and strength of H-bonds (Table 3) versus the corresponding iodine bond valence sums ( Table 2).  Hence, the overall crystal packing is dictated by the nature of the inter-chain interactions, mediated by hydrogen bonds from the molecular cations. Details of H-bonding are shown in Table  3. However, despite the quite distinct nature of the molecular cations, in terms of size, shape, and Hbonding options, each of the structures adopts a similar relative packing of the inorganic chains, which results in quite similar unit cell metrics (Table 1) and is shown in more detail in Figures 3-5. Moreover, the space group symmetries for [IqH]PbI3 and [BzH]PbI3 are the same, and, in this sense, these two may be regarded as isostructural, although in the former case each organic moiety has only one H-bond donor atom and hydrogen-bonds to only one adjacent inorganic chain, whereas in the latter, the corresponding moiety bridges two adjacent chains via two distinct H-bond donor atoms. The symmetry of [4MiH]PbI3 differs, exhibiting more subtle distortions due to the enhanced effects of the inter-chain interactions, whereby each organic moiety bridges three adjacent inorganic chains via the two H-bond donors. This leads to a slightly different displacement of adjacent     UV-Vis absorbance spectra were carried out for all three powder samples: [IqH]PbI3, [4MiH]PbI3, and [BzH]PbI3 at wavelengths between 200 nm to 900 nm ( Figure 6). The absorption spectra revealed that all three compounds featured similar peaks at ~385 nm (3.2 eV) and ~415 nm (3.0 eV). Interestingly, [BzH]PbI3 has an extra absorption peak at ~493 nm (2.5 eV), leading to a band gap significantly lower than the other two, as derived from the Tauc-Plot (inset Figure 6) It can be noted that the band gaps of all three new compounds we report here are lower than the two known examples of the same structure type: [C10H7CH2NH3]Pbl3 (absorption peak is at ~401 nm) [19], and (C7H7N2)PbI3 (band gap is Eg = 2.44 eV) [20].

Conclusions
In conclusion, we have prepared three new examples of an unusual [PbI3] chain consisting of edge-shared PbI6 octahedra. This type of chain has been previously seen only rarely in both inorganic and hybrid lead halides. Structural distortions within the [PbI3] chains, and the crystal packing of the chains themselves, can be rationalized to some extent, based on the hydrogen-bonding requirements of the organic moieties. However, the contrasting nature of the three amines used here suggests that structural and compositional features of molecular 'templates' that might direct the crystallization of this type of [PbI3] chain, and thus favor the crystallization of these structure types rather than competing APbI3 (or other) polymorphs, may be difficult to predict. Further work is merited in exploring related examples of organic amines.
Author Contributions: Y.G. and L.Y. carried out all the chemistry, crystallography. and further characterization. P.L. coordinated the project and writing of the paper, with the approval of all authors.
Funding: This research was funded by the China Scholarship Council, grant number 201603780005" the University of St Andrews. Raw data pertaining to this work are available at https://doi.org/10.17630/bb979bce-5a77-45cf-b0c4-dfa7bd5c4588.

Conflicts of Interest:
The authors declare no conflicts of interest.