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

Abstract

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.

1. 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 corner-sharing 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, we represent three further examples of this rare structure type amongst hybrid organic–inorganic lead halide materials: [IqH]PbI₃, [4MiH]PbI₃, and [BzH]PbI₃, ([IqH⁺] = isoquinolinium, [4MiH⁺] = 4-methylimidazolium, and [BzH⁺] = benzotriazolium). 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].

2. Experimental Section

2.1. Chemicals

Lead (II) iodide (PbI₂, ≥98%), hydroiodic acid (HI, 57%, w/w aqueous solution, stabilized with 1.5% hypophosphorous acid), isoquinoline (C₉H₇N, 97%), and 4-methylimidazole (C₄H₆N₂, 98%) were purchased from Alfa Aesar, Lancashire, UK. Benzotriazole (C₆H₅N₃, 99%) was purchased from Sigma Aldrich, Dorset, UK. All chemicals were directly used without further purification.

2.2. Synthesis

For [IqH]PbI₃, (C₉H₈NPbI₃), isoquinoline (0.18 mL, 1.5 mmol), and PbI₂ (0.922 g, 1 mmol) were dissolved in conc. HI (12 mL) with moderate heating. By cooling for a few hours, yellow, needle-shaped crystals were obtained. These were filtered and washed with diethyl ether (yield 72.0% based on PbI₂). The powder x-ray diffraction and Rietveld refinement are given in Figure S1. Elemental analysis: (anal. calc. (%) for [IqH]PbI₃: C, 15.05; H, 1.12; N, 1.95. Found: C, 15.20; H, 1.08; N, 2.04).

For [4MiH]PbI₃ (C₄H₇N₂PbI₃), 4-methylimidazole (0.329 g, 4 mmol) and PbI₂ (0.922 g, 2 mmol) were dissolved in conc. HI (8 mL) with moderate heating. By cooling for a few hours, yellow, needle-shaped crystals were obtained. These were filtered and washed with diethyl ether (yield 22.5% based on PbI₂). The powder x-ray diffraction and Rietveld refinement are given in Figure S2. Elemental analysis: (anal. calc. (%) for [4MiH]PbI₃: C, 7.16; H, 1.05; N, 4.17. Found: C, 7.17; H, 0.98; N, 4.12).

For [BzH]PbI₃ (C₆H₆N₃PbI₃), benzotriazole (0.476 g, 4 mmol) and PbI₂ (0.922 g, 2 mmol) were dissolved in conc. HI (12 mL) with moderate heating. By cooling for a few hours, yellow, needle-shaped crystals were obtained. These were filtered and washed with diethyl ether (yield 47.2% based on PbI₂). The powder x-ray diffraction and Rietveld refinement are given in Figure S3. Elemental analysis: (anal. calc. (%) for [BzH]PbI₃: C, 10.18; H, 0.85; N, 5.93. Found: C, 10.22; H, 0.80; N, 5.97).

2.3. 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-K_α 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 F² 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 Figure 1, Figure 2, Figure 3, Figure 4 and Figure 5. A Flack parameter of 0.357(5) for [IqH]PbI₃ suggests an inversion twin component, but refinement of this did not significantly improve the fit.

Powder X-ray diffraction data were collected on a PANalytical EMPYREAN diffractometer using Cu K_α1 (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).

3. Results and Discussion

Crystallographic details for the three new compounds are given in

Table 1. Crystal and structure refinement data for [IqH]PbI₃, [4MiH]PbI₃, and [BzH]PbI₃ at 173 K.

Compound	[IqH]PbI3	[4MiH]PbI3	[BzH]PbI3
Formula	C9NH8PbI3	C4N2H7PbI3	C6N3H6PbI3
Formula Weight	718.05	671.01	708.03
Crystal System	Orthorhombic	Monoclinic	Orthorhombic
Space Group	P212121	P21/c	P212121
a/Å	4.6946(2)	4.6110(2)	4.6102(2)
b/Å	12.8898(9)	22.2560(16)	12.4712(9)
c/Å	23.2093(16)	11.7926(8)	22.2047(16)
β/o	-	98.801(9)	-
V/Å3	1404.45(15)	1195.94(13)	1276.65(14)
Z	4	4	4
MEASURED Ref	11890	12122	13081
Independent Ref	2473	2717	2893
	[R(int) = 0.0399]	[R(int) = 0.0631]	[R(int) = 0.0556]
GOOF	1.067	0.952	0.749
Final R Indices (I > 2σ(I))	R1 = 0.0187	R1 = 0.0272	R1 = 0.0207
	wR2 = 0.0399	wR2 = 0.0544	wR2 = 0.0373
Flack Parameter	0.357(5)		0.006(5)

. 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 Pb²⁺ environments leads to considerable distortions of the PbI₆ octahedra, as detailed in Figure 2. For [IqH]PbI₃, [4MiH]PbI₃, and [BzH]PbI₃, respectively, the Pb–I bond lengths were in the ranges of 3.0063(6)–3.5061(6), 3.0329(5)–3.4849(5), and 3.1197(7)–3.3709(7) Å, the I–Pb–I bond angles in the ranges of 85.582(14)–95.198(16), 82.133(13)–95.401(14), and 84.807(17)–99.555(18)°, and the Pb–I–Pb bond angles in the ranges of 91.303(15)–92.714(15), 87.357(14)–92.440(13), and 87.953(17)–94.323(16)° at 173 K. The distortions were further quantified using conventional polyhedral distortion indices and compared to those of the previously known examples of hybrid compounds displaying the same type of [PbI₃]_∞ chain in

Table 2. Calculated bond length distortions ^$ and bond angle variance ^$ and bond valence sums ^£ for [IqH]PbI₃, [4MiH]PbI₃, and [BzH]PbI₃ at 173 K, and previously reported [C₁₀H₇CH₂NH₃]Pbl₃ (298 K) and (C₇H₇N₂)PbI₃ (173 K).

Compound	[IqH]PbI3	[4MiH]PbI3	[BzH]PbI3	[C10H7CH2NH3]Pbl3 [19]	(C7H7N2)PbI3 [20]
Δd (× 10−4)	21.30	17.32	9.34	18.68	8.34
σ2	10.70	13.88	16.66	9.92	12.38
Σν (Pb)	1.82	1.86	1.79	1.80	1.78
Σν (I1)	0.54	0.50	0.40	0.54	0.42
Σν (I2)	0.56	0.66	0.70	0.48	0.64
Σν (I3)	0.71	0.70	0.68	0.78	0.72
$\mathsf{\Delta }d=\left(\frac{1}{6}\right){{\displaystyle \sum }}^{}{\left[\frac{d_n-d}{d}\right]}^2$					Equation (1)
${\sigma }^2={\displaystyle {\displaystyle \sum }_{i=1}^{12}}\frac{{({\theta }_i-90)}^2}{11}$					Equation (2)
${\nu }_{ij}=exp\left(\frac{R_0-d}{b}\right)$					Equation (3)

^$ The bond length distortion of the octahedra in each composition at both 173 and 298 K was calculated using Equation (1) [31], where d is the average Pb–I bond distance and d_n are the six individual bond distances. The bond angle variance of each octahedron from the ideal 90° of an undistorted structure was calculated using Equation (2) [32], where θ_i is the individual I–Pb–I angle. ^£ The bond valence was calculated using Equation (3) [33], where d is the individual bond length, R₀ is a constant for a particular bond type, here R₀ = 2.78 Å for the Pb–I bond, and b = 0.37 Å, a universal constant. The bond valence sum, Σν_i, is the summation of the individual bond valences.

. It is apparent that there is a considerable range of Δd and σ² distortions amongst this family. For comparison, the two known inorganic analogues, δ-CsPbI₃ and RbPbI₃, have Δd values of 10.61 and 8.81 and σ² values of 19.0 and 20.6, respectively (Table S6). It is generally the case in these structures that a larger Δd corresponds to a smaller σ², 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. Hydrogen bonds for [IqH]PbI₃, [4MiH]PbI₃, and [BzH]PbI₃ at 173 K (Å and °).

Compound	D-H...A	d(D-H)	d(H...A)	d(D...A)	<(DHA)
[IqH]PbI3	N(1)-H(1)...I(1)	0.86	3.24	3.852(7)	130.0
	N(1)-H(1)...I(2)	0.86	2.94	3.626(7)	138.3
[4MiH]PbI3	N(1)-H(1)...I(2)	0.86	3.04	3.612(7)	126.3
	N(1)-H(1)...I(1)#4	0.86	3.07	3.812(6)	145.2
	N(2)-H(2)...I(1)#5	0.86	2.96	3.700(7)	145.2
[BzH]PbI3	N(1)-H(1)...I(1)#5	0.86	2.90	3.598(7)	139.1
	N(3)-H(2)...I(1)	0.86	2.79	3.490(7)	139.2

Symmetry transformations used to generate equivalent atoms: #4 x, −y + 1/2, z − 1/2; #5 x + 1, y, z − 1 ([4MiH]PbI₃). #5 −x + 1, y + 1/2, −z + 3/2 ([BzH]PbI₃).

). 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).

#1 x − 1, y, z; #2 x − 1/2, −y + 1/2, −z + 1 ([IqH]PbI₃)
#1 x − 1, y, z; #2 x + 1, y, z ([4MiH]PbI₃)
#1 x − 1, y, z; #2 x − 1/2, −y + 3/2, −z + 1 ([BzH]PbI₃).

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 H-bonding 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 Figure 3, Figure 4 and Figure 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 [PbI₃]_∞ chains, such that the shortest inter-chain I–I distance in [4MiH]PbI₃ is 4.23 Å, compared to 4.31 Å in δ-CsPbI₃[11] 4.72 Å in [IqH]PbI₃ and 4.39 Å in [BzH]PbI₃.

It is of interest to compare the crystal packing within these three new examples to those of the two previously known examples. (C₇H₇N₂)PbI₃ [20] is essentially isostructural with [IqH]PbI₃ and [BzH]PbI₃, with the same space group and similar unit cell metrics. Indeed, the H-bonding scheme also very closely mimics that in [BzH]PbI₃. The organic moiety in (C₇H₇N₂)PbI₃ is benzimidazolium, which differs from benzotriazolium only in the substitution of the ‘central’ N atom by C–H. This minor change to a ‘hydrogen-bond-inactive’ part of the molecule clearly does not influence the crystal packing. In contrast, [C₁₀H₇CH₂NH₃]Pbl₃ [19] contains 1-naphthylmethylamine, which has more limited H-bonding opportunities, leading to a quite different style of crystal packing to those observed here.

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). The derived band gaps were 2.36 eV, 2.28 eV, and 2.16 eV for [IqH]PbI₃, [4MiH]PbI₃, and [BzH]PbI₃, respectively. 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 E_g = 2.44 eV) [20].

4. 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.