Crystalline Mixed Halide Halobismuthates and Their Induced Second

Article pubs.acs.org/cm

Crystalline Mixed Halide Halobismuthates and Their Induced Second Harmonic Generation Xiaoxin Zheng, Yang Liu,* Guangfeng Liu, Jie Liu, Xin Ye, Quanxiang Han, Chao Ge, and Xutang Tao* State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, P. R. China S Supporting Information *

ABSTRACT: Mixed halide coordination has been widely used to finely tune the properties of inorganic and inorganic− organic hybrid compounds, especially for emerging perovskites materials. Despite the increasing number of reports on preparation methods and the affected functionalities, the peculiar and precise role of the doping halogens in structural regulation of the crystals and the resulting variations on the basic properties remain to be addressed. Here, to shed light into the “black box”, a new series of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.847, and 1) single crystals were grown from the mixed halide solvents by the temperature lowering method. The correlation between the inclusion amounts of Br in the final crystals with the halide concentrations in the precursors is discussed from different perspectives. The two kinds of halogens share the same position in the mixed halide system, with every crystallographically independent halide site possessing different halogen occupancies. The mixed halide coordination exhibits a regulated effect on the distortion of the anion octahedra. Optical absorption, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and the second harmonic generation (SHG) measurements have confirmed that, with increased Br inclusion, [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 crystals exhibit a regulated effect on their bandgaps, thermal stabilities, and SHG capacities. halide crystals for infrared transmitting materials, Cs2HgI2Cl2,18 inorganic perovskites cesium lead halide,19 and especially the inorganic−organic hybrid perovskites methylammonium lead halide (MAPbX3, X stands for different or a mixture of halogens) systems.20−32 From both experimental measurements and theoretical calculations, coexistence of two types of halide elements may effectively influence the resulting chemical and physical properties. Mixed halide in the anionic groups does not change the overall packing mode of the species in crystals; thus, most of the mixed halide compounds are in the same space group as the compounds containing monohalogen. This strategy provides a platform to finely tune the functionalities of materials based on a constant matrix, which is conducive to a detailed and precise analysis of the relationship between the structures and properties. As mentioned above, mixed halide in the inorganic− organic hybrid perovskites effectively tunes their optical bandgaps,33,34 photoluminescence abilities,35,36 carrier diffusion length,24,25,27,28,37,38 and subsequently their photovoltaic performances.22,23,39 However, a deep understanding about the specific effect of the doping halogens on the structural features of the halides, and how they affect or determine the functionalities of the mixed halide perovskites, remains one of the

1. INTRODUCTION The inorganic−organic hybrid compounds, in which the organic and inorganic components crystallize together according to stoichiometric ratios, have been the subject of increased attention.1−7 In principle, the inorganic−organic hybrid crystals combine the advantages of both inorganic and organic materials, endowing them with more adjustable species diversity and higher thermal stability and mechanical processability. Inorganic− organic hybrid compounds composed of metal-containing cluster ions and organic ligand molecules, such as carboxylates, phosphonates, or amines,8 normally regulate their structural and functional properties by incorporation of different components. Among the inorganic−organic hybrid crystals, a class of compounds with a crystal structure of perovskite has obtained a significant breakthrough as brilliant light absorbers in solar cells and other optoelectronic applications in recent years.9 The inorganic−organic hybrid perovskites were found to possess suitable band-gaps, high extinction coefficients, and high carrier mobility.10,11 As a result, the halide perovskite solar cells have seen an increase in power conversion efficiency from 9.7%12 to ∼20%13,14 in just two years.15 The increase in speed is largely due to a strategy of mixed halide perovskites. Mixed halide, a chemical method to finely tune the structural features of many coordination compounds, has been found to have a dramatic effect on the optical and electrical properties of the halide compounds, such as tetrahalogenozincate (II) complexes,16 zirconium clusters,17 strontium barium halide scintillators, thallium © 2016 American Chemical Society

Received: April 21, 2016 Revised: May 18, 2016 Published: May 19, 2016 4421

DOI: 10.1021/acs.chemmater.6b01622 Chem. Mater. 2016, 28, 4421−4431

Article

Chemistry of Materials most uncertain questions. Researchers have employed firstprinciples modeling to examine the elusive effect of the mixed halide in perovskite solar cells.20,40 Experimental studies about the detailed structural regulation of MAPbI3−xClx and MAPbBr3−xClx with different compositions of halogens have also made important advances recently.37,41 On the other hand, systematic investigations about the effect of mixed halides on other halides compounds are far more lacking. Thus, the fundamental understanding of the connections between the halogen ratio of the mixed halide and the resulting structural changes and the subsequent effects on their properties are quite limited, on the basis of a large number of inorganic−organic hybrid compounds. In addition, for the perovskites MAPbCl, MAPbBr, and MAPbI, their crystal structures are inconsistent even at the same temperature,42−44 and they have different doping concentration limits of Cl into the bromide and/or iodide perovskites,37 making this system not an ideal model for studying the subtle structural regulation on account of a stable crystal packing mode. Thus, in this paper, we investigate another kind of metal halide inorganic−organic hybrid compound as a mixed halide model: the halobismuthate materials. The chemistry of the bismuth(III) halides has been explored for several decades owing to the strong tendency of bismuth to act as a halide ion acceptor and its extensive structural diversity.45−53 For the elements of VA group, there are numerous mixed-halide species of phosphorus, while the mixed halides of bismuth are comparatively rare due to the apparent decreasing tendency to form mixed-halide species upon leaving the group.54 Here, we systematically studied a series of bismuth haide-based crystals, tridiethylammonium hexahalogen bismuthate ([NH2(CH2CH3)2]3Bi(Cl1−xBrx)6), to illustrate the role of mixed halide in affecting the crystal structures and the resulting properties. The crystals of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 contain {BiX6} octahedras, which are similar to the {PbX6} octahedra in the perovskites. It is worth noting that recent studies showed that the bismuth halide perovskite (CH3NH3)3(Bi2I9) can be used as a light absorber in solar cells and shows photovoltaic properties.53 Moreover, in the series of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 crystals, the halogen doping concentration has no upper limit, and all of them share an undisputed and consistent space group. These features overcome obstacles existing in the lead perovskites for the structural analysis. We successfully grew a series of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.847, and 1) single crystals from the mixed halide solvents by the temperature lowering method. The structural and optical characterization on these crystals were explored in detail, revealing the systematic structural regulation over the ratio of Br/Cl, and the effect on the distortion of the {BiX6} octahedron, which resulted in different nonlinear optical capacities of the materials.

The synthesized salts were recrystallized twice from the corresponding mixed solvent to improve the purity. The final single crystals of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 were grown from the saturated solutions of the purified products of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 in the corresponding HCl/HBr mixed solvent, by the slow-cooling method from 50 to 30 °C in a sealed vial. Powder X-ray Diffraction Analysis. Powder X-ray diffraction was performed on a Bruker D8 ADVANCE X-ray diffractometer equipped with a diffracted beam monochromator set for Cu Kα radiation (λ = 1.54056 Å) in the 2θ range of 10−60°, with a step size of 0.02° and a step time of 0.04 s at room temperature. Single-Crystal X-ray Diffraction Analysis. A block-shaped single-crystal (with a size of about 0.2 mm × 0.2 mm × 0.2 mm) of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 synthesized and grown from the mixed solvents with different HCl/HBr ratios was selected for singlecrystal X-ray data collection on a Bruker SMART APEXII CCD area detector of a D8 goniometer at 100 K. Data were collected using graphite-monochromated and 0.5 mm Mono Cap-collimated Mo Kα radiation (λ = 0.71073 Å) with the ω scan method. Data were processed with the INTEGRATE program of the APEXII software for reduction and cell refinement. Multiscan absorption corrections were applied by using the SCALE program for the area detector. The structure was solved by direct methods and refined by the full-matrix least-squares method on F2 (SHELX-97).56 All non-H atoms were refined anisotropically. Hydrogen atoms were placed in idealized positions and included as riding with Uiso (H) = 1.2Ueq (C). Elements Concentration Analysis. The concentrations of Cl− and Br− ions in the obtained crystals were measured by the X-ray fluorescence (XRF) analysis method (Rigaku, ZSX primus II). UV−vis Diffuse Reflectance Analysis. UV−vis diffuse reflectance spectra were recorded for the dry-pressed disk samples by a Shimadzu UV2550 recording spectrophotometer in the wavelength range of 300− 700 nm. BaSO4 was used as a reference material. Reflectance spectra were converted to absorbance with the Kubelka−Munk function.57 Second-Harmonic Generation Analysis. Powder second harmonic generation (SHG) measurements were performed on a modified Kurtz-NLO58 system. Since SHG efficiencies are known to depend strongly on particle size, polycrystalline samples were ground and sieved into the following particle size ranges: 45−58, 58−75, 75−109, 109−120, and 120−150 μm. The samples were pressed between glass microscope cover slides and secured with tape in 1 mm thick aluminum holders containing an 8 mm diameter hole. To make relevant comparisons with known SHG materials, crystalline KH2PO4 (KDP) were also ground and sieved into the same particle size. The samples were then placed in a light-tight box and irradiated with a pulsed laser. The measurements were performed with a Q-switched Nd:YAG laser at the wavelength of 1064 nm. A cutoff filter was used to limit background flash-lamp light on the sample. An interference filter (530 ± 10 nm) was used to select the second harmonic for detection with a photomultiplier tube attached to a RIGOL DS1052E 50 MHz oscilloscope. This procedure was then repeated using the standard nonlinear optical materials KDP, and the ratio of the second-harmonic intensity outputs was calculated. No index-matching fluid was used in any of the experiments. Thermal Stability Analysis. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) for the [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.847, and 1) crystals were carried out using a TGA/DSC/1600HT analyzer (METTLER TOLEDO Instruments). The sample was placed in an Al2O3 crucible and heated at a rate of 10 K/min from room temperature to 670 K under flowing nitrogen gas.

2. EXPERIMENTAL SECTION Materials Preparation. [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.847, and 1) crystals were obtained from the precursor solutions by spontaneous crystallization according to the previous report.55 All of the starting materials used were analyticalgrade reagents from TCI Chemical Company without further purification. The raw [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 products were prepared by dissolving [NH2(CH2CH3)2]Cl and Bi2O2CO3 with a molar ratio of 6:1 in the mixed solvent HCl/HBr. The mixed solvents HCl/HBr were prepared by mixing 36% HCl and 47% HBr, with the molar ratio of HCl/HBr = 1/0, 0.8/0.2, 0.6/0.4, 0.4/0.6, 0.2/0.8, and 0/1, respectively. The solution was stirred for 3 h at room temperature, and the solvent was evaporated by vacuum distillation.

3. RESULTS AND DISCUSSION Materials Preparation and Elements Concentration. The halobismuthate compounds were frequently synthesized via supplying additional halide ions to bismuth(III) trihalides (BiX3).48 The additional halide ligands are believed to terminate forming the extended Bi−X−Bi bridge linkages, leading to a reduction in dimensionality from a 3-dimensional framework 4422


Article

Chemistry of Materials structure to a 0-dimensional discrete octahedral [BiX6]3− ion by dismantling the corner sharing in BiX3 octahedra.59 Another often-used synthetic route to halobismuthate materials involves the reaction of BiX3 with alkylamine and a haloacid (HX) serving as both X- source and proton donor.54 On account of a higher propensity to the formation of mixed haloanions of bismuth by feeding a haloacid containing a different halogen with the one in bismuth trihalide, we adopted a modified synthetic pathway employing bismuth subcarbonate Bi2O2CO3 and [NH2(CH2CH3)2]Cl as the starting materials to react in a mixed solvent HCl/HBr. The mixed solvents HCl/HBr, serving as both proton donor and, more importantly, the mixed halide resources, were prepared by mixing 36% HCl and 47% HBr with different molar ratios. Because the specific stoichiometries of the halobismuthate are dependent on the relative concentration of the haloacid in the starting HCl/HBr mixed solvents, we can utilize this to regulate the relative content of Cl/Br in the target crystals. The synthetic conditions are shown in Table 1. We used six kinds of mixed solvents with the molar

[NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 show different halogen contents, with x = 0, 0.135, 0.255, 0.385, 0.847, and 1, respectively. Furthermore, the element concentrations of Br and Cl in each crystal were measured by an XRF (X-ray fluorescence) spectrometer. As shown in Table 1, the XRF results are in close agreement with the data from single crystal analysis, confirming the mixed halogens in the crystals. Figure S2 demonstrates the relationship between the halogen concentrations in the precursor solvents and those in the resulting crystals. We can see the halogen content is not in strict consistency with the halogen concentration in the mixed solvents. Nevertheless, in general, more specific halogen will be included in the crystals from the solvent with a higher concentration of the corresponding halogen. For example, when the Br concentration of the mixed solvent is 0.4, the Br/(Br + Cl) ratio (x) in the [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 is only 0.255; when the Br concentration of the mixed solvent increases to 0.8, the x value in the crystal becomes 0.847, bigger than that in the precursor. We attribute the deviation to the removal of the lower portion constituents during the crystallization process. That is, when the bromide constitutes the minority of the ions, it will be more likely to be excluded into the crystals during crystallization, making the content of bromide in crystals even lower than in the solutions, and when the bromide constitutes the majority of the ions, its content in crystals is higher due to the removal of chloride. In addition, the different affinities of Cl and Br in forming the coordination compound would be another non-negligible factor that influences the Br/Cl ratios in crystals. The different Br/Cl ratios in the single crystals from the precursor solutions remind us to be cautious about the halogen concentration of the in situ formed films of mixed halide perovskites. Previously reported Br/Cl ratios in the spincoated CH3NH3Pb(Cl1−xBrx)3 films were regarded as the same as that of the precursors.23,35 However, the variation of the Br/Cl ratio during the spontaneous crystallization of the CH3NH3Pb(Cl1−xBrx)3 single crystals also appears.41 Thus, we must discriminate the halogen doping concentration of the mixed halide compounds from that of the feeding precursors, especially in case of the disturbance of the unreacted halides to the elemental analysis result.

Table 1. Synthetic Conditions (Solvent Ratios and Raw Materials) and the Bromine Content (Detected by XRF and Single Crystal XRD) of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 Crystals x no.

solvent HCl/HBr

1 2

1/0 0.80/0.20

3

0.60/0.40

raw material

XRF 0 0.128 0.250

NH2(CH2CH3)2Cl + Bi2O2CO3

4

0.40/0.60

0.379

5

0.20/0.80

0.845

6

0/1

1

XRD (single crystal) 0 0.135 (x1 = 0.213, x2 = 0.053)a 0.255 (x1 = 0.389, x2 = 0.121)a 0.385 (x1 = 0.557, x2 = 0.214)a 0.847 (x1 = 0.931, x2 = 0.762)a 1

a

x1 and x2 are the bromine content of (Cl1−xBrx)1 and (Cl1−xBrx)2, respectively.

ratio of HCl/HBr = 1/0, 0.8/0.2, 0.6/0.4, 0.4/0.6, 0.2/0.8, and 0/1, respectively. Consequently six new halobismuthatethe were successfully synthesized. The synthesized [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 salts were purified via recrystallization in the corresponding HCl/HBr mixed solvent. The final single crystals were grown from the saturated solutions by the slow-cooling method from 50 to 30 °C in sealed vials. The tri(diethylammonium) hexahalobismuthates have good crystallization capacity; our recent report shows that [NH2(CH2CH3)2]3BiCl6 can crystallize into bulk single-crystals with a size of several centimeters by the slow-cooling method in halide acid (Figure 2a).55 Single crystals with differnt halide compositions are shown in Figure S1. The specific halogen concentration in each crystal was determined through crystallographic structural refinement by a full matrix least-squares against F2 method within SHELXTL.56 Preliminary refinement of all the six independent halides as fully occupied atoms resulted in unreasonable displacement ellipsoids when refined as either Br or Cl, suggesting the statistical Br/Cl disorder on these sites. After assignment of them as mixed Br/Cl positions, free refinement of the occupancies leads to constraint to sum to unity. The final refined occupancy ratios for the crystals grown from different mixed solvents are shown in Table 1. From the mixed solvents with the molar ratio of HCl/HBr = 1/0, 0.8/0.2, 0.6/0.4, 0.4/0.6, 0.2/0.8, and 0/1, single crystals of

Figure 1. Powder XRD of [NH2(CH2CH3)2]3 Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.847, 1). The peaks in the dashed rectangle box indicate the stepwise increase of the interplanar distance of (022̅) along with the x = 0 to x = 1. 4423


Article

Chemistry of Materials Structure Analysis. Systematic studies were performed on the structures of the series of mixed halide halobismuthate crystals. First the X-ray diffraction (XRD) of the ground powders of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.847, 1) crystals were measured. Figure 1 shows the XRD patterns. We can see the diffraction peaks of all the crystals show a similar profile, indicating that they belong to an identical space group. What is notable is that the corresponding diffraction peaks of each crystal shift to a slightly lower angle as the x value increases. This means that, with an increase of Br inclusion, the interplanar distances of the corresponding crystal planes expand continuously. Taking the (022̅) plane as an example, we can see that, without Br substitution (x = 0), its diffraction peak (2θ) is located at 16.57°, corresponding to an interplanar distance of 5.35 Å; when x = 0.385, the diffraction peak shifts to 16.35°, corresponding to an interplanar distance of 5.42 Å, and when all the Cl ions are replaced by Br (x = 1), the diffraction peak shifts to 16.02°, corresponding to an interplanar distance of

5.53 Å. These results are consistent with the single crystal data analyzed below, which show the expansion of crystal lattices of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 crystals due to the incorporation of more bigger Br ions. Structural analyses based on single crystal XRD determination were carried out on the series of crystals [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.847, 1). Unlike the mixed halide perovskites and other halobismuthate compounds,54 where different halide coordinations may change the crystal structures, here all of the crystals belong to the trigonal system with the same space group of R3c, including the pure chloride and bromide bismuthates. Details of the crystallographic data are shown in Table 2. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via https://summary.ccdc.cam.ac.uk/structuresummary-form. The structures were deposited in Cambridge Structural Database (CSD) with numbers: 1444702−1444706 and 1031018. The unit cell parameters of a and c are in the range

Table 2. Detailed Single-Crystal Structural Information of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 no.

1

2

3

4

5

6

x (single crystal XRD)

0

0.135

0.255

0.385

0.847

1

empirical formula

C12H36BiCl6N3

C12H36BiBr0.81Cl5.19N3




C12H36BiBr6N3

formula weight

644.12

681.17

710.81

747.86

866.42

910.88

temperature (K)

100(2)

wavelength (Å)

0.71069 trigonal, R3c

crystal system, space group unit cell dimensions (Å) a = 14.699(4)

a = 14.739(5)

a = 14.796(5)

a = 14.848(5)

a = 15.076(5)

a = 15.166(5)

b = 14.699(4)

b = 14.739(5)

b = 14.796(5)

b = 14.848(5)

b = 15.076(5)

b = 15.166(5) c = 19.733(5)

c = 19.102(5)

c = 19.170(5)

c = 19.268(5)

c = 19.363(5)

c = 19.631(5)

volume (Å3)

3574.2(17)

3607(2)

3653(2)

3697(2)

3864(2)

3931(2)

Z, calculated density (mg/m3)

6, 1.795

6, 1.882

6, 1.939

6, 2.015

6, 2.234

6, 2.309

absorption coefficient (mm−1)

8.072

9.292

10.195

11.335

14.704

15.878

F(000)

1884

1974

2046

2136

2424

2532

crystal size (mm)

0.40 × 0.30 × 0.30

0.20 × 0.20 × 0.20

0.20 × 0.20 × 0.20

0.20 × 0.20 × 0.20

0.20 × 0.20 × 0.10

0.20 × 0.20 × 0.20

θ range for data collection (deg)

2.67 to 27.49

2.66 to 27.53

2.64 to 27.51

2.63 to 27.49

2.60 to 28.31

2.58 to 27.53

limiting indices

−18 ≤ h ≤ 19, −19 ≤ k ≤ 18, −24 ≤ l ≤ 24

−19 ≤ h ≤ 18, −19 ≤ k ≤ 19, −24 ≤ l ≤ 24

−19 ≤ h ≤ 19, −19 ≤ k ≤ 19, −24 ≤ l ≤ 24

−19 ≤ h ≤ 19, −19 ≤ k ≤ 19, −25 ≤ l ≤ 25

−20 ≤ h ≤ 19, −19 ≤ k ≤ 20, −25 ≤ l ≤ 25

−19 ≤ h ≤ 19, −19 ≤ k ≤ 19, −25 ≤ l ≤ 25

reflections collected/ unique

12907/1816 [R(int) = 0.0278]

12522/1826 [R(int) = 0.0454]

13094/1848 [R(int) = 0.0241]

13369/1874 [R(int) = 0.0278]

14287/2090 [R(int) = 0.0436]

13867/2013 [R(int) = 0.0399]

completeness to θ = 27.49

100.00%

100.00%

99.90%

99.90%

99.10%

99.90%

0.1956 and 0.1406

0.2580 and 0.2580

0.2350 and 0.2350

0.3210 and 0.1570

0.1434 and 0.1434

2090/1/71

2013/1/70

absorption correction max. and min. transmission

semiempirical from equivalents 0.2102 and 0.2102

full-matrix least-squares on F2

refinement method data/restraints/ parameters

1816/1/70

1826/1/72

1848/1/72

goodness-of-fit on F2

1.092

1.067

0.938

0.912

0.969

0.911

final R indices [I > 2σ(I)]

R1 = 0.0109, wR2 = 0.0295

R1 = 0.0154, wR2 = 0.0379

R1 = 0.0105, wR2 = 0.0276

R1 = 0.0123, wR2 = 0.0299

R1 = 0.0186, wR2 = 0.0332

R1 = 0.0146, wR2 = 0.0302

R indices (all data)

R1 = 0.0109, wR2 = 0.0295

R1 = 0.0155, wR2 = 0.0380

R1 = 0.0107, wR2 = 0.0277

R1 = 0.0129, wR2 = 0.0302

R1 = 0.0213, wR2 = 0.0338

R1 = 0.0156, wR2 = 0.0304

absolute structure parameter

−0.009(6)

0.016(7)

−0.001(5)

−0.007(5)

−0.019(7)

−0.032(6)

largest diff. peak and hole (e. Å−3)

0.783 and −0.392

1.073 and −1.082

0.432 and −0.330

0.551 and −0.295

0.903 and −0.502

0.508 and −0.371

4424

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Chemistry of Materials

Figure 2. (a) Single crystal of [NH2(CH2CH3)2]3BiCl6 (Reproduced from ref 55 with permission from the Royal Society of Chemistry). (b) Crystal structural plot of the components of [NH2(CH2CH3)2]3Bi(Cl0.615Br0.385)6. (c) Bi(Cl0.615Br0.385)63− octahedron. Symmetry transformations used to generate equivalent atoms: a: −y + 2, x − y + 1, z; b: −x + y + 1, −x + 2, z. (d) View of the crystal packing along the a axis. (e) View of the crystal packing along the c axis. The Bi(Cl0.615Br0.385)63− octahedron is shown in lavender. Color scheme: Bi (III), purple; (Cl0.615Br0.385), brown; C, gray; N, blue; H, white.

the {BiX6} octahedrons are isolated by diethylammonium cations; no Bi polyhedral chains exist. The [BiX6]3− anions stack more closely in the (1000) plane than in the (0001) plane, resulting in different thermal properties of the crystals in different directions.55 To more fully understand the distortion of the [BiX6]3− octahedron, especially the influence of the mixed halide on the structural variation, we analyzed the relationship between the content of bromine inclusion with the Bi−X bond lengths and the difference between the two kinds of bond lengths. As shown in Figure 3a, the increasing trends of the bond length of the longer and shorter Bi−X bonds are different. This makes the deviation of the two bonds along X1−Bi−X2 (Δbond length) first decrease and then increase, with a minimum value at the medium Br inclusion (Figure 3b). We believe the variation of the Δbond length is related to the asymmetrical distribution of Br in the two halogen sites of X1−Bi−X2. As described above, the shorter and longer Bi−X bonds contain different Br content. A higher Br content in the shorter Bi−X bond makes it a little longer and vice versa. Thus, we can see with a medium overall Br inclusion the difference of Br content in the two kinds of bond (ΔBr) reaches its maximum, inducing the smallest bond length difference. The angle of X1−Bi−X2 also changes as a function of Br inclusion. As shown in Figure S4, being inversely related to the changing trend of Δbond length, the bond angle reaches its maximum at a medium Br inclusion. Figure 4 shows the details of the evolution trend of a {BiX6} octahedron, where we can compare the gradual change of the bond length and bond angle with the increase of Br inclusion in one frame. There is no doubt that the deviation of Bi−X bond lengths and X1−Bi−X2 angles would determine the magnitude of the distortion of the anion octahedron. Due to the complexity induced by the mixed occupancy of the halide coordination, it is difficult to figure out the induced distortion of each octahedron exactly. While the direction and magnitude of the distortion of the octahedra can be quantified by determining the statistical local dipole moments, this method uses a bond-valence approach to calculate the direction and magnitude of the dipole moments.66−71 Using this methodology, we were able to calculate the dipole moment of each Bi(Cl1−xBrx)6

of 14.699−15.159 and 19.102−19.713 Å, respectively, resulting in the cell volume V ranging from 3574.2 to 3923.2 Å3. The variation of cell dimensions a and c and the cell volume V, as a function of bromine inclusion content, are shown in Figure S3. We can see both a and c increase almost linearly with the ratio of bromide. Figure 2 shows the single crystal XRD structure of the mixed halide bismuthates, where we choose a crystal with x = 0.385, that is, [NH2(CH2CH3)2]3Bi(Cl0.615Br0.385)6 as an example. The compound is composed of protonated diethylammonium cations and [BiX6]3− anions (Figure 2b). The {BiX6} octahedrons are isolated and 0-connected because of the large excess of the haloacid in the reaction solutions. We can see that all of the six halogens in an octahedron are mixed sites. According to the previous studies, there are two cases for mixed halide coordination: one is the different halogens occupy different and fixed positions;18,60,61 the other one is different halogens share a same position, which means that all the different halogens can appear on that position, with an occupancy sum being equal to unity. The two different mixed coordinations can be distinguished by the stoichiometric ratio of the halogens, where the latter one is normally a fractional number standing for the statistical probability of the occupancy of specific halogen. This fractional occupancy is dominant in the lead coordination of the well-known mixed halide perovskites systems.20−32,41 For the mixed halide bismuthates, both of the cases occur.54 Here, all of the crystallographically independent halide sites in the anion are affected by halide mixing. According to the structural symmetry, they were divided into two classes with different bond lengths. As shown in Figure 2c, different bond lengths also result in different halogen occupancies, where the shorter Bi−X bonds (2.701 Å) are accompanied by a higher Br occupancy (0.557) and the longer Bi−X bonds (2.906 Å) have a lower Br occupancy (0.214), thus leading to a final refined composition of the anion being “Bi(Cl0.615Br0.385)6”. Along the two opposite Bi−X bonds, e.g., (Cl0.443Br0.557)1−Bi−(Cl0.786Br0.214)2, a skewing with an angle of 172.90° makes an obvious distortion of the [BiX6]3− octahedron. Figure 2d,e illustrates the packing motifs of [NH2(CH2CH3)2]3Bi(Cl0.615Br0.385)6 crystal viewed along the a- and c-axis, respectively. It can be seen clearly that 4425


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Figure 3. (a) Changing of the bond lengths of the two Bi−X bonds, Bi−(Cl1−xBrx)1 and Bi−(Cl1−xBrx)2, as a function of bromine inclusion. (b) Changing of the deviation between the two bonds along (Cl1−xBrx)1−Bi−(Cl1−xBrx)2 (Δbond length) and the difference of Br content in the two halogen sites (ΔBr) in [NH2(CH2CH3)2]3 Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.847, 1) as a function of bromine inclusion. The (Cl1−xBrx)1 and (Cl1−xBrx)2 are of directly opposite positions.

Figure 4. Changing trend of the Bi(Cl1−xBrx)63− octahedra in [NH2(CH2CH3)2]3 Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.847, 1) single crystals with increasing bromine inclusion. The bond lengths and angles along X1−Bi−X2 are labeled. Spheres with green and brown colors stand for the occupancy of Cl and Br, respectively.

The distortion of the {BiX6} octahedron also leads to the variation of the interactions between BiX63− anions and diethylammonium cations. As shown in Figure S5, in the pure chloride and bromide systems, only one kind of N−H···Cl (or Br) and N···Cl (or Br) bond exists. However, in the mixed halide systems, all the halogen bonds including N−H···Cl/Br and N···Cl/Br bonds appear, which should affect the distortion of the octahedrons in reverse to some extent. The distortion of the octahedra will affect a host of important properties such as piezoelectricity, ferroelectricity, and nonlinear optics.62−65 In addition, the crystal structural alteration brought by mixed halide must induce variation of materials properties, which will be discussed below.55,62−65

octahedron. (Each halide site was defined statistically from the occupancy of the different halogens. For details, see Table S1.) The results show relatively large differences of the dipole moments with different halogen ratios, which should be due to the deviation of Bi−X bond lengths and X1−Bi−X2 angles. The change in the dipole moments of the Bi(Cl1−xBrx)63− octahedra with respect to bromine inclusion is shown in Figure 5. We can see that, with a medium Br inclusion, the {Bi(Cl0.615Br0.385)6} octahedron is of the largest dipole moment, which is about 3 and 16 times that of {BiCl6} and {BiBr6} octahedra, tively. This data means that the mixed halides could make an obvious contribution to the distortion of {BiX6} octahedron. 4426


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series of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0. 847, 1) crystals. The normalized spectra are shown in Figure 6, which reveal an excitonic peak followed by a continuum absorbance at higher energy. We noticed that, with the increase of Br inclusion, the UV−vis absorbance edge redshift from ∼380 to ∼442 nm, indicating an estimated band gap from ∼3.26 to ∼2.81 eV (the inset (b) of Figure 6). The gradual shifts of the absorption with the increasing Br inclusion are manifested by the color of the [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 powders. As shown in the inset (a) of Figure 6, the color of the crystalline powders deepen stepwise as the x value increases from 0 to 1, with the [NH2(CH2CH3)2]3BiCl6 single crystal being colorless and the [NH2(CH2CH3)2]3BiBr6 being pale yellow. This observation is also consistent with observations of the CH3NH3Pb(Br1−xClx)3 perovskites, where different Br/Cl ratios tune the band gaps of the crystals and films.24,35,41 Thermal Stability. To understand the connections between thermal properties and the Br/Cl ratios of the [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.847, and 1) crystals, differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were performed. As shown in Figure 7, endothermal peaks on the DSC curves corresponding to the melting points shift to higher temperatures with an increase in Br inclusion; the TGA curves also demonstrate elevating decomposition temperatures along with higher x values for the [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0. 847, 1) crystals. These results show a definite effect of Br doping to improve the thermal stability of the [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 crystals. Second-Harmonic Generation (SHG). Because all the series of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 crystals belong to the noncentrosymmetric R3c space group, we measured their second-order NLO (Nonlinear Optical) properties, aiming to correlate the crystal structural distortion induced by mixed halides with their SHG capacities. Figure 8a shows the SHG signal intensity versus particle size for various [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.0.847, 1) crystals. As is shown, the SHG intensity increases with particle size and plateaus at a maximum value, indicating the phase-matchable nature of the materials. What is notable is that the SHG signal intensities of samples with higher Br inclusion increase much greater than those with lower Br inclusion. This comparison is clearer for the different samples in the same particle range from

Figure 5. Change in dipole moments of the Bi(Cl1−xBrx)63− octahedra in [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.847, 1) single crystals versus bromine inclusion.

Figure 6. Diffuse reflectance UV−visible absorption spectra of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.847, 1) crystalline powders. Inset (a) is the photo of the [NH2(CH2CH3)2]3 Bi(Cl1−xBrx)6 powders with different x values. Inset (b) shows the changing of the bandgaps of each [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 crystal deduced from the absorption spectra.

Optical Absorption Properties. To study the effect of mixed halide on the materials properties, UV−vis diffuse reflectance optical absorption spectra were first collected on the

Figure 7. DSC (a) and TGA (b) thermograms of [NH2(CH2CH3)2]3 Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.847, 1) crystalline powders. 4427


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Figure 8. (a) Second-harmonic intensity of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 1) crystals as a function of particle size. (b) Second-harmonic intensity of various [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 (x = 0, 0.135, 0.255, 0.385, 0.847, 1) and KDP powder in the same particle range (109−120 μm).

109 to 120 μm (Figure 8b). We can see that the pure chloride bismuthate generates a SHG intensity of about 1.9 times that of KDP sample with the same particle range. The mixed halide bismuthate crystals generate a stronger SHG signal with an increase in Br inclusion, with a maximum value of 3.2 times of KDP for the pure bromide bismuthate. The SHG efficiency of the mixed halide bismuthates fall in between the pure chloride and bromide bismuthates but do not exceed both of them. Here, the changing trend of the SHG effect is inconsistent with that of the distortion of the [BiX6]3− octahedra. According to Chen’s anionic group theory, the SHG response in an inorganic material is attributable to the anionic groups.72−74 The main responsible factor in the halide bismuthates should be the distortion magnitude of the [BiX6]3− octahedral induced by mixed halide. We speculated that the inconsistency may come from the large volume and complex structure of the inorganic− organic hybrid system. The organic cations themselves, and the synergistic effect including variational interactions between organic cations and inorganic ions, may contribute to the NLO effect. Thus, it is insufficient to determine the magnitude of the NLO effect just by the distortion of the anionic groups for the inorganic−organic hybrid materials. Nevertheless, these results still reveal concrete judgment for the effect of mixed halide coordination on the SHG capacities. More importantly, we believe the tunability implemented via mixed halide resources may apply to other inorganic−organic hybrid materials.

[NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 as a model to study the role of mixed halide in affecting the crystal structures and the properties of inorganic−organic hybrid compounds. We found several detailed but important clues: one is that the inclusion ratio of the second halogen into a mixed halide crystal is not linearly correlated with the ratio in the mixed halide resources. Normally, the minority ions are more likely to be excluded into the crystals. Second, in this mixed halide system with fractional stoichiometric ratio, the two halogens share the same position, with every crystallographically independent halide site possessing different halogen occupancies. Optical absorption, TGA, DSC, and the SHG measurements have confirmed that, with increased Br inclusion, [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 crystals exhibit a regulated effect on their bandgaps, thermal stabilities, and SHG capacities. The results shed light on the hidden structural mechanisms of the mixed halide inorganic− organic materials, promising useful directions for the precise chemical methods needed to modulate the properties of materials via mixed halide resources.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01622. Photo of single crystals of [NH 2 (CH 2 CH 3 ) 2 ] 3 Bi(Cl1−xBrx)6 (x = 0.255, 0.847, 1); the relationship of x value in [NH2(CH2CH3)2]3Bi(Cl1−xBrx) 6 single crystals with the halogen concentrations in the precursor solvents; the single crystal XRD cell dimensions and volumes of [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 as a function of bromine inclusion; the bond angle of (Cl1−xBrx) 1−Bi−(Cl1−xBrx)2 in [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 as a function of bromine; the interactions between Bi(Cl1−xBrx)63− anions and diethylammonium cations of various [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 crystal structures. (PDF)

4. CONCLUSION Mixed halide coordination in many materials has established important applications as an effective functionality regulator, represented by the most promising mixed halide perovskites photovoltaics. Chloride doping in the MAPbI3−xClx perovskites have been proven to effectively enhance the stability and carrier diffusion length. However, in these systems, many underlying mechanisms related to the structural changing caused by mixed halide are far behind the applications due to the lack of systematic structural studies. In this paper, we choose 4428


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The crystal date file of various [NH2(CH2CH3)2]3Bi(Cl1−xBrx)6 crystals. (CIF) The method of dipole moment calculations. (XLSX)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Yuqiao Zhou and Professor Ning Ye for their kind help in the second-harmonic generation analysis. We thank the National Natural Science Foundation of China (grant no. 51321091, 51227002, 51303095, and 51272129) and the Program of Introducing Talents of Disciplines to Universities in China (111 program no. b06015).

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