Synthesis, crystal structure, and properties of a

Synthesis, crystal structure, and properties of a perovskite-related bismuth phase, (NH4)3Bi2I9 Shijing Sun, Satoshi Tominaka, Jung-Hoon Lee, Fei Xie, Paul D. Bristowe, and Anthony K. Cheetham Citation: APL Mater. 4, 031101 (2016); doi: 10.1063/1.4943680 View online: http://dx.doi.org/10.1063/1.4943680 View Table of Contents: http://scitation.aip.org/content/aip/journal/aplmater/4/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Structural phase transition causing anomalous photoluminescence behavior in perovskite (C6H11NH3)2[PbI4] J. Chem. Phys. 143, 224201 (2015); 10.1063/1.4936776 Synthesis, structural and optical properties of perovskite type CH3NH3PbI3 nanorods AIP Conf. Proc. 1665, 080034 (2015); 10.1063/1.4917938 Publisher's Note: “Purple photochromism in Sr2SnO4:Eu3+ with layered perovskite-related structure” [Appl. Phys. Lett. 102, 031110 (2013)] Appl. Phys. Lett. 102, 099903 (2013); 10.1063/1.4792374 Purple photochromism in Sr2SnO4:Eu3+ with layered perovskite-related structure Appl. Phys. Lett. 102, 031110 (2013); 10.1063/1.4788752 Strong reddish-orange light emission from stress-activated Srn+1SnnO3n+1:Sm3+ (n = 1, 2, ∞) with perovskite-related structures Appl. Phys. Lett. 101, 091113 (2012); 10.1063/1.4749807

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APL MATERIALS 4, 031101 (2016)

Synthesis, crystal structure, and properties of a perovskite-related bismuth phase, (NH4)3Bi2I9 Shijing Sun,1 Satoshi Tominaka,2 Jung-Hoon Lee,1 Fei Xie,1 Paul D. Bristowe,1 and Anthony K. Cheetham1,a 1

Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, United Kingdom 2 International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Ibaraki 305-0044, Japan

(Received 29 January 2016; accepted 29 February 2016; published online 15 March 2016) Organic-inorganic halide perovskites, especially methylammonium lead halide, have recently led to remarkable advances in photovoltaic devices. However, due to environmental and stability concerns around the use of lead, research into lead-free perovskite structures has been attracting increasing attention. In this study, a layered perovskite-like architecture, (NH4)3Bi2I9, is prepared from solution and the structure solved by single crystal X-ray diffraction. The band gap, which is estimated to be 2.04 eV using UV-visible spectroscopy, is lower than that of CH3NH3PbBr3. The energy-minimized structure obtained from first principles calculations is in excellent agreement with the X-ray results and establishes the locations of the hydrogen atoms. The calculations also point to a significant lone pair effect on the bismuth ion. Single crystal and powder conductivity measurements are performed to examine the potential application of (NH4)3Bi2I9 as an alternative to the lead containing perovskites. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). [http://dx.doi.org/10.1063/1.4943680]

Renewable energy sources such as solar, hydroelectric, and wind power have attracted increasing attention in recent years due to growing global energy demands as well as environmental considerations. Not surprisingly, therefore, research and development into promising new materials and devices that improve the use of renewable energy, for example, solar cells and eco-friendly light emitting diodes, have been under the spotlight in both academia and industry. Among these initiatives, a relatively young class of hybrid materials, namely, organo-lead trihalide perovskites, has recently impressed the solar cell community with its outstanding performance in photovoltaic devices. In particular, solar cells using methylammonium lead iodide (CH3NH3PbI3) have seen their power conversion efficiencies increase from less than 4% in 20091 to over 20% now.2 The CH3NH3PbI3 perovskite offers a number of advantages including large absorption coefficients, high charge carrier mobilities, as well as excellent solution processability at lower temperatures than its counterparts based on inorganic compounds.3,4 However, the commercialization of organo-lead halide materials also faces a number of challenges arising from the toxicity of lead and the instability of CH3NH3PbI3 in moist air.5,6 Snaith et al. and Kanatzidis et al. have reported the replacement of Pb by other Group IV metals, specifically Sn7,8 and Ge.9 There has also been a work on lead-free ferroelectrics which has also explored metal-organic frameworks with the perovskite architecture.10,11 Beyond the Group IV metals, our attention is drawn to the possibility of using bismuth as an alternative to lead in organo-halide perovskite architectures. Bismuth is used in medicine, where it is available in a tablet form to treat upset stomach, heartburn, nausea, and diarrhea.12 However, the conventional hybrid perovskites have the formula ABX3 (where A is a monovalent amine

a Author to whom correspondence should be addressed. Electronic mail: [email protected]

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cation, B is a divalent metal, and X is a halide ion) and hence it is challenging to accommodate Bi3+ instead of Pb2+ at the B site. When replacing PbI2 by BiI3 with the same synthesis condition as CH3NH3PbI3, we obtained methylammonium bismuth iodide, (CH3NH3)3Bi2I9, which is a 0D dimer at room temperature (see Figs. S1(a) and S2(a) and Table S1)13 and isostructural with (CN3H6)3Sb2I9 and (CN3H6)3Bi2I9,14 as well as Cs3Bi2I9 at this temperature.15 As reported extensively by Mitzi,16 the dimensionality of perovskite-related phases varies from 0D to 3D depending on the sizes of the ions, forming isolated polyhedra, chains, layers, or three dimensional architectures. Recently, layered perovskites have been reported in solar cell devices, demonstrating band gap tuning by changing the sizes of the cations and the dimensionality of the framework. For example, devices using the layered perovskites, (PEA)2(MA)2(Pb3I10) (band gap of 2.06 eV, PEA = phenylethylammonium, and MA = methylammonium) have achieved a power conversion efficiency of 4.73%.17 The applications of these wide band-gap layered perovskite materials encouraged us to explore layered perovskites with the ABX4 structure where, in this case, A is a monovalent cation, B is a trivalent metal, and X is a monovalent anion.18 In attempts to synthesize a layered structure isostructural to NH4FeF4,19 a 2D perovskite-related phase, (NH4)3Bi2I9 was obtained, which is isostructural to hexagonal Rb3Bi2I9.20,21 This structure type is also known in Sb-based compounds, such as (NH4)3Sb2I922 and Rb3/Cs3Sb2I9,23 and Mitzi et al. have characterized Cs3Sb2I9 thin-films24 and reported their potential for photovoltaic applications. In addition, (CH3NH3)3Bi2I9 has recently been made into solar cell devices and achieved efficiency of over 1%,25,26 and alkaline metal based bismuth iodides, A3Bi2I9 (A = K, Rb, and Cs), have been studied in the context of photovoltaic applications for the first time.21 In the present study, we report the crystal structure, characterization, and physical properties of (NH4)3Bi2I9, which can be harvested from a solution under the same experimental conditions as CH3NH3PbI3. The experimental X-ray structure is compared with density functional theory (DFT) calculations, and the thermal behavior is examined to assess the stability. Conductivity and optical measurements were also performed to explore the potential of (NH4)3Bi2I9 as an alternative to the lead-containing perovskites. Single crystals of (NH4)3Bi2I9 were prepared by solution methods. 1 mmol of bismuth iodide (Sigma-Aldrich) was dissolved in 0.3 ml of hydroiodic acid (57% aqueous solution, Sigma-Aldrich) by heating in an oil bath. At 90 ◦C, a stoichiometric amount of ammonium iodide (Sigma-Aldrich) was added to the solution and the mixture was then held for 3 h at this temperature. Dark red crystals (see Fig. S2(b))13 were then filtered out without allowing the solution to cool to room temperature. The crystals were dried under vacuum overnight. Red fine powder could be obtained by grinding at room temperature. Powder X-ray diffraction measurements were performed at room temperature to confirm the purity of a ground bulk sample (see Fig. S3).13 A single crystal X-ray diffraction study was carried out using an Oxford Diffraction Gemini E Ultra diffractometer with an Eos CCD detector. Mo Kα radiation (λ = 0.7107 Å) was used. The temperature range from 120 K to 380 K was explored under nitrogen flow. Data collection and data reduction were performed with CrysAlisPro (Agilent Technologies). An empirical absorption correction was applied using the Olex2 platform,27 and the structure was solved with ShelXT28 using direct methods and refined with ShelXL29 by least squares methods. All non-hydrogen atoms were refined anisotropically (see Fig. 1). A summary of the crystal structure information is shown in Table I (CCDC deposition number 1434984). Optical characterization was performed on a PerkinElmer Lambda 750 UV-Visible spectrometer in the reflectance mode with a 2 nm slit width (see the supplementary material for experimental details).13 The measurement of (NH4)3Bi2I9 was repeated with several sample rotations of 45◦. Thermogravimetric analysis (TGA) was carried out under continuous nitrogen flow using a TA Instruments Q-500 series thermal gravimetric analyzer. 6.31 mg of the sample was held on a platinum pan. The sample was heated at a rate of 10 ◦C min−1 up to 800 ◦C, with significant weight loss being observed from around 240 ◦C (see Fig. S4).13 CHN analysis was performed to confirm the elemental composition; observed: H: 0.80 wt. %, N: 2.5 wt. %; calc: H: 0.75 wt. %, N: 2.6 wt. %. All the DFT calculations employed the generalized gradient approximation (GGA) implemented with projector augmented-wave (PAW)30,31 pseudopotentials as supplied in the Vienna ab


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FIG. 1. Crystal structure of the (NH4)3Bi2l9 phase along (a) a-axis, (b) b-axis, and (c) c-axis. Black lines mark the unit cell edges. (d) Two corner sharing BiI6 octahedra. The silver, purple, and blue spheres represent bismuth ions, iodine ions, and nitrogen atoms in NH4 ions, respectively.

initio Simulation Package (VASP).32–35 The effect of spin-orbit coupling was included during band structure and density of states calculations. The effects of van der Waals dispersion interactions were included during structural and electronic relaxation.36 The following parameters were adopted: (i) a 3 × 5 × 2 Monkhorst-Pack k-point mesh37 and (ii) a 500 eV plane-wave cutoff energy. The numbers of valence electrons treated explicitly were as follows: 15 for Bi (5d106s26p3), 7 for I (5s25p5), 5 for N (2s22p3), and 1 for H (1s1). All structural relaxations were performed with a Gaussian broadening of 0.05 eV.38 The ions were relaxed until the forces on them were less than 0.01 eV A−1. All schematic representations of the crystal structures were generated using the VESTA program (see Fig. 2(a)).39 The conductivities were measured using the single crystal conductivity method.40,41 In brief, single crystal samples were filtered and dried in a vacuum oven overnight. The dried crystals were mounted on two Au microelectrodes which were designed to have a 20 µm gap by lithography. The electrode was placed in a dark chamber with dry nitrogen flow. The conductivities of several crystals were measured by the AC impedance method using the Gamry Interface electrochemical instrument at 10 mV amplitude from 1 MHz to 0.1 Hz at 20 ◦C and the results were reproducible. We also performed powder conductivity measurements on a pellet sample. The pellet was formed at 0.1 GPa from dry powder, and then, before measuring conductivities, the pellet was kept in dry nitrogen for a week in order to remove adsorbed water which can cause ionic conduction, probably due to hydrated interparticle spaces.42 TABLE I. Crystallographic data and refinement of (NH4)3Bi2I9 from single crystal X-ray diffraction. Empirical formula Temperature (K) Crystal system Space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) Volume (Å3) Z ρ calc (g cm−3) µ (mm−1) F(000) Crystal size (mm3) GOF Reflections collected Independent reflections Final R indexes [I ≥ 2σ(I)] Final R indexes [all data]

N3H12Bi2I9 300 Monoclinic P21/n 14.609 5(4) 8.142 7(3) 20.909 4(5) 90 90.917(2) 90 2 487.06(12) 4 4.312 25.29 2 656 0.20 × 0.17 × 0.11 1.001 10 154 5 325 [Rint = 0.025 2, Rsigma = 0.044 1] R1 = 0.036 2, wR2 = 0.071 0 R1 = 0.065 3, wR2 = 0.082 3


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FIG. 2. (a) Optimized geometry of the layered (NH4)3Bi2I9 structure; the dotted lines indicate hydrogen-bonds between I and ammonium cations. (b) Band structures along the k vectors of the high symmetry line of the first Brillouin zone. (c) Partial density of states indicating the Bi s, Bi p, I p, and N p orbitals.

(NH4)3Bi2I9 crystallizes in the monoclinic space group P21/n with lattice parameters a = 14.6095(4) Å, b = 8.1427(3) Å, c = 20.9094(5) Å, and β = 90.917(2)◦ (see Table I). The framework of (NH4)3Bi2I9 consists of bismuth iodide layers stacked in a closed-packed fashion, forming hexagonal layers in the ab plane, as shown in Figs. 1(a) and 1(c). Each Bi3+ atom is coordinated by six I− ions in a distorted octahedral environment, as shown in Fig. 1(d), with Bi–I bond distances ranging from 2.93 Å to 3.27 Å (three closer and three further away). The BiI6 octahedra are corner sharing and there are Bi atoms filling two-thirds of the available BiI6 octahedral cavities. As seen in Fig. 1(b), the structure of (NH4)3Bi2I9 is hence related to the conventional 3D perovskites, as it is the structure obtained after removing every third octahedral layer along the ⟨111⟩ direction of a cubic ABX3 perovskite. Unlike Cs3Sb2I9, which forms both 0D dimer and 2D layered polymorphs,25 our variable temperature XRD studies on (NH4)3Bi2I9 reveal no evidence of a phase transition and the layered structure is stable across the temperature range 120 K–380 K (see Figs. S5 and S6 and Table S2 for changes in the unit cell and coefficients of thermal expansion, respectively).13 The DFT results can be compared with the experimental X-ray study. The calculated lattice parameters obtained at zero Kelvin were a = 14.6094 Å, b = 8.1617 Å, c = 20.8595 Å, and β = 90.61◦, in good agreement with our experimental values. The optimized atomic positions, listed in Table S3,13 agree very well with our experimental results. In addition, we have successfully predicted the preferred positions of the H atoms, which is impractical using X-ray diffraction due to their low electron density compared with Bi and I. In fact, the positions of the H atoms conform with the space group P21/n. In Fig. 2(a), hydrogen-bonds are indicated by the dotted lines (assuming that hydrogen-bonding is present when H–I < 3 Å). Interestingly, the hydrogen-bond

FIG. 3. Isosurface plots of the PCD of (a) the HOMOs and (b) the LUMOs of layered (NH4)3Bi2I9. The color saturation levels are between 0 (blue) and 0.0008 e/Å3 (red).


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interactions play a key role in stabilizing the layered architecture. Namely, hydrogen-bonds provide links between the BiI6 octahedra and thereby stabilize the whole structure. As shown in Fig. 2(b), the computed band gap of (NH4)3Bi2I9 is 1.46 eV and it has a direct band gap (note the band gaps calculated by DFT method are typically smaller than the experimental values, see below). Our DFT calculations give relatively flat valence and conduction bands, which we ascribe to the heavy effective masses of the electrons and holes. Fig. 2(c) shows the orbital-resolved partial density of states (PDOS) for the Bi s, Bi p, I p, and N p orbitals. The two prominent features of the PDOS are: (i) weak overlap between the Bi 6s orbital PDOS and the I 5p orbital PDOS for energies between −0.5 and 0 eV below the valence-band edge, and (ii) overlap between the Bi 6p orbital PDOS and the I 5p orbital PDOS for energies between 1.5 and 3 eV above the conduction-band edge. These orbital levels can be further visualized in real space using isosurface plots of the computed partial-charge density (PCD) of the highest band below the valence-band edge and the lowest band above the conduction-band edge. Fig. 3 shows that the highest occupied molecular orbital (HOMO) is mainly composed of Bi 6s–I 5p antibonding states, and the lowest unoccupied molecular orbital (LUMO) is mainly composed of Bi 6p–I 5p antibonding states. Interestingly, the HOMOs clearly reveal the stereo-active Bi 6s lone-pair orbitals, as shown in Fig. 3(a). Because of these lone-pair electrons, the Bi ions are displaced with respect to the centers of the BiI6 octahedra (see Tables S4 and S5 for experimental bond lengths and angles).13 Both the sharp increase near the observed absorption edge and the DFT prediction support the direct band gap of (NH4)3Bi2I9. Experimentally, the band gap of the powder sample was measured to be 2.04 eV (see Fig. 4 for the band gap determination), and this is lower than the band gap of the 0D dimer bismuth phase (CH3NH3)3Bi2I9, which was measured to be 2.11 eV (see Figs. S1(b) and S1(c)).13 The band gap of layered (NH4)3Bi2I9 is also lower than the 3D perovskite, CH3NH3PbBr3, which was reported with a band gap of 2.20 eV.43 In order to further examine the potential of (NH4)3Bi2I9 as a photovoltaic material, measurements of its conductivity were performed. Several crystals were tested, and we confirmed that they are, at least intrinsically, insulating, judging by the high resistivity, which is >3.1 × 108 Ωm, at 20 ◦C measured along a diagonal line of a hexagonal plate (ab plane), 35 µm thick and 178 µm wide, mounted on a 20 µm gap (over the detection limit of 1 TΩ as found in previous works40,42). Considering the fact that closely related structures have similar band gaps to our compound and can work as the absorption layer for photovoltaics, these compounds might be extrinsically conductive. In fact, the AC impedance data for the powder sample show a low resistivity of 0.42 Ωm (20 ◦C) without phase shifts below 10 kHz (see Fig. S8),13 indicating electrical conduction (not ionic). Since the powder resistivity is very low, we tried to measure temperature-dependent conductivities in order to estimate the activation energy for the conduction. However, the resistivity increased after the measurements, up to 1.5 Ωm at 20 ◦C (note the value mentioned above is around the lowest value), and thus we consider that the conductivity is unstable. Following these measurements, we found by using an electrometer that the conductivity is not from the bulk red material but from shiny black material on the surface. This was confirmed by removing the surface coating and measuring

FIG. 4. (a) Normalized absorption spectrum and (b) Tauc plot with band gap calculated assuming a direct band gap. The measurements were repeated with sample rotations to confirm consistency. No absorption was observed in the infrared up to 1500 nm (see Fig. S7).13


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the conductivity of the freshly exposed red material. We therefore speculate that surface defects act as carriers, though further details on the origin of the conductivity are still under investigation. In summary, a dark red bismuth phase, (NH4)3Bi2I9, with a 2D layered perovskite-like architecture was synthesized in solution and characterized by X-ray diffraction. DFT calculations were in good agreement with the experimental X-ray diffraction results and also revealed the locations of the hydrogen atoms. The band gap of the material was measured to be 2.04 eV. The results of single crystal conductivity measurements suggest that the bulk material is insulating, but that pellets made from powders show good conductivity due to surface effects from making the pellets. X-ray photoelectron spectroscopy and thin film fabrications are under investigation to provide a better understanding of the application of this bismuth based perovskite family in photovoltaic devices. This work was supported by the Cambridge Overseas Trust, the China Scholarship Council, and the Winton Programme for the Physics of Sustainability at the University of Cambridge. We acknowledge an Advanced Investigator Award to A.K.C. from the European Research Council (ERC) and support for S.T. from the World Premier International Research Center Initiative on “Materials Nanoarchitectonics (WPI-MANA),” MEXT, Japan. We thank Alan Dickerson (Department of Chemistry, Cambridge) for CHN analysis and Dr. Gregor Kieslich (Department of Materials Science and Metallurgy) for help with the structure solution. The DFT calculations were performed at the Cambridge HPCS and the UK National Supercomputing Service, ARCHER. Access to the latter was obtained via the UKCP consortium and funded by EPSRC under Grant No. EP/K014560/1. All data accompanying this publication are directly available within the publication and the accompanying electronic supplementary information. 1

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