Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
From Isolated Anions to Polymer Structures through Linking with I2: Synthesis, Structure, and Properties of Two Complex Bismuth(III) Iodine Iodides Tatiana A. Shestimerova,† Natallia A. Yelavik,†,⊥ Andrei V. Mironov,† Alexey N. Kuznetsov,†,‡ Mikhail A. Bykov,† Anastasia V. Grigorieva,§ Valentina V. Utochnikova,†,∥ Leonid S. Lepnev,∥ and Andrei V. Shevelkov*,† †
Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1-3, Moscow 119991, Russia N.S. Kurnakov Institute of General and Inorganic Chemistry, RAS, 119991 Moscow, Russia § Department of Materials Sciences, Lomonosov Moscow State University, Moscow 119991, Russia ∥ P.N. Lebedev Physical Institute, RAS, 119333, Moscow Russia ‡
S Supporting Information *
ABSTRACT: We report the synthesis, crystal structures, and optical properties of two new compounds, K18Bi8I42(I2)0.5·14H2O (1) and (NH4)7Bi3I16(I2)0.5·4.5H2O (2), as well as the electronic structure of the latter. They crystallize in tetragonal space group P4/ mmm with the unit cell parameters a = 12.974(1) and c = 20.821(3) Å for 1 and a = 13.061(3) and c = 15.162(7) Å for 2. Though 1 and 2 are not isomorphous, their crystal structures display the same structural organization; namely, the BiI6 octahedra are linked by I2 units to form disordered layers in 1 and perfectly ordered chains in 2. The I−I bond distances in the thus formed I−I−I−I linear links are not uniform; the central bond is only slightly longer than in a standalone I 2 molecule, whereas the peripheral bonds are significantly shorter than longer bonds typical for various polyiodides, which is confirmed by Raman spectroscopy. The analysis of the electronic structure shows that the atoms forming the I−I−I−I subunits transfer electron density from their occupied 5p orbitals onto their vacant states as well as onto 6s orbitals of bismuth atoms that center the BiI6 octahedra. This leads to low direct band gaps that were found to be 1.57 and 1.27 eV for 1 and 2, respectively, by optical absorption spectroscopy. Luminescent radiative relaxation was observed in the near-IR region with emission maxima of 1.39 and 1.24 eV for 1 and 2, respectively, in good agreement with the band structure, despite the strong quenching propensity of I2 moieties.
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INTRODUCTION The discovery of prominent photovoltaic properties of hybrid and inorganic iodoplumbates with the perovskite crystal structure has led to major progress in the field of lightharvesting materials for solar cells and caused a transition from the classical Grätzel solar cells to so-called all-solid-state (or simply all-solid) or perovskite cells.1−4 Complex lead iodides show high photovoltaic efficiency exceeding 20%, but their use in large-scale solar cells may be limited due to the toxicity of lead and its great bioavailability because of the solubility of iodoplumbates in water. A search for alternative solar power conversion materials for all-solid solar cells is in progress now,5,6 and one of the directions is to explore complex bismuth iodides. Methylammonium bismuth iodide (CH3NH3)3Bi2I9 is an easily fabricated compound that seems to be an analogue of methylammonium lead iodide; at least it shows the octahedral environment of bismuth by six iodine atoms. This compound displays a sharp absorption band near 500 nm and a long © XXXX American Chemical Society
photoluminescence decay time; however, photovoltaic efficiency does not exceed 0.2% for a solar cell made with use of mesoporous TiO2.7−9 Replacement of methylammonium by alkali-metal cations leads to compounds with the general formula A3Bi2I9 (A = K, Rb, Cs, NH4) exhibiting two different structure types.10,11 Although the photovoltaic efficiency was not reported in the literature, it was shown that the compounds with potassium, rubidium, and ammonium cations, which possess a perovskite-like two-dimensional crystal structure, are promising candidates for light-harvesting materials, whereas Cs3Bi2I9 having isolated bioctahedral Bi2I93− anions is less attractive. Furthermore, it was demonstrated that for a given type of structure the optical properties were only slightly sensitive to the nature of a charge-balancing cation as long as the latter does not affect the dimensionality of the structure. Received: January 30, 2018
A
DOI: 10.1021/acs.inorgchem.8b00265 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
a JEOL JSM 6490LV scanning electronic microscope equipped with an INCA x-Sight module. For each sample, spectra were acquired at 10 points and the average was taken as a resulting sample composition. Cobalt metal was used as a standard. For 1, the K:Bi:I ratio was found to be 26:12:62 with the estimated uncertainty of 1 molar %, which is very close to the calculated ratio of 24:12:64, whereas in the case of 2 only the Bi:I ratio of 18:82 was determined, which agrees fairly well with the expected value of 15:85. The low accuracy of determination is explained by the less than ideal alignment of not perfectly flat crystals on the holder and slow degradation of the samples under high vacuum. Crystal Structure Determination. Single crystals of 1 and 2 were picked from the respective products and mounted on a CAD4 diffractometer with the κ geometry and Ag Kα X-ray source. The sets of experimental data were corrected for Lorenz and polarization factors and absorption. The structures were solved by direct methods (SIR2002 program package).23 The solution of structure 1 revealed bismuth and iodine atoms arranged in pairs of edge-shared and vertexshared BiI6 octahedra. Successive Fourier synthesis (JANA200024) revealed three potassium atoms. One of them, namely K3, appeared to be too close to the symmetrically related atom (2.88 Å) and its occupancy was set to 1/2 (0.54 in the refinement). In addition, an unknown peak was found 2.84 Å away from I4. Such a distance is common for the In− polyanion; therefore, it was introduced as I10 and its occupancy was refined as 1/3 and further fixed at this level. Refinement in the anisotropic approximation of atomic displacement parameters (ADP) and successive difference Fourier synthesis gave three peaks 2.7−3.2 Å from potassium atoms, which were considered as oxygen atoms of water molecules. Finally, a small maximum was found in the ab plane of the unit cell 1.93 Å from I10 and 2.83 Å from I3. Some distances to symmetry-related peaks were also less than 2 Å. This peak was considered as iodine atom I11 with partial occupancy. Occupancies of I10 and I11 were refined, and it was found that the sum of occupancies of two I10 and four I11 symmetry-related positions is equal to 1.02(4). Thus, only one of these atoms may exist in each unit cell at the time. Successive difference Fourier synthesis revealed several weak peaks which were interpreted as oxygen atoms (water molecules), but in this case electroneutrality of the structure was not fulfilled due to the lack of potassium. Fourier maps were calculated by the maximum entropy method. The oxygen position at z = 1/2 (see Figure S3 in the Supporting Information) appeared to be symmetry disordered and widely spread. The refinement of this position as potassium (including occupancy) resulted in an excess of this cation. Finally, two atoms were refined in this region: namely, one potassium and one oxygen. Taking into account the rather short distance to symmetry-related atoms their occupancies were set to 1/4. Both atoms were refined in the isotropic approximation. The refined occupancies of other atoms deviated from unity within 3σ and were fixed to 1 in the final refinement. The refinement of water molecules as rigid bodies was unstable and was not included in the final refinement. A similar scheme was applied to the structure solution of 2. Direct methods revealed that its crystal structure is built of isolated octahedra in the ab plane of the unit cell and edge-shared octahedra on each ac (bc) plane. Isolated octahedra in the origin are linked by I2 groups along the c axis. Difference Fourier synthesis revealed several atoms which might be interpreted as oxygen (water molecules) or nitrogen (ammonia). Some of them formed the zigzag chain in the central part of the unit cell about 2.9 Å apart, similar to the structure of 1. Such a distance may be considered as weak hydrogen bonds, which may exist in the case of alternation of nitrogen and oxygen atoms. Two symmetry-related peaks were found on the a (b) edge of the unit cell about 2 Å apart. Such a distance is too short for both ammonia and water molecules; therefore, its occupancy was set to 1/2. According to the requirement of electroneutrality, this position was considered as nitrogen N1. At this stage the ADP of I7 appeared to be significantly higher than for other iodine atoms. This atom is relatively close (3.38(2) Å) to a statistically distributed N1 atom, and it may be the reason for a large variation of the I7 position in the ac plane. According to ADP (U11 = 0.128(2), U33 = 0.098(1) Å2) this site was separated into two lying 0.5 Å apart. Their ADP parameters were constrained to avoid large correlation, and their occupancies were set to 1/2. The
The same conclusion was drawn for another type of complex bismuth iodide containing a one-dimensional BiI4− anion composed of edge-shared {BiI6} octahedra.12,13 These compounds show a variety of non-perovskite structure types, but as long as the BiI4− 1D anion remains the same in all of them, they display similar band gaps between 1.75 and 1.85 eV, with the exception of compounds with organic cations that exhibit rather strong intermolecular bonding, resulting in higher band gaps.14,15 It was shown that KBiI4·H2O is a promising lightharvesting material for all-solid solar cells owing to the ease of preparation, relative thermal and chemical stability, and optical absorption properties. The recent discovery of intriguing optical and photoluminescence properties of CsBi3I1016 and AgBi2I717 prove that the perovskite structure is not a necessary condition for a compound to be a candidate for a lead-free light harvester and that there may exist other ways to link BiI6 octahedra together to achieve efficient transport of photoexited charge carriers across a material. The analysis of the literature shows that, despite the great number of compounds demonstrating 1D or 2D Bi−I substructures, there exist only two modes of forming extended structures out of simple building units. One of them works only in the case of cluster compounds of low-valence bismuth, where Bi−Bi bonds ensure the formation of 1D structures.18,19 For bismuth(III) complex iodides, creation of one- or two-dimensional structures is associated with different patterns of vertex- and edge-shared BiI6 octahedra, and the exact Bi−I substructure depends largely on the shape, size, and charge of a countercation.20−22 In this paper we present two new compounds, K18Bi8I42(I2)0.5·14H2O (1) and (NH4)7Bi3I16(I2)0.5·4.5H2O (2), which can be formally considered as featuring extended fragments formed by linking BiI6 octahedra by I2 moieties. We discuss their synthesis, reactivity, and crystal and electronic structure as well as optical absorption and luminescence properties.
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EXPERIMENTAL SECTION
Starting Materials. Used as starting materials were Bi (granules, 99.99%), I2 (analytical grade), KI (analytical grade), NH4I (analytical grade), HI 57% water solution (pure), and H2O (distilled). BiI3 was synthesized from the elements as described elsewhere.12 Synthesis. For the preparation of compounds 1 and 2, 3 mL of an aqueous solution containing 57 wt % of a mixture of HI and I2 in a molar ratio of 1:1 was added to the starting reagents with a total weight of 0.2 g (2KI:BiI3 for 1 and 7NH4I:3BiI3 for 2). In the case of the potassium-containing system, two types of crystals, black and red, form after 3 weeks of storage. The product was dried at room temperature, and black crystals of compound 1 were separated manually from the admixture. Compound 2 was obtained as the only product in the form of large square black crystals that grew within 3 weeks. They were separated from the residual solution by decanting and left to dry at room temperature. Powder XRD Analysis. Powder XRD analysis was performed at room temperature using a Rigaku D/MAX-2500 diffractometer with a rotating anode, in a reflection mode, with Cu Kα (λ = 1.540598 Å) radiation and graphite monochromator. For the analysis, crystals were finely crushed in an agate mortar, and the resulting powder was fixed on a holder using Scotch tape. XRD patterns showed that compound 2 was obtained phase-pure, whereas compound 1 contained a minor admixture of potassium iodide. The positions and intensities of the peaks matched well the respective diffraction peaks calculated on the basis of the single-crystal data (see Figures S1 and S2 in the Supporting Information). EDS Analysis. To determine the elemental composition of the obtained samples, crystals with a flat surface were picked, mounted on a graphite substrate using graphite adhesive tape, and investigated with B
DOI: 10.1021/acs.inorgchem.8b00265 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Summary of Crystallographic Information for 1 and 2 1
2
K18Bi8I42(I2)0.5·14H2O
composition cryst syst space group a, Å c, Å V, Å3 Z dcalc radiation/wavelength, Å temp, K cryst shape cryst size, mm color diffractometer scan mode absorption correction θ range (data collection) range of h, k, l Rint structure soln structure refinement refinement on R/Rw (I > 3σ(I)) GOF no. of params Nref positive/negative Δrmax, e Å−3
(NH4)7Bi3I16(I2)0.5·4.5H2O tetragonal P4/mmm
12.974(1) 20.821(3) 3504.8(7) 1 3.830
13.061(3) 15.162(7) 2586(1) 2 3.834 Ag Kα/0.56083 295 prismatic 0.19 × 0.13 × 0.07 0.21 × 0.13 × 0.10 black CAD4, graphite monochromator ω/θ Gaussian (crystal shape) 2−23 0 → h → 18, 0 → k → 18, 0 → l → 28 0 → h → 18, −18 → k → 0, −3 → l → 21 0.036 0.030 SIR200223 JANA200024 F 0.031/0.04 0.029/0.041 1.02 1.00 87 67 995 1019 0.80/−0.84 1.95/−1.99
Table 2. Interatomic Distances for 1 and 2 K18Bi8I42(I2)0.5·14H2O atoms Bi1−I (vertex shared, I9) Bi3−I (vertex shared, I8) Bi2−I (edge shared, I3 × 2) I3−I11 I4−I10 I10−I10 I11−I11 K1−O1 K2−O1 K3−O2 K4−O2 K4−O41 O2−O41
(NH4)7Bi3I16(I2)0.5·4.5H2O
distance, Å I7, 3.075(1) × 4; I1, 2.906(4); I9, 3.196(1) I2, 3.072(1) × 4; I4, 2.951(6); I8, 3.231(3) I3, 3.277(1) × 2; I5, 3.067(1) × 2; I6, 2.932(1) × 2 2.84(4) 2.85(1) 2.74(1) 2.77(6) 2.79(1) 2.81(1) × 2 2.71(1) 2.85(8) 2.78(8) 2.88(7)
atoms
distance, Å I2, 3.088(1) × 4; I3, 3.022(3) × 2
Bi1−I (isolated, at the origin) Bi2−I (isolated, in the center) Bi3−I (edge shared, I6 × 2) I3−I8
I5, 3.065(1) × 2; I6, 3.260(1) × 2; I71, 2.985(5) × 2; I72, 2.933(5) × 2 3.168(4)
I8−I8
2.782(3)
O1−N2 O1−N3
2.92(1) 2.93(1)
refined occupancies of other atoms deviated from full within 3σ and were fixed to unity in the final refinement except for O2. Its refined value was found to be very close to 1/2 and was fixed in the final refinement. The refinement of water molecules and ammonia as rigid bodies was unstable and was not used in the final refinement. A summary of experimental and crystallographic information for the studied compounds is given in Table 1, and interatomic distances are provided in Table 2. Atomic parameters are given in Tables S1 and S2 in the Supporting Information. Raman Spectroscopy. The Raman spectrum for 1 was recorded on a Renishaw In Via spectrometer with a laser wavelength of λ 514 nm (Ar, 50 mW) with capacity varied via ND (neutral density) filters in an interval of 0.00005−100%. The backscattering geometry with a
I1, 3.081(1) × 4; I4. 3.033(2) × 2
Leica DMLM confocal microscope (100′ lens) was used for measurements performed at room temperature in air. The focus distance was 250 mm, and the size of the laser beam was 20 μm. The CCD camera (1024 × 368 pixels) was used as a detector. The scale calibration was done using monocrystalline silica (521.5 cm−1) as a standard sample. WiRE 3.4 software was used for data processing. Optical and Photoluminescence Spectroscopy. Optical diffuse reflectance spectra were recorded using a PerkinElmer Lambda 950 UV−vis spectrometer with an attached diffuse reflectance accessory. Measurements were performed at 298 K in the spectral range of 250− 1200 nm, with a scanning rate of 2 nm/s using finely ground polycrystalline samples. The data were processed using the Kubelka− Munk theory approximation in the two types of coordinates: [(k/s) C
DOI: 10.1021/acs.inorgchem.8b00265 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry hν]1/2 − hν and [(k/s)hν]2 − hν, with hν along the x axis, where k is the absorption coefficient, s is the scattering coefficient, and h is the Planck constant.25 The k/s relation known as a remission function was calculated using the refraction data as k/s = (1 − R)2/2R, where R is the absolute diffuse reflectance.26 The Kubelka−Munk function is better linearized in [(k/s)hν]2 − hν coordinates. The choice of this function is further supported by the calculated electronic structure, which suggests direct band gaps. Extrapolation to k = 0 gives an approximate optical Eg value of the material. Emission spectra were measured with an Ocean Optics S2000 multichannel spectrometer with a diode laser (λex 465 nm) as an excitation source at 77 K. The spectra were corrected for the wavelength response of the system. Emission spectra of solid samples were measured with an Ocean Optics S2000 multichannel spectrometer with a diode laser (λex 465 nm) as an excitation source at 77 K. The efforts to register spectra at room temperature were not successful due to insufficient luminescence intensity. The spectra were corrected for the wavelength response of the system. Computational Details. Band structure calculations were performed for a slightly simplified model of compound 2 on a density functional theory (DFT) level utilizing the projector augmented wave pseudopotential approach (PAW)27 as implemented in the VASP code.28 The PBESol exchange-correlation functional29 of the GGA type was used in the calculations. The valence space for Bi, I, and K atoms consisted of 17, 7, and 9 electrons, respectively. The energy cutoff was set at 500 eV with a 10 × 10 × 10 (75 irreducible k-points) Monckhorst−Pack30 k-point mesh. Bader charge density was calculated according to the literature.31−34 Three modifications were made to the unit cell of 2 to perform periodical calculations: (i) water molecules were omitted, which is a viable approximation since there are no short distances between the oxygen atoms and no heavy atoms in the cell, which implies no covalence, and dispersion interactions are not included in the GGA functionals; (ii) semioccupied atomic positions of the two I7 atoms were averaged into one fully occupied I7; (iii) since the hydrogen atoms were not localized, we have approximated NH4+ cations by K+ cations, which have a very close ionic radii. This can be done since, as we have shown in ref 12, the cations do not provide any substantial contribution to the electronic structure near the Fermi level, and their interaction with other structural units in such systems is essentially ionic. In order to look closely at the interactions of the iodine atoms I8, connecting two BiI63− octahedra, we have performed calculations on the model of the one-dimensional chain [(BiI63−)−I−I−(BiI63−)], isolated from the structure and consisting of two octahedra comprising Bi1, I2, and I3, and two I8 atoms (see Table S2). To achieve the best possible accuracy of the description, we have utilized the all-electron full-potential linearized augmented plane wave method (FP-LAPW) as implemented in the ELK code35 and a fully relativistic approach, including spin−orbit coupling being taken into account. The muffintin sphere radii for the respective Bi, I, and K atoms are (in bohr): 2.80, 2.60, and 2.40. The maximum moduli for the reciprocal vectors kmax were chosen so that RMTkmax = 8.0. The electron localizability indicator (ELI-D),36−38 QTAIM charge density, and Laplacian39 of electron density were calculated using the DGrid package.40 The calculations were performed using the an Intel Core i7 based laboratory cluster and the MSU Lomonosov supercomputer.41 Visualization and topological analysis were performed using the ParaView package.42
Figure 1. Crystal structure of 1: (left) view slightly off the b axis; (right) group of BiI6 octahedra around I2 units that have three alternative orientations. Color code: bismuth, magenta; iodine in vertices of BiI6 octahedra, cyan; iodine of I2 units, blue; potassium, green; water oxygen, red; central I−I bonds, green; peripheral I−I bonds, yellow.
Figure 2. Crystal structure of 2: (left) view of the unit cell; (right) fragment of a chain composed of BiI6 octahedra and I2 units that is running along the c axis. Color code: BiI6 octahedra, magenta; iodine, cyan; ammonium nitrogen, blue; water oxygen, red.
range from 2.91 Å for terminal iodine atoms to 3.28 Å for bridging atoms. Pairs of octahedra appear to be further linked by the I2 units by forming contacts to iodine atoms of the octahedra. The intraunit I−I distance ranges from 2.74 to 2.78 Å, whereas links to the BiI6 octahedra are slightly longer, from 2.83 to 2.85 Å. The I2 units are disordered in such a way that three different positions are possible, as shown in Figure 1, only one of which is present at the time. Potassium atoms and water molecules form the cationic part of the structure. Two types of cations are observed; they are perfectly ordered [K3(H2O)2]3+ zigzags with a K−O bond distance of 2.81 Å and severely distorted [K2(H2O)2]2+ units with the K−O bond length varying from 2.71 to 2.85 Å. Within the latter cationic unit, an O−O separation of 2.88 Å is observed, which can be assigned to O−H···O hydrogen bonds. In contrast to 1, in the crystal structure of 2 the BiI6 octahedra either share edges or remain unshared. The isolated octahedra are quite regular, with the Bi−I distances ranging from 3.02 to 3.08 Å, whereas the edge-shared octahedra are
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RESULTS AND DISCUSSION K18Bi8I42(I2)0.5·14H2O (1) and (NH4)7Bi3I16(I2)0.5·4.5H2O (2) are unique compounds that combine BiI63− anions and I2 units. Their crystal structures are shown in Figures 1 and 2. The basic building unit in both compounds is the BiI6 octahedron. In the crystal structure of 1 these octahedra are joined into pairs via two different modes: i.e., either by sharing opposite vertices or by sharing edges. All octahedra are distorted; the Bi−I distances D
DOI: 10.1021/acs.inorgchem.8b00265 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
delocalized σ-bonding is typically observed near 110 cm−1.44 The lowest energy band at 184 cm−1 can be assigned to the only active vibration of I2 in Raman spectra. Lower wavenumbers in comparison to the I2 molecule (214 cm−1 50) stem from association of I2 with additional iodine atoms in a linear I−I−I−I moiety. The remaining two bands at 134 and 146 cm−1 can be attributed to Bi−I valence modes, both terminal and bridging, which are normally observed in this range of wavenumbers for various iodobismuthates.51 Acquisition of the Raman spectrum for 2 was unsuccessful due to decomposition of the sample under green laser irradiation of a required power. Both compounds contain solvent water, which makes them relatively unstable toward heating (see Figures S4 and S5 in the Supporting Information). Compound 1 starts losing water at 130 °C. The dehydration is accompanied by releasing I2. The X-ray diffraction analysis of the sample after heating to 285 °C for 1 h under a dry nitrogen atmosphere confirms the formation of a mixture of crystalline KI and K3Bi2I9. Further heating leads to decomposition of K3Bi2I9 that ends at about 415 °C with complete evaporation of volatile BiI3 and leaves KI as the sole solid product. Decomposition of 2 starts at about 70 °C with the loss of all water molecules, leading to the amorphous, according to X-ray analysis, compound with the apparent composition (NH4)7Bi3I16. Taking into account that an analogous thallium compound was reported in the literature52 and that Tl+ and NH4+ have very close radii, we speculate that the formation of isomorphous compounds is quite possible. Further heating to about 200 °C leads to the decomposition of the amorphous phase with evolution of volatile BiI3 and formation of (NH4)3Bi2I9; however, decomposition of the latter phase already starts around 265 °C and concludes at 330 °C, leaving NH4I as a residue. Slower heating (5 instead of 10 K min−1) did not allow resolving signals pertaining to two overlapping decomposition steps. Optical absorption spectra recorded from polycrystalline powders of 1 and 2 are shown in Figure 4. Both compounds are black, and the Kubelka−Munk function plot analysis constructed in (αhν)2 coordinates give Eg values of 1.57 and 1.27 eV, respectively, which is in good agreement with the observed color. We note that these values are significantly smaller than those reported for various iodobismuthates that do not contain strong I−I bonds.12−16 To shed some light on the origin of such small band gaps, we performed quantum chemical calculations of the band structure for 2, which was selected because of the lack of considerable disorder in the I2 arrangement in comparison to the crystal structure of 1. The calculated total (TDOS) and projected (PDOS) densities of states near the Fermi level for the model structure of (K)7Bi3I16(I2)0.5, which in our view is a good approximation of the structure of (NH4)7Bi3I16(I2)0.5 (2) (see Computational Details), are shown in Figure 5. As seen from the DOS plots, the compound is predicted to be a narrow-gap semiconductor, with a gap value of ca. 1.2 eV, which agrees very well with the observed gap of 1.27 eV and confirms the validity of the used model. Iodine atom 5p states provide dominant contribution near the Fermi level and essentially form the top of the valence band as well as provide strong contribution to the bottom of the conduction band. The latter band is almost equally contributed by the 6p electrons of bismuth; however, they feature far less prominently in the valence band, which indicates that these 6p states must be at least partially vacant. In addition, as was previously shown,12 alkali-metal cations (and, by
distorted, with d(Bi−I) ranging from 2.95 Å (terminal) to 3.27 Å (bridging). The I2 units appear to link isolated octahedra forming chains running along the c axis, as depicted in Figure 2. The I−I distances within I2 moieties (2.78 Å) are significantly shorter than those between I2 and BiI6 (3.17 Å). The cationic part of the structure consists of (NH4)3(H2O)2 similar to the K3(H2O)2 zigzags and additional NH4+ cations. A comparison of the I−I distances between the iodines belonging to different structural units (BiI6 octahedra and I2 moieties) in both compounds points to the slightly different nature of the I2 units and indicates their bridging function. Within the I2 units, the I− I distances of 2.74−2.78 Å are only slightly longer than d(I−I) = 2.72 Å in elemental iodine.43 Longer contacts (2.83−3.17 Å) link I2 units with the BiI6 octahedra. The I−I−I−I bridge thus formed is linear; it formally resembles the well-known I42− anion. However, the bond distances are not similar to those reported in the literature for the latter polyanion. Whereas the central bond distance is almost the same as in the structures of 1 and 2, the peripheral distances are typically between 3.3 and 3.4 Å,44 which is significantly longer than in the structure of 1 and considerably longer in comparison to those in 2. Our analysis of the electronic structure and bonding for 2 (vide infra) is also not in favor of the I42− representation. Summarizing all structural features, we note that in both compounds the BiI63− octahedra linked by the I2 units form the anionic framework whereas either K+ (in 1) or NH4+ (in 2) composes the cationic substructure together with water molecules, thus completing the crystal structure. However, purely on the basis of structural data and connectivity, the nature of the bond between BiI63− and I2 cannot be established, and thus it cannot be deduced whether they form polymer chains that can be regarded as true 1D fragments. This is investigated in the section concerning bonding analysis (vide infra). To the best of our knowledge, these are the first example of a crystal structure of complex bismuth iodine iodides. In contrast to iodobismuthates, bromine analogues are known from the recent literature.45,46 In general, polyiodides of p metals are much less abundant in comparison to those of transition metals, where a variety of polyiodide patterns is known ranging from single I−I units47 to planar and 3D networks.48,49 The Raman spectrum recorded for 1 consists of four bands in the far-IR region (Figure 3). The high-energy band at 108 cm−1 lies in the area typical for polyiodide anions, although it is not easy to assign it to a particular I−I bond; it should be noted that symmetrical stretching of the I3− anion that features
Figure 3. Raman spectrum for 1. E
DOI: 10.1021/acs.inorgchem.8b00265 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
transferred from these states to the mixed bismuth and iodine p states in the conduction band (Figure 6). Considering the fact that I3 and I8 atoms connect Bi1-centered octahedra, this might imply that the electron transfer is preferably achieved along this line, and therefore electron transport is somewhat anisotropic. One the one hand, this assumption appears to be corroborated by the energy dispersion plot (Figure 7), where we see that the smallest gap between valence and conduction bands (ca. 1.2 eV) is at X and M points, and the band gap is formed by the bands along the X−M direction, which apparently corresponds to the direction of the (BiI6)−I2− (BiI6) chains. A low degree of band dispersion along this direction can be an indication of a low-dimensional structure. On the other hand, the energy dispersion in the M− direction is almost negligible (with M− orthogonal to X−M), which makes the description in terms of 1D polymer chains less likely. Moreover, while band dispersion along the −R−X−Z path differs from that along the X−M− direction, the difference in maximum energy is covered by ca. 0.15 eV, which is a rather small range. Band density near the Fermi level also does not differ dramatically. In addition, the maximum band gap (Z point) is ca. 1.55 eV; thus, there is only ca. 0.35 eV difference with the minimum value. For instance, for the layered perovskite (IC6)2[PbI4],53 the difference in the top valence band energies along the k path is ca. 0.55 eV, and the difference between minimum and maximum gaps between valence and conduction bands is ca. 1.6 eV. Obviously, the connection between anisotropic conductivity and energy dispersion is not a simple one. Nevertheless, on the basis of this comparison we cannot unequivocally state that the structure in question is highly anisotropic, although a certain amount of anisotropy is undoubtedly present. It must be noted, though, that our analysis of band dispersion is of a qualitative nature, since proper calculations of electron effective mass tensor would require a very high level of theory and fully relativistic approach. On comparing our band
Figure 4. Kubelka−Munk plots for 1 (top) and 2 (bottom).
extension, ammonium cations) do not contribute in any significant way to the valence band. For potassium this indicates essentially empty 4s states, which is consistent with its cationic role in the structure. A more detailed look at the iodine PDOS shows that the very top of the valence band (between 0 and ca. −0.5 eV) essentially consists of the contributions from I3 and I8 p states, and thus we can assume that the charge carriers are
Figure 5. Total (lines) and projected (filled areas) DOS near the Fermi level for the model compound 2 (left to right): Bi, I, K. F
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Figure 6. PDOS for I3 (left) and I8 (right).
anomalously low negative charge on the I8 atoms and slightly higher, but still rather low in comparison to other iodines, negative charges on the I3 atoms. Since all other iodine atoms as well as all the bismuth atoms fall into the same range chargewise, we might take this as an indication that some charge density is transferred from the I3 atoms to the I8 atoms constituting the I2 unit. Since such a charge distribution between iodine atoms is quite unequal, it does not agree well with the possibility of the formation of a I42− polyanionic fragment by I3 and I8 and renders more plausible the role of the I2 units as linking units. To gain more insight into the nature of the bonding between the iodine atoms connecting the Bi1-centered octahedra, we have initially investigated the topology of the electron localization function (ELF)54−56 on the basis of VASP calculations. Its analysis did not reveal any non-atom-centered attractors between the iodine atoms (see Figure S6 in the Supporting Information). This might indicate a lack of strong covalency exhibited as the absence of attractors corresponding to covalent bonds. It must be noted, however, that the ELF topology in this case might be affected by the use of pseudopotentials rather than all-electron approach, as the latter is preferable for ELF construction. For a more detailed study, we have then modeled an isolated one-dimensional chain of Bi1-centered octahedra, formed by I2 and I3, and connected through the I3 atoms to the I8 pair of atoms, taken from the structure, which would allow us to use a higher level of theory and fully account for relativistic effects. However, the topological analysis of the ELI-D, calculated for such a chain, once again did not reveal any nonatomic attractors between
Figure 7. Energy dispersion curves near the Fermi level for the model compound 2.
structure with the calculated band dispersion of a pseudo-3D [mepy]BiI4 compound,14 containing well-defined 1D [BiI4]− chains of edge-sharing BiI63− octahedra, we do observe similarities such as low band dispersion. However, in our case it is not obvious whether such a clear 1D arrangement exists, and the only candidate is the potential chain of BiI63− linked by the I2 units. In order to investigate the validity of treating this chain as a (BiI6)−I−I−(BiI6) 1D polymer, we have performed bonding analysis using various approaches. Atomic charges, calculated according to Bader’s QTAIM approach, are given in Table 5. Of immediate note are the Table 5. Bader Atomic Charges for the Model Compound 2a charge a
Bi1
Bi2
Bi2
I1
I2
I3
I4
I5
I6
I7
I8
K
+0.95
+1.03
+1.02
−0.61
−0.58
−0.36
−0.58
−0.62
−0.54
−0.52
−0.17
+0.85
Low negative charges on iodine atoms are given in boldface. G
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final argument in favor of not treating the four iodines in the (BiI6)−I−I−(BiI6) as an I42− unit. On the other hand, the appearance of the smaller maxima between I3 and I8 not only makes it unlikely that the two I8 atoms form a completely neutral I2 molecule but also provides a handy explanation for the stability of the whole (BiI6)−I−I−(BiI6) fragment and the lack of any orientational disorder in this chain. Thus, on the basis of our analysis, we can describe this linear fragment as a combination of conventional BiI63− octahedra and essentially molecular, but slightly charged, I2 fragments linked between each other by weak covalent (possibly halogen) bonds. Therefore, due to the lack of strong covalency along the chains and considering the weaker nature of bonding between the I2 and BiI63− units, in comparison to the intramolecular interactions, we would prefer not to describe (BiI6)−I−I− (BiI6) fragments as truly 1D groups, and thus we cannot unequivocally regard the structure of the whole compound as 1D. The remaining question is whether compounds 1 and 2 are good candidates for light harvesting in all-solid solar cells. The answer to this question is directly connected to the presence of the optically active excited state, which must possess low energy so that light can be absorbed efficiently across the solar spectrum.59 In the case of 1 and 2 this demand is fulfilled, and molar extinction coefficients, though they are impossible to measure, must be high, as these materials appear deep black. The origin of the excited state in halide perovskites is usually of a charge transfer nature,60 which is also the case in 1 and 2 according to the calculated electronic structure. However, the relaxation of such a charge transfer state may take place through a chemical reaction leading to compound decomposition. In particular, the literature shows61 that the light-excited LMCT state relaxation in BiI3 in I2/CH3CN media resulted in I−I bond formation reactions within a nanosecond time span. Therefore, to ensure the possibility of the halide perovskite to perform in a solar cell, it is important that the excited state could relax through charge separation. In the case of organic dyes this is ensured by the long lifetime of the excited state measured upon luminescence decay, as well as the presence of the luminescence itself.62 Naturally, luminescence spectroscopy was utilized in the present case to ensure the possibility of radiative relaxation of 1 and 2. We succeeded in measuring luminescence spectra in only powder at low temperature, which is most likely connected with the concentration quenching. The low intensity of the emission precludes performing lifetime measurements. In addition, measurement in solution is impossible because the structures of both 1 and 2 are not preserved when they are subjected to dissolution. Luminescence spectra of 1 and 2 powders measured at 77 K upon excitation by a 465 nm diode laser are presented in Figure 10. Each of them contains a single band in the near-IR range, in perfect agreement with the low band gap energy measured by optical spectroscopy, and the emission maxima correspond to 1.39 and 1.24 eV for 1 and 2, respectively. The maxima are shifted to lower energy in comparison to those observed for iodobismuthates that do not contain I−I bonds. In those compounds emission peaks are observed at 1.9−2.1 eV.16,63 This observation correlates well with the difference in the band structure and reflects that the LMCT transfer involves states of iodine atoms forming the I−I2−I chain. The low intensity of the registered bands is connected with the effective quenching of the compound luminescence, which is due to the presence of the iodine species. Indeed, iodine in
iodines (Figure 8). However, we can clearly see higher values of the function between the I8 atoms, in comparison to the I3−I8
Figure 8. ELI-D cross-section along the I3−I8 line of the (BiI6)−I−I− (BiI6) chain.
area, which might indicate stronger interactions between the former. Calculated Bader charges are +0.86 (Bi1), −0.65 (I2), −0.33 (I3), and −0.18 (I8), which agrees well with the 3D calculations. The lack of bond attractors in the topology of ELI-D once again points toward the lack of strong covalency in the bonding between iodine atoms. Thus, we can safely rule out the representation of the (BiI6)−I−I−(BiI6) fragment as a uniform covalently bonded chain. This situation is often encountered when halogen bonding is involved, which is a special kind of bonding associated with fairly subtle shifts in the electron density57 that are difficult to observe using conventional means. To study such an effect, we have employed the topological analysis of the Laplacian of electron density.39 This is the function L = −∇2ρ(r) which shows the regions of electron concentration (∇2ρ(r) < 0) and electron depletion (∇2ρ(r) > 0). It is known to work well for the lighter elements but is considered less informative from the fifth period on.58 However, we have managed to observe some features, which might be associated with the high level of theory used, particularly a fully relativistic approach. As we see from Figure 9, there are two areas of charge concentration along the I3−I8 line.
Figure 9. Topology of electron density Laplacian (L = 0.04) for the (BiI6)−I−I−(BiI6) chain.
The larger area, between the two iodine atoms I8, corresponds to the interaction between these atoms, while the smaller nonsymmetrical areas indicate the interaction between I3 and I8. We cannot assign a quantitative estimation to these maxima; however, it would not be too speculative to assume that the larger maximum corresponds to a stronger interaction between the iodines forming the I2 unit. This is a H
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light-harvesting properties, which is further corroborated by the observation of luminescence in the near-IR region. The emission maxima correspond to 1.39 and 1.24 eV for 1 and 2, respectively, in good agreement with the band structure. Luminescence is observed despite the strong quenching propensity of I2 moieties, which we attribute to a strong involvement of the I2 units in the crystal structures, owing to bonds that are far stronger than the typical I···I interactions in multifarious polyiodides. This first observation of luminescence in iodide polyiodide complexes and their optical absorption properties, details of the electronic structure, and facile synthesis are strong evidence for the potential of such compounds as light-harvesting materials in all-solid perovskite solar cells and open a new strategy for designing light-harvesting complex halides that do not contain toxic elements.69
Figure 10. Luminescence spectra of 1 and 2 on excitation by a diode laser (465 nm).
various forms (I2, I3−) is known as one of the most efficient quenchers of luminescence of various species.64−66 Two different mechanisms of luminescence quenching by iodine have been discussed. One is associated with the presence of resonance forms, which is similar to the universal quenching properties of NO2 and N3 groups,67,68 whereas another consists of effective excitation light absorption due to high absorption in the near-UV range.64 The luminescence of even highly emitting materials is usually completely quenched in the presence of iodine; therefore, the presence of the detectable luminescence of 1 and 2 is strong evidence of the photoactive excited state. We also note that the luminescence intensity of 2 is higher than of 1, which may be connected with the different degree of disorder. Probably the perfectly oriented I−I−I−I species in 2 result in lower luminescence quenching in comparison to disordered moieties in 1; however, this requires further investigation.
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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.inorgchem.8b00265. Atomic parameters, X-ray powder diffraction patterns, thermal analysis curves for 1 and 2, electron density maps by the maximum entropy method for 1, and the topology of ELF (η = 0.6) for the unit cell of model structure 2 (PDF) Accession Codes
CCDC 1586245−1586246 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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CONCLUSIONS We have prepared two new complex bismuth iodides that contain additional I2 units along with common BiI6 octahedra. The compo unds K 1 8 Bi 8 I 4 2 (I 2 ) 0 . 5 ·14H 2 O (1 ) and (NH4)7Bi3I16(I2)0.5·4.5H2O (2) are not isomorphous but display similar organization of their crystal structures, in which I2 moieties link BiI6 in such a way that either anionic layers or formal chains form. Our experiments and calculations show that the I2 units are involved in bonding. The I−I bond within the I2 unit appears slightly weaker in comparison to a standalone I2 molecule, whereas the I−I bond between the I2 moiety and BiI6 octahedron gains some electron density, ensuring a prominent BiI6−I2 interaction at a distance ranging between 2.83 and 3.17 Å, which is noticeably shorter than in various polyiodides. Both compounds are low-gap semiconductors with optical band gaps of 1.57 and 1.27 eV for 1 and 2, respectively. The analysis of the band structure shows that the electron transfer is slightly anisotropic and is preferably achieved along the direction that includes (BiI6)−I2−(BiI6) chains, in which the atoms forming the I−I2−I fragment transfer electron density from their occupied 5p orbitals onto their vacant orbitals as well as to 6s orbitals of bismuth atoms that center the BiI6 octahedra. However, our bonding analysis shows the lack of strong covalency between (BiI6) fragments and linking I2 units, which prevents us from treating the (BiI6)−I2−(BiI6) units as uniform polymer chains and from treating the whole structure as truly 1D. The presence of ligand-to-metal charge transfer (LMCT) and black color of compounds 1 and 2 point to their potential
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AUTHOR INFORMATION
Corresponding Author
*E-mail for A.V.S.:
[email protected]. ORCID
Andrei V. Shevelkov: 0000-0002-8316-3280 Present Address ⊥
N.A.Y.: Faculty of Technical Chemistry, Technical University of Vienna, Getreidemarkt 9, 1060 Wien, Austria.
Funding
This work was supported by the Russian Ministry of Education and Science, contract no. RFMEFI6131X0053 (14.613.21.0053). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We wish to thank Mrs. T. Filippova for assistance with powder XRD experiments and Dr. V. Y. Verchenko for EDXS analysis. The use of MSU supercomputer center resources is acknowledged within the framework of the MSU Development Program.
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DOI: 10.1021/acs.inorgchem.8b00265 Inorg. Chem. XXXX, XXX, XXX−XXX
