High-Pressure Study of Perovskite-Like Organometal Halide: Band

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Letter

High Pressure Study of Perovskite-Like Organometal Halide: Band Gap Narrowing and Structural Evolution of [NH-(CH)-NH]CuCl 3

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Qian Li, Shourui Li, Kai Wang, Zewei Quan, Yue Meng, and Bo Zou J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02786 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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The Journal of Physical Chemistry Letters

High Pressure Study of perovskite-like Organometal Halide: Band Gap Narrowing and Structural Evolution of [NH3-(CH2)4-NH3]CuCl4 Qian Li,†, £, # Shourui Li,§, Kai Wang,† Zewei Quan,£ Yue Meng,# Bo Zou*, †

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State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China

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Department of Chemistry, South University of Science and Technology of China, Shenzhen,

Guangdong 518055, China #

High Pressure Collaborative Access Team (HPCAT), Geophysical Laboratory, CIW, Argonne, IL

60439, USA §

National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China

Academy Of Engineering Physics, Mianyang 621900, China

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ACS Paragon Plus Environment


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ABSTRACT Searching for nontoxic and stable perovskite-like alternatives to lead-based halide perovskites for photovoltaic application is one urgent issue in photoelectricity science. Such exploration inevitably requires an effective method to accurately control both the crystalline and electronic structures. This work applies high pressure to narrow the band gap of perovskite-like organometal halide, [NH3-(CH2)4NH3]CuCl4 (DABCuCl4), through the crystalline-structure tuning. The band gap keeps decreasing below ~12 GPa, involving with the shrinkage and distortion of CuCl42-. Inorganic distortion determines both band gap narrowing and phase transition between 6.4 and 10.5 GPa. And organic chains function as the spring cushion, evidenced by the structural transition at ~0.8 GPa. The supporting function of organic chains protects DABCuCl4 from phase transition and amorphization, which also contributes to the sustaining band gap narrowing. This work combines crystal structure and macroscopic property together, and offers new strategies for futher design and sythesis of hybrid perovskite-like alternatives. TOC GRAPHIC

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Organometal halide perovskites (OMHPs), especially hybrid lead-based perovskites, have recently obtained intense studies owing to their remarkable combination of electronic and photovoltaic properties, developing as the most promising absorbers for the next generation of solar cells.1-3 However, commercialization of perovskite solar cells still faces great challenges that the toxicity of metal elements (Pb, Cd, and Hg-based perovskites) and the unstable structure (Sn or Bi-based perovskites) exposed to light, humidity, heat and external force. Finding other nontoxic and stable perovskite-like alternatives with corresponding structure and electronic properties becomes one urgently solved problem for photovoltaic and optoelectronic applications.4, 5 In order to achieve this goal, it is essential to get a deeper insight into the structural nature and structure–property relationships of perovskite-like materials. Furthermore, efficiency of solar cell device is strongly influenced by the structure and band gap of absorber. However, most OMHPs still possess the much bigger band gap than the optimum band gap energy of 1.34 eV in Shockley-Queisser theory.6, 7 It is also particularly needed for photoelectricity to explore an effective method to accurately narrow the band gap and tune the structure of perovskite-like material. Comparing with traditional chemical approaches, pressure emerges as an efficient and environment-friendly method to control OMHPs structures and properties. Though high pressure exploration of OMHPs is just beginning, numerous intriguing responses, such as phase transitions, amorphization, piezochromism, increased resistance, insulator−semiconductor transformation and band gap shifting, are all observed at high pressure.8-16 Actually, high pressure study on OMHPs not only provides intriguing properties of OMHPs, but also presents their structural nature and structure-property relationships, providing guidance for ambient OMHPs exploration reversely. Among all the nontoxic alternatives of OMHPs, Cu-based composites are particularly interesting. On one hand, the Jahn-Teller effect of Cu2+ makes the Cu-based structure more flexible and tunable.17 On the other hand, the nontoxic bivalent copper ions are capable of keeping stable in aerobic 3



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environment, forming compounds with absorption coefficient in wide visible region.18 Based on these considerations, we choose one Cu-based perovskite-like organometal halide of [NH3-(CH2)4-NH3]CuCl4 (DABCuCl4) for high pressure research. Unlike classical octahedron arrangement, in DABCuCl4, JahnTeller effects lead to the square planar CuCl42- coordinated clusters, forming the flexible perovskite-like inorganic layers (Figure S1).19 And in [NH3(CH2)nNH3]CuCl4 (n≥4) series, conformation of organic chains plays more important roles for whole structure.20 It is expected that, the long organic chains of DABCuCl4 can help us to explore the individual function of organic sheets and its influences on optical properties with increasing pressure. Meanwhile, comparing with C‒H…Cl, the two NH3 groups within the organic chains provide stronger N‒H…Cl hydrogen bonds, which may benefit the structural stabilization during compression.21 Furthermore, DABCuCl4 possesses the band gap of ~2.45 eV at ambient condition, which is comparable to that of famous OMHP material of CH3NH3PbBr3, making it a potential candidate for series components in optoelectronic applications.22 Herein, we report the high pressure evolution of electronic and crystalline property of DABCuCl4 by the means of in-situ UV-vis absorption, Raman and angle-dispersive synchrotron X-ray diffraction (ADXRD) experiments. At high pressure, DABCuCl4 exhibits persistent band gap narrowing, accompanied by the phase transitions. Meanwhile, the spring function of organic chains and band gap determined function of inorganic coordination are captured at high pressure. Our work combines the crystal-structure changes and macroscopic-property tuning together, which is helpful for the further synthesis and design of stable and environment-friendly perovskite-like materials. Considering the important role of band gap in photoelectricity applications, UV-vis absorption and optical micrographs experiments are performed to show the pressure effects on optical properties of DABCuCl4. At ambient conditions, DABCuCl4 crystalizes in room temperature (RT) phase with quasibidimensional structure of P21/a symmetry. And the ~2.45 eV band gap of RT phase leads to the yellowish green color at ambient conditions. The absorption spectra of DABCuCl4 (Figure 1a) include 4


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three absorption peaks for electron transitions of internal crystal field of d levels of Cu (Cu_dxz, yz → Cu_dx2-y2, Cu_dxy, yz → Cu_dx2-y2, Cu_dz2 → Cu_dx2-y2), as well as one absorption edge of ligand to metal charge transfer (CT) from Cl 3p state of the valence band to Cu 3d orbital.18, 23, 24 Redshifts of CT absorption edge are observed, involving with the piezochromic phenomenon (yellow → red brown →black) below ~12 GPa (Figure 1c and 1d). And the band gap of DABCuCl4 decreases from 2.44 eV to 2.05 eV, showing the powerful ability of pressure on band gap narrowing. CT band edge is strongly affected by the local structure of CuCl42-.25 The obvious discontinuity in band gap shifts at ~6.5 GPa reflects the possible phase transition from RT phase to the high pressure (HP) phase. This transition should involve with the local-structure changes around CuCl42- moieties. And the broad d-d absorption peaks reveal linear blueshifts without abrupt changes (Figure 1b), associating with the continuous compression of structure and insensitive response of crystal field to the possible structural transition.25 During further compression, the absorption signal becomes too weak to determine the precise band gap value. But d-d transitions and CT edge still keep their initial blue and red shifts, respectively, and tend to merge with each other, as shown in Figure S2. DABCuCl4 exhibits the characteristics of an indirect band gap of sharp absorption increase with shallow absorption region.5 At high pressure, no obvious evidence is captured for the transition of band gap type. Meanwhile, comparing with state-of-the-art hybrid perovskites, Cu-based hybrid perovskites reveals the much lower light absorption and charge transport properties.18 High pressure treatment further decreases the absorption coefficient of DABCuCl4 dramatically, which is not benefit for its absorber applications. Nevertheless, the band gap narrowing of DABCuCl4 may contribute to the improvement of its electronic conductivity, so does the charge mobility property for solar cell applications.8 Furthermore, unlike the band gap broadening of short-chains OMHPs (such as CH3NH3PbBr3), DABCuCl4 keeps sustaining band gap narrowing up to ~12 GPa, which may result from the stable nature of DABCuCl4.22 5



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Figure 1. (a) High pressure UV-vis absorption spectra of DABCuCl4, showing the crystal field (d-d) and ligand-to-metal charge transfer (CT) transitions. (b) – (c) The positions of d-d transitions and band gap as a function of pressure. The dashed line presents the possible phase transition pressure. (d) Optical micrographs of the piezochromic phenomenon in DABCuCl4. In order to obtain more information on CuCl42- (the band gap related moieties), high pressure Raman experiments are performed to explore the local structural variations. The identification of Raman modes is according to the previous studies of similar compounds.8 In Raman spectra, Cu-Cl modes are applied to deduce the CuCl42- coordination evolution. Below 5.9 GPa (Figure 2a and 2b), Cu-Cl modes of DABCuCl4 exhibit normal blue shifts, originating from the contraction of interionic distances. During further compression, a new peak marked by the arrow arises at 5.9 GPa, two Cu-Cl modes emerge into one at 7.6 GPa and another two new modes appear around 150 cm-1 at 10.2 GPa. Such variations are coincident with the absorption results, confirming the possible phase transition to HP phase between 5.9 6


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GPa and 10.2 GPa. Above 10.2 GPa, no obvious discontinuity is observed until the highest pressure, reflecting the stabilization of HP phase.

Figure 2. (a) − (b) Representative Raman patterns of Cu-Cl modes of DABCuCl4 and the corresponding frequency shifts as a function of pressure. (c) − (e) Selected N-H stretching modes of DABCuCl4 with respect to pressure. (f) Plots of N-H stretching peak positions versus pressure. The part with shadow presents the transition range from RT to HP phase. High pressure responses of hydrogen bonding also provide the indirect evidence for CuCl42behaviors. Since hydrogen bonding connects the CuCl42- and [NH3-(CH2)4-NH3]2+ together, responses of N-H (electron donors in N‒H…Cl) stretching mods are applied to explore the relationships between two building blocks. As summarized in Figure 2c, υ(NH) modes exhibit normal blue shifts with the mergence of υ3 and υ4, which indicates that the hydrogen bonds in DABCuCl4 are strong and to be sustaining strengthened by high pressure.26 Importantly, in the pressure range of 5.9 – 10.2 GPa, υ5 loses its intensity gradually, and υ1 shows a dramatic discontinuity in the pressure coefficients (Figure 2d and 7



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2f). The unobvious variations and the adjusted intensities of υ(NH) modes imply that the hydrogen bonding networks are only distorted slightly during the structural transition. Such distortion may arise from the slightly local-structure changes around CuCl42-, such as tilting, rotation or distortion.10 Above 10.2 GPa, hydrogen bonding network keeps stable (Figure 2e), since the remaining three modes maintain their initial distribution in both intensities and positions. High pressure Raman experiment supports the proposed absorption results that DABCuCl4 undergoes a structural transformation from RT phase to HP phase between ~6 GPa and ~10 GPa. And the phase transition involves with the slightly distortion of inorganic layers.

Figure 3. (a) Representative ADXRD patterns at selected pressure up to ~2.9 GPa of DABCuCl4. (b)-(c) Structural transformation from HT phase to RT phase. Based on absorption and Raman results, high pressure ADXRD experiments are also performed to ensure the DABCuCl4 structural evolution. At ambient conditions, DABCuCl4 only crystalizes in RT phase, forming single crystals. And for ADXRD experiments, DABCuCl4 crystals have to be ground into powdered sample with a few micrometres. As depicted in Figure S3, grinding of RT phase causes the inevitable structural transition from RT to high temperature (HT) phase.27 However, pressure purify 8


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the mixed-phases powder to pure RT phase again by the compression to 0.8 GPa (Figure 3a). As shown in Figure 3b and 3c, the phase transition from HT to RT phase is determined by the organic chains, their conformation changes form all-trans (ttt) to left-handed (gtg’). Such centrosymmetrical adjusting leads directly to the shortening of molecular, reduction of interlayer distance, and larger offset of inorganic layers. Meanwhile, the phase transition is also accompanied by the modification of hydrogen bonding scheme, from “monoclinic” (two Cl atoms in axial and one Cl atom in equatorial) configuration to “orthorhombic” (one Cl atoms in axial and two Cl atoms in equatorial) configuration.28 During this transition, the inorganic sheets are protected, presenting the high pressure function of organic chains. That is, organic layers act as the spring cushion between inorganic layers, stabilizing the whole construction below ~0.8 GPa.

Figure 4. (a) – (d) High pressure evolution of ADXRD patterns, lattice parameters and unit cell volume. The parts with shadow present the phase transition range. (e) Refinement of ADXRD pattern collected at 12.6 GPa. Green lines reflect the refined peak positions and the blue line represents the differences between observed (black) and simulated (red) profiles. 9



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After the first phase transition up to 5.5 GPa, as shown in Figure 4a, all diffraction peaks shift to higher angles without abrupt discontinuity, suggesting the stabilization and sustaining contraction of RT phase. Furthermore, the diffraction patterns dramatically changes in the pressure range of 6.4 – 10.5 GPa, including the splitting of (1 1 -1) diffraction peak at 6.4 GPa and the emergence of two new peaks around (0 0 1) diffraction peak at 7.7 and 10.5 GPa, respectively. All these changes prove the proposed phase transition from RT phase to HP phase in this pressure range. The transition extends over broad pressure range of ~ 4 GPa, implying the high-energy hindrance between the two phases.29 Since the diffraction peaks are not broadened obviously across the phase transition, it is inferred the transition is uncorrelated with structural amorphization. Moreover, variations only appear around (0 0 1) and (1 1 -1) Millar planes, which are associated with inorganic layers (Figure S4). It is therefore conceivable that this transition is determined by the distortion of inorganic parts, just as the results deduced from absorption and Raman experiments. From diffraction patterns, we are able to obtain high pressure evolution of lattice parameters, as well as the candidate structure of HP phase. As shown in Figure 4b, 4c and 4d, for the contraction of whole structure, the unit cell axes and volume are all decreased below 5.5 GPa, just as expected. However, the axial angle β is gradually enlarged, reflecting the more obvious offset of the inorganic layers at high pressure. In transition range, abrupt discontinuities of lattice parameters and volume collapse are observed. Here, the diffraction pattern of 12.6 GPa is applied to assign the HP phase symmetry. Pawley refinement (Figure 4e) shows the indexed lattice parameters of HP phase at 12.6 GPa are monoclinic P2 symmetry, with a = 10.213(5) Å, b = 4.443(2) Å, c = 9.674(8) Å, β = 115.149(0)̊. The P–V data is fitted based on the third-order Birch-Murnaghan equation of state (see details in SI), obtaining the bulk modulus B0 = 27.7(3) GPa for RT phase, and B0 = 61.2(3) GPa for HP phase. B0 depends on the density of the material, and the relatively higher B0 presents the less compressible nature of HP phase. 10


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Figure 5. High pressure evolution of (a) Cu-Cl bond lengths and (b) angles in RT phase. Structural responses of CuCl42- coordination are used to understand the interplay between crystal structure and macroscopic property. As despited in Figure 5a, pressure introduces inhomogeneous contraction of CuCl42-. The shrinkage of Cu-Cl bonds possesses different pressure coefficients. Bond lengths of Cu-Cl1 and Cu-Cl2 become almost equal at ~6.5 GPa, indicating the quenching of Jahn-Teller effect at high pressure. And the gradual distortion of CuCl42-, from square-planar to rhombus-planar coordination, is evidenced by the reduction of Cl1-Cu-Cl2 bond angle. Meanwhile, the decreased angles of Cu1-Cl2-Cu2 present the in-plane approaching of CuCl42- coordination at high pressure (Figure 5b).30 No obvious tilting of CuCl42- is observed for RT phase. Shrinkage and distortion are able to introduce distribution changes of electronic cloud in CuCl42- coordination, which just determines the band gap narrowing during compression.22 This is also coincident with the results in both absorption and Raman experiments. Furthermore, it is deduced that such distortion may benefit from the flexibility of CuCl42-, thus promotes the band gap tuning under high pressure. Combining all the experimental results together, the mechanisms of DABCuCl4 behaviors are inferred as follows. With increasing pressure, contraction of interatomic distances introduces the enhancement of hydrogen bonding and van der Waals force between organic and inorganic layers.31 In 11



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order to balance the increased free energy, HT phase gradual transfers to RT phases below ~0.8 GPa, involving with the conformation changes of organic chains from left-handed to all-trans. In this pressure range, the [NH3-(CH2)4-NH3]2+ chains function as spring cushion, protecting the CuCl42- layers and supporting the whole structure.32 With further compression to ~6.4 GPa, the contraction of RT phase cannot support the external force anymore, DABCuCl4 transfers to the HP phase between 6.4 and 10.5 GPa. The transition from RT phase to HP phase is determined by the distortion of inorganic layers and hydrogen bonding networks. RT phase keeps stable all the way up to ~6.4 GPa, possessing the much higher transition pressure than most short-chain OMHPs (such as 0.35 GPa of CH3NH3PbI3, 0.4 GPa of CH3NH3PbBr3, and 0.7 GPa of CH3NH3SnI3).11, 12, 22 Obviously, supporting function of long organic chains assists the structural stabilization of DABCuCl4 in low pressure range. Furthermore, the flexible CuCl42- inorganic moieties function as the band gap controller at high pressure. The sustaining distortion and shrinkage of CuCl42- coordination lead to the significant band gap narrowing, which is of pivotal importance for the optoelectronics applications. Importantly, the band gap of DABCuCl4 keeps narrowing all the way up to ~12 GPa, which is different from the high pressure behaviors of CH3NH3Pb-based perovskites. Band gap broadening of CH3NH3Pb-based perovskites results from destruction of architectures.22 The much longer organic chains in DABCuCl4 provides the spring cushion within structure, protecting the structure from amorphization, so as to promote the narrowing of band gap. The length of organic chains can influence the structure and band gap behaviors at high pressure, which is the new insight for perovskite-like materials. In this work, RT phase of DABCuCl4 was prepared according to the previous reports.19 The DAC, equipped with 0.4 mm diamond culets, was adopted for generating high pressure. The sample was loaded into a 0.14 mm diameter aperture, which is drilled in the centre of the preindented T301 gasket. Silicon oil (Aldrich, ~150 mPa.s) is applied as pressure-transmitting medium. The ruby fluorescence technique was applied for the pressure calibration.33 All the experiments were conducted at room 12


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temperature. Ocean Optics QE65000 Scientific grade spectrometer was applied to detect absorption signal. The experiments were measured by the transmittance method. And high-pressure Raman spectra were detected using ARC-SP 2558 spectrograph (Princeton Instruments) equipped with charge-coupled detector (CCD) PyLon: 100B. The excitation line was 532 nm from the diode pumped solid-state (DPSS) laser. ADXRD measurements were conducted on 4W2 beamline at the High Pressure Station of the Beijing Synchrotron Radiation Facility (BSRF), using an image plate area detector (Mar345) and focused beamsize of 20 × 30 µm2. Portions of the works were performed at the High Pressure Collaborative Access Team's (HPCAT’s) 16 IDB beamline facility of the Advanced Photon Source (APS). In an agate mortar, flawless DABCuCl4 crystals of RT phase were selected and ground to powder with a few micrometres. CeO2 was chosen as the standard for geometry calibration. Bragg diffraction rings were converted into plots of intensity versus 2θ using software Fit2D.34 Further analysis of XRD data was performed using commercial software Materials Studio 5.0. To sum up, we successfully narrow the band gap of DABCuCl4 through high pressure treatment. At high pressure, band gap narrowing arises from the shrinkage and distortion of CuCl42- coordinated environment. Inorganic distortions determine both the band gap responses at high pressure and the phase transition from RT phase to HP phase between 6.4 and 10.5 GPa. Meanwhile, organic chains function as spring cushion to protect inorganic layers, evidenced by the phase transition from HT phase to RT phase at ~ 0.8 GPa. Such supporting function leads to the stable structure in low pressure range, and protects DABCuCl4 from amorphization, which also contributes to the sustaining narrowing of band gap. This work demonstrates the powerful ability of pressure in property and structure tuning, and offers a deeper understanding of structure – property relationships in atomic level. It also provides some new strategies for the further design and synthesis of nontoxic and stable OMHPs materials, the next generation photovoltaic material.

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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications Website at DOI: Structural, experimental and fitting details of DABCuCl4.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. ORCID Bo Zou: 0000-0002-3215-1255 Author Contributions Q.L. and B.Z. designed and performed experiments and analyzed data. S.L., K.W., and Z.Q. assisted in performing experiments. B.Z. provided intellectual input. Q.L. and B.Z. wrote the manuscript.

Notes The authors declare no competing financial interest.

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ACKNOWLEDGEMENTS This work is supported by the National Science Foundation of China (NSFC) (No.91227202, 21673100, 11604141), Shenzhen fundamental research programs (Nos. JCYJ20160530190717385), the Changbai Mountain Scholars Program (No. 2013007) and Program for Innovative Research Team (in Science and Technology) in University of Jilin Province. ADXRD measurements were performed at 4W2 HPStation, Beijing Synchrotron Radiation Facility (BSRF), which is supported by Chinese Academy of Sciences (Grant KJCX2-SW-N20, KJCX2-SW-N03). Portions of work were performed HPCAT’s beamline facility (Sector 16), of the Advanced Photon Source at Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974 and DOE-BES under Award No. DE-FG02-99ER45775, with partial instrumentation funding by NSF. The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The support provided by China Scholarship Council (CSC) during a visit of Qian Li to HPCAT is also acknowledged.

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absorption 126x93mm (300 x 300 DPI)



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Raman spectra 98x64mm (300 x 300 DPI)


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ADXRD spectra and structural evolution 76x33mm (300 x 300 DPI)



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structural evolution at high pressure 89x45mm (300 x 300 DPI)


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bond length evolution 60x25mm (300 x 300 DPI)



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Ambient structure of RT phase 32x12mm (300 x 300 DPI)


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absorption above 12 GPa 112x171mm (300 x 300 DPI)



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mixed phases 63x50mm (300 x 300 DPI)


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planes 94x50mm (300 x 300 DPI)



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TOC 50x50mm (300 x 300 DPI)


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High-Pressure Study of Perovskite-Like Organometal Halide: Band

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