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Letter pubs.acs.org/JPCL

Effects of Nonhydrostatic Stress on Structural and Optoelectronic Properties of Methylammonium Lead Bromide Perovskite Rong Zhang,† Weizhao Cai,† Tiange Bi,‡ Niloofar Zarifi,‡ Tyson Terpstra,‡,§ Chuang Zhang,† Z. Valy Verdeny,† Eva Zurek,*,‡ and Shanti Deemyad*,† †

Department of Physics and Astronomy, University of Utah, 115S 1400E, Salt Lake City, Utah 84112, United States Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14260-3000, United States § Department of Chemistry, School of Arts and Science Education, D’Youville College, 591 Niagara Street, Buffalo, New York 14201, United States ‡

J. Phys. Chem. Lett. 2017.8:3457-3465. Downloaded from pubs.acs.org by 93.179.91.178 on 08/13/18. For personal use only.

S Supporting Information *

ABSTRACT: We report synchrotron X-ray diffraction, photoconductivity, and photoluminescence investigations of methylammonium-lead-bromide (MAPbBr3) under various stress conditions, supported by density-functional-theory (DFT) calculations. The properties of MAPbBr3 show substantial dependence on the hydrostatic conditions. While nonhydrostatic compression of MAPbBr3 leads to amorphization above 2.4 GPa, under quasi-hydrostatic (Ar) and hydrostatic (He) pressure, the sample remains in crystalline phases. A sequence of phase transitions between two cubic phases and orthorhombic Pnma phase is observed when using Ar, or no pressure-transmitting-medium (PTM). In helium-PTM only transitions between the two cubic structures and a new isostructural phase transition with a large volume collapse to a third cubic-phase at 2.7 GPa was observed. The photoluminescence measurements indicate a pressureinduced band gap-narrowing in the cubic phase I, and a blue-shift in the orthorhombic structure. DFT calculations illustrate that the dynamics of the organic molecules and the inorganic lattice, coupled via the N−H···Br hydrogen-bonding interactions, affect the Pb−Br distance and the bandgap evolution under pressure.

P

Application of high pressure is a major tool in condensed matter for tuning the electronic and optical properties without chemical modification.9 The material response to stress also provides important information for device fabrication even at ambient pressure; since nonuniform stress components can be present due to thermal expansion mismatch with the surrounding and/or substrates. The prominent strain induced by external pressure has initiated tremendous interest in stimulus-sensitive hybrid materials.10 Their structural response has been shown to depend on several factors such as framework flexibility and guest cations.11−13 In addition, piezochromic behavior is reported in the analogues compounds, illustrating that the electronic structures as well as crystal structure can be tuned by external pressure.14,15 The high pressure response of structural and band gap changes of hybrid halide perovskites with substitution of organic cations and metal atoms have been reported recently.14−27 These results manifest that similar to the temperature-induced distortions, the OIP framework that is built from PbX6 octahedra tilts considerably under pressure, leading to a sequence of structural phase transitions. In addition, the organic cations in the free voids, which interact with the main framework via H-bonds, also affect the electronic properties through the lattice distortions via spin−orbit

erovskites encompass a large class of materials with the general ABX3 formula, where the B cation is 6-fold coordinated surrounded by anions in an octahedral arrangement (BX6), whereas the A cation is 12-fold coordinated. Many technologically important compounds that exhibit high temperature superconductivity, colossal magnetoresistance, charge ordering and spin-dependent-transport belong to this family. As such, perovskites have been the subject of intensive studies in past years.1−3 Recently, synthetic hybrid organic−inorganic perovskites (OIPs) based on methylammonium lead halide (MAPbX3, MA = CH3NH3, X = Cl, Br and I) have shown superior properties for applications in photovoltaic technology. These materials can be synthesized at a low cost, and have strong solar absorption and excellent power conversion efficiency that currently reaches ∼22%. The unique optical and electronic properties of these compounds can be achieved through tuning the electronic structure by chemical substitutions of both organic and inorganic components.4−8 However, the structural instability of these compounds for relatively small variations in temperature, as well as their chemical instability in the presence of moisture poses a challenge for their application. In addition, due to the presence of lead in their structures, these compounds are not environmentally friendly. Therefore, understanding the factors that contribute to the chemical and structural instabilities of MAPbX3, as well as designing a lead-free equivalent of these compounds, is currently a milestone in photovoltaics. © 2017 American Chemical Society

Received: May 31, 2017 Accepted: July 10, 2017 Published: July 10, 2017 3457

DOI: 10.1021/acs.jpclett.7b01367 J. Phys. Chem. Lett. 2017, 8, 3457−3465

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The Journal of Physical Chemistry Letters

Figure 1. Structures of various MAPbBr3 phases observed under high pressure. (a) Phase I, cubic phase having space group Pm3m ̅ viewed approximately along the [001] direction. (b) Phase II, cubic structure having space group Im3̅ viewed approximately along the [001] direction. (c) Phase III, which is observed here when He was used as the PTM, and is isostructural to phase II, viewed approximately along the [001] direction. (d) Projection of phase IV orthorhombic structure with space group Pnma along the [010] direction. The Glazer symbols are added for the corresponding phases. Color code: green Pb atoms, orange Br atoms. For clarity, the MA cations are not shown.

al. used He as a PTM and observed the Pm3̅m−Im3̅ and Im3̅amorphous phase transitions at ∼0.9 and 2.8 GPa, respectively.38 Other studies show that when silicone oil was employed as a pressure medium, the Pm3m ̅ phase converts to the Im3̅ phase at ∼0.5 GPa.37,39 While several reports assume that the MAPbBr3 phase transitions are not sensitive to the hydrostaticity of the PTM, a comparison between the results of the aforementioned studies questions this assumption. Although the pressure dependence of methylammonium-leadhalides structures has been previously reported, the effect of nonuniform stress is not known. In this work, we present a systematic study of MAPbBr3 crystal under high pressure, and investigate the interplay between the various phases and photophysical properties under hydrostatic conditions exerted by PTM of helium and argon and without a PTM, respectively. We have used synchrotron Xray diffraction, photoluminescence (PL), and photoconductivity spectroscopies in a diamond anvil cell, supported by DFT calculations to elucidate the mechanism of the band gap changes and structural phase transitions under pressure. In addition, first-principles molecular dynamics (FPMD) calculations have also been performed to illustrate the influence of the MA molecules dynamics on the instantaneous and timeaveraged band gaps of the various structural phases.

coupling.28 DFT calculations on MAPbX3 have demonstrated that the orientations of the organic MA cations influence the dispersion relations of the conduction and valence bands.29,30 Crystalline samples of methylammonium lead bromide (CH3NH3PbBr3, abbreviated here as MAPbBr3) are orange in color since their band gap is ∼2.3 eV at ambient conditions.31 The room-temperature structure of MAPbBr3 is cubic with space group Pm3̅m and Z = 1 and shows a diversity of structures during cooling.32−34 The tilt of the PbBr6 octahedra can be described by Glazer notation as a0a0a0 (Figure 1a).35 The framework of all the structures of MAPbBr3 consist of vertex-sharing PbBr6 octahedra, in which the disordered/ ordered MA cations occupy the three-dimensional channels and hydrogen-bond to the host framework of the PbBr6 octahedra. The pressure dependence of the various MAPbBr3 structural phases and the related optoelectronic properties have been studied by several groups. Deuterated MAPbBr3 samples pressurized using isopropanol as the pressure-transmittingmedium (PTM) have been shown to undergo a structural phase transition from Pm3̅m phase to Im3̅ phase at ∼0.9 GPa, and become amorphous above ∼2.8 GPa.36 High pressure studies without a PTM by Wang et al. showed that the Pm3̅m phase transforms to Im3̅ at ∼0.4 GPa, followed by an orthorhombic Pnma phase at ∼1.8 GPa.37 Recently, Jaffe et 3458


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Figure 2. (a) Lattice parameter a of MAPbBr3 compressed in He obtained from single crystal diffraction data. Formula-unit volume (V/Z) as a function of pressure is shown in the bottom panel. The lines through the data points are second-order Birch−Murnaghan equation-of-states fit to the volume (V/Z) data in phases I−III. The unit-cell dimensions of phases I and II obtained from Ar, and phase I at ambient pressure from the literature, are added for comparison. The insets show a prominent piezochromism behavior of a single crystal in a DAC chamber at 0.6 and 3.0 GPa compressed in He. (b) Evolution of Pb−Br coordination bond length and Pb−Br−Pb angle in MAPbBr3 as a function of pressure. The insets show the dihedral angle of Pb−Br−Pb relative to the ab plane and a distortion of the PbBr6 octhedra under pressure during the phase transition. Vertical dashed lines in both (a) and (b) indicate two phase transitions at ∼0.85 and ∼2.7 GPa of MAPbBr3 pressurized in He.

zero-pressure bulk modulus B0 of 12.2(8) GPa for phase I, and 13.5(6) and 16.1(9) GPa for phases II and III, respectively; this indicates that MAPbBr3 becomes increasingly harder under compression (Figure 2a). In addition, as reported in a recent study, the slow kinetics emerge in the phase transformations of MAPbBr3.23 Therefore, herein we have kept the sample below 1.0 GPa for 24 h; however, no other phase transitions were observed. The external pressure induces an intriguing structural response of the observed phases. For example, in phase I, the Pb−Br distance reduces by ∼1.8% at 0.75 GPa and the PbBr6 octahedra are vertex-shared so that the Pb−Br−Pb angle is equal to 180°. The Pb−Br−Pb bending angle suddenly drops to 165.8(4)° when the phase I−II transition occurs, and it reduces considerably through the whole phase II region (by ∼6.7° up to 2.4 GPa). Meanwhile, the Pb−Br distances are shortened due to the isotropic compression of the crystal, i.e., its length decreases by ∼1.5% upon compression up to 2.4 GPa (Figure 2b). An additional abrupt increase in the bending angle was observed after the phase II−III transformation occurs. We postulate that the guest MA cations in the inflating voids play a key role in the distortion of the PbBr6 octahedra within the main framework, as illustrated in recent cases of MAPbI3 and a 3D metal−organic framework.13,15 High pressure enhances the H-bonds and affects the interacting PbBr6 octahedral framework. The rotation of the PbBr6 octahedra within phases II and III can also be directly reflected in the dihedral angle (φ) of

To investigate the structural phase transitions under high hydrostatic pressure conditions, we first studied MAPbBr3 using He as a PTM. At room temperature, helium sustains hydrostatic conditions up to 20 GPa.40 As shown in Figure 2, the ambient-pressure cubic phase I (Pm3̅m) of the MAPbBr3 persists during monotonic compression up to 0.85 GPa. Above this pressure, a discontinuous phase transition to phase II (Im3̅) with a small unit volume collapse (V/Z) of ∼1.0 Å3 was observed. In addition to the I → II phase transition, another distinct phase transition to phase III was observed at 2.7 GPa, which was accompanied by a large volume drop of ∼4.4 Å3 and a discontinuous change of the lattice parameter (Figure 2 and S1). This structure had not been observed in previous studies.36,38 We established that the phase II−III transformation is isostructural: both phases have the same space group symmetry and similar lattice parameter, a. Moreover, the color of the single crystal changes from orange to light yellow (Figure 2a). When the pressure was released to ambient pressure, we found that the phase transitions and color of the crystal sample are completely recoverable (Figure S1). The lattice parameter of phase II is about twice that of phase I, hence the unit-cell volume becomes 8 times larger (Z = 8), and the tilt character changes to a+a+a+ (Figure 1). In phase I region, we find a to be isotropically reduced by ∼1.8%, whereas in phase II a shrinks by ∼3.0% (Figure 2 and Table S1). The second-order Birch−Murnaghan equation of state was used to fit the V(P) data of the three obtained phases. The fit yields the 3459


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under similar conditions.37 We observed that the first phase transition from phase I → II occurs at ∼0.4 GPa with a clear volume collapse (Figures S3 and S4).39 The second phase transition from the cubic phase II to the orthorhombic phase IV took place when the pressure increased above ∼1.5 GPa (the peak splitting at ∼4.3° and 8.8° in Figures 3c and S3). In agreement with previous nonhydrostatic studies beyond 2.8 GPa, we observed a gradual amorphization, and the MAPbBr3 sample loses its crystalline character (the diffraction image and broad peaks in Figure S3). The compression evolution of the unit-cell volume is initially consistent with the single crystal data in He, but then gradually deviates from the trend observed in He (Figure S4). The discrepancy is caused by the stress components from nonhydrostatic conditions exerted on the sample grains. The distinct difference between the pressure induced phase transitions of MAPbBr3 under various environments demonstrates that the inhomogeneous strain has a strong effect on its structural stability. The phase stability of MAPbBr3 in helium is quite different as compared to the other media: for example, no phase IV was observed at elevated pressures. A mixed phase II and IV presents in Ar PTM when the pressure is above ∼1.0 GPa. Since Ar solidifies at 1.4 GPa at 300 K, the solid environment (pseudohydrostatic) may hinder the complete transformation from II → IV.40,41 Amorphization, on the other hand is the consequence of severe nonhydrostatic conditions caused by uniaxial stress components. In an effort to better evaluate this effect, we have used highly uniaxial pressure conditions on a single crystal sample that was bridged between the walls of the gasket and otherwise immersed in helium as a pressure medium (Figure S5). In this case, the sample undergoes a phase transition to phase IV at 0.4 GPa, which was the lowest pressure measured; and transformed to an amorphous phase at 1.7 GPa, which strongly suggests that both transitions to noncubic structures and amorphization are consequences of uniaxial stress. We observed that the color of the sample under pressure changes from orange to light yellow (Figure 4a). The piezochromism of the MAPbBr3 sample reveals discrete band gap changes under pressure, which can be estimated from the PL spectrum. The PL spectrum was previously investigated under different hydrostatic conditions, e.g., using paraffin as a PTM42 and also without a PTM.37 The pressure-induced shift in the PL band directly depends on the structural phases of MAPbBr3. We measured high pressure PL spectra from MAPbBr3 single crystals in argon (488 nm) up to 3.1 GPa. The PL band shows a red-shift in phase I region (0−0.9 GPa), which most likely originates from the contraction of the Pb−Br bond lengths. Whereas further compression in the mixed phase of II and IV region (above 0.9 GPa) leads to a distinct PL blueshift from 2.33 → 2.39 eV during compression from 1.2 → 2.0 GPa (Figure 4a). Such an energy change illustrates that the rotation of the PbBr6 octahedra in different phases may strongly affect the MAPbBr3 electronic structure. In addition, we measured the PL spectrum of MAPbBr3 in helium as PTM. Below 1.0 GPa, when the sample is in the cubic phase I, the PL band red-shifts upon compression. Further compression causes an abrupt PL blue-shift. Also the sample color exhibited similar trends as when Ar was used as a PTM (Figure 4b). The MA cations become ordered, and the PbBr6 octahedra largely tilt within phase IV compared to the cubic phase II.33 The MA cations interact with the main framework with N−H···Br interactions leading to the tilt of the

Pb−Br−Pb relative to its perpendicular plane, e.g., ab plane (Figure 2b). The dihedral angle φ is equal to 90° in phase I, and it is abruptly reduced by ∼17.6° at 0.9 GPa and continues to decrease gradually under compression (phase II). On the other hand, in phase III all the dihedral angles increase above 2.7 GPa (Figure 2b). In a second series of experiments, we used argon as a PTM and the structural phase diagram was determined using both high pressure single crystal and powder X-ray measurements. The results of powder and single crystal studies were in excellent agreement for all pressure points. At room temperature argon crystallizes at 1.4 GPa, but below this critical pressure it maintains perfect hydrostatic conditions. We found that under these conditions the cubic phase I is stable up to ∼1.0 GPa. At 1.1 GPa a structural transformation to a mixture of phase II and phase IV (space group Pnma, Z = 4) takes place (Figures 3 and S2). This mixed phase persists at least to our

Figure 3. Le Bail fit of the X-ray data of MAPbBr3 compressed in (a) Ar at 1.6 GPa, (b) He at 1.6 GPa, and (c) without PTM at room temperature. The black circles are the measured scattering intensity, and the red solid line represents the fit to the data. The vertical bars indicate Bragg reflection positions of phase II and IV together with difference profiles (blue lines) shown at the bottom.

experimental limit of 11.9 GPa (Figure S2 and Table S2). The single crystal diffraction data yields the lattice parameters a = 11.5628(10) Å for phase II, and a = 8.1744(11) Å, b = 11.545(12) Å, c = 8.1773(9) Å for phase IV at 1.0 GPa (Table S2). In phase IV, the Pb−Br−Pb bending angles appreciably deviate from 180°, i.e., 158.8° and 170.1° at 1.0 GPa. For proper comparison with previous studies, we performed X-ray measurements of MAPbBr3 in the absence of a PTM. Our observations here are consistent with the previous studies 3460


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power) while simultaneously collecting the PL spectrum. Figure S6 shows the photocurrent increase up to 0.7 GPa prior to the phase transition (i.e., phase I → II). With increasing pressure, the photocurrent abruptly decreased above 1.0 GPa and became hardly visible at 2.0 GPa. We postulate that the initial increase of the photocurrent originates from the enhancement of the charge carrier mobility under pressure due to the band gap narrowing (Figure S6b). Pressure-induced structural changes are responsible for the reduction of photocurrent for pressures above 1.3 GPa (from the above discussion, MAPbBr3 transforms to phase IV). This phenomenon is consistent with the PL blue-shift under pressure (i.e., band gap widening). DFT calculations were carried out to develop an understanding of the band gap evolution of the MAPbBr3 phases as a function of pressure. Previous first-principles calculations have illustrated that the rotations of the organic cations are facile at room temperature. Importantly, the favorable hydrogenbonding interactions between the N−H and the halogen atoms result in a coupling of the dynamics of the organic molecules and the lead-halogen lattice.43,44 It is well-known that the bottom of the conduction band largely consists of Pb 6p character, whereas the top of the valence band is mainly comprised of p character from the halogen atom. Because the Pb-halogen distances, which are influenced by the cation dynamics, are key in determining the electronic structure, the band gap fluctuates with time. FPMD simulations have illustrated that the top of the conduction band, and the band gap itself, can vary by 0.07−0.20 eV at 320−350 K within various phases of MAPbI3 at ambient pressures.45,46 Therefore, to obtain realistic trends, it is crucial to average the calculated band gaps over a number of structures that are accessible at the experimental pressure and temperature conditions employed. Toward this end we have carried out FPMD calculations of the Pm3̅m, Im3̅ and Pnma phases as a function of pressure at 300 K. We employed the canonical ensemble wherein the volume was fixed, and the stresses at each MD step were nonuniform. Therefore, the pressures that were chosen for the different phases corresponded to those observed experimentally without a PTM (Figure 4c). The band gaps were calculated every 100th MD step (Figure S9) and averaged (Figure S7 and Table S5). The lattice dynamics, which is caused by a coupling of the motion of the MA molecules and the Pb−Br lattice via the N−H···Br interactions, has a significant impact on the instantaneous band gaps. The average range of the band gaps was calculated to be 0.71, 0.55, and 1.58 eV for the Pm3̅m, Im3̅, and Pnma phases, respectively. The larger variation of the gaps (the minimum, maximum and range in Table S5) computed here as compared to those in refs 45 and 46 likely stems from the fact that the previous work kept the positions of the Pb atoms fixed to avoid potential phase transitions from occurring during the MD run. A plot of the band gap and the shortest Pb−Br distance as a function of time in Figure S9 is suggestive of an imperfect correlation between the two observables. Whereas the average gaps of the Pm3̅m and Im3̅ phases remained relatively constant as a function of pressure, the gap of the Pnma phase increased. Typically, increasing pressure leads to a shorter interatomic distance that results in a larger degree of orbital overlap, increased band broadening, and a concomitant decrease in the band gap. For example, for the related CsSnX3 perovskites, it has been illustrated that decreasing the lattice constant decreases the gap.47 Thus, the increase of the band gap with increasing pressure observed for Phase IV (Pnma), as illustrated in Figure 4c, is unexpected. The

Figure 4. Proposed phase boundaries of MAPbBr3 compressed in various PTM using λ = 488 nm excitation laser (a) in argon (Ar), (b) in He, and (c) without a PTM at room temperature. The black squares indicate the excitonic transition energy obtained from the PL band (right panels). The insets in the left panels of a and b show the optical macrographs of a MAPbBr3 single crystal compressed in Ar and helium, together with the polycrystalline sample without PTM in panel c.

PbBr6 octahedra, which is most likely responsible for the electronic structures of MAPbBr3.28 We also measured the PL spectrum of a MAPbBr3 powder sample under pressure without a PTM. At 0.2 GPa, the PL spectrum shows a broad peak centered at 543 nm, from which we estimate a band gap of 2.28 eV; very close to the band gap value at ambient pressure (2.3 eV).31 In the phase I region (0− 0.9 GPa), a red shift of the PL band was observed, which likely originates from the decrease of the Pb−Br bond length. An apparent blue-shifted PL spectrum is present in the phase IV region when the pressure exceeds ∼1.5 GPa and the PL band weakens with further increasing pressure, making it difficult to detect due to the compression-induced amorphization. Moreover, we performed photoconductivity measurements of MAPbBr3 single crystals in mineral oil illuminated with green laser light (532 nm peak intensity and ∼0.6 nm width, 4 mW 3461



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Pnma phase is the only one that can retain perfect symmetry when the CH3NH3+ cation is added to the inorganic lattice, and indeed our calculations (Figure S10 and S11) illustrate that the band gap and Pb−Br distances within the perfect symmetry crystal at 0 K decrease with increasing pressure. On the other hand, when the finite temperature lattice dynamics is taken into account we find that the band gap increases, in agreement with the experimental results. Within the 1.8−3.0 GPa pressure range, the shortest Pb−Br distances in the perfect symmetry structure are ∼0.16−0.26 Å longer than the average of the shortest Pb−Br distances obtained in the MD simulations. Whereas the average Pb−Br distances in the static phase do not differ much from the minimum Pb−Br bond lengths, at finite temperatures the former are at least 0.3 Å longer than the latter (Figure S8), suggesting that the lattice dynamics has a profound impact on the structure and distortions of the icosahedra. At finite temperature, the average of the minimum Pb−Br contacts remains relatively constant within the pressure range considered. This, coupled with the larger geometrical distortions introduced by temperature and dynamic effects, and inhomogeneous pressure gradients within the structure are important factors leading to the blue-shift in the band gap. Importantly, these results suggest that the evolution of the band gap as a function of pressure for the various phases is likely to be temperature dependent. In summary, we have shown that the structural and optoelectronic properties of the photovoltaic material MAPbBr3 are highly sensitive to the presence of nonuniform stress. High-pressure single crystal X-ray diffraction measurements using He as a PTM demonstrate that the I → II and II → III phase transitions occur at ∼0.85 and ∼2.7 GPa, respectively, and no amorphization is observed up to 4.8 GPa. If Ar is employed as a PTM, phase I converts to a mixture of phase II and IV. Similar to helium studies and in contrast to nonhydrostatic measurements, the compression of MAPbBr3 in Ar does not lead to amorphization up to the experimental limits of 12.0 GPa. The PL study under pressure demonstrates that the pressure-induced changes of the PL band are consistent with the structural changes under compression. A PL redshift (band gap narrowing) was observed for MAPbBr3 in Phase I followed by an abrupt change to a PL blueshift upon transition to the orthorhombic phase IV. These pressure-induced optoelectronic changes are associated with the H-bonds and the distortion of the PbBr6 octahedra within the MAPbBr3 structures. FPMD simulations illustrate that the lattice dynamics, which is influenced by the coupled motion of the Pb−Br lattices and the MA molecules via the hydrogen bonds, has a profound impact on the instantaneous band gap under pressure. Strikingly, whereas the band gap of the perfect symmetry Pnma phase at 0 K is predicted to decrease as a result of pressure induced band broadening, the band gap obtained by averaging the results obtained from snapshots of the FPMD simulations at 300 K increases with increasing pressure, in agreement with the experimental findings. The stimuliresponsive character of MAPbBr3 in different environments under pressure sheds light on optimizing their performance and assisting in the design of perovskites-based optoelectronic devices. The presence of stress even at ambient pressure is common due to mismatch between the thermal expansions coefficients of the films and substrate in devices, and we show here that is an important consideration in designing photovoltaic devices that operate in a broad temperature range.

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METHODS

Single crystals of MAPbBr3 were prepared by the solvent vapor exchange method. PbBr2 and MABr (Sigma-Aldrich, molar ratio 1:1) were dissolved in dimethylformamide (SigmaAldrich, 5 mL, 0.5 M), and stirred for 3 h. The vial containing as-prepared solution was placed in a beaker with 2-propanol (20 mL) as the antisolvent. The beaker was then covered and placed in a dark place for 2−7 days. The crystal size can vary from 0.1 mm to 5 mm depending on the growth time. A symmetrical diamond anvil cell with piston cylinder design (DAC) and a triangle Merill-Bassett design DAC were used to generate high pressure. Pressure was applied using three different pressure transmitting conditions under helium, argon and without a pressure medium. A stainless steel gasket was preindented to ∼70 μm in thickness, and a ∼ 150 μm diameter hole served as the sample chamber for all the X-ray measurements. The pressures were determined by the ruby fluorescence method.48 A single crystal of MAPbBr3 together with two ruby chips for pressure calibration were loaded into a DAC chamber. For loading the pressure medium, the gasket was sealed using high pressure loading of dense helium and argon. In the case of argon, some of the pressure medium loading was performed using a cryogenic loading method. While the initial structure of the sample is different at low temperature where the cryogenic loading is done, but no differences were observed between the results of two pressure loading methods. High pressure powder X-ray diffraction data were collected at the 16 ID-B beamline of the High Pressure Collaborative Access Team (HPCAT) at the Advanced Phonon Source (APS), Argonne National Laboratory (λ = 0.4066 Å). The diffraction data were analyzed by the Le Bail fitting method using the GSAS-EXPGUI package.49 High-pressure single X-ray diffraction data using He and Ar as pressure media were collected at beamline 13-BM-C of the Advanced Photon Source, Argonne National Laboratory with the X-ray wavelength of 0.434 Å. Diffraction data were analyzed using the ATREX IDL software package.50 Polarization, Lorentz, and absorption corrections were applied to the fit peaks. The unit cell and orientation matrix were determined in RSV for each data set. Lattice parameters were refined in RSV using a least-squares fitting procedure. Due to the low completeness of the diffraction data at high pressure, only the positions of Pb and Br atoms were determined for some pressure points. The data were refined by using the ambientpressure structure as the starting model or solved by direct methods with the aid of SHELXL-97.51 High-pressure photoluminescence measurements of MAPbBr3 were carried out also under helium, argon and without a PTM. The excitation laser line of 488 nm was used in those experiments. We performed photoconductivity measurements of the photoinduced current versus voltage characteristics on MAPbBr3 crystals as a function of pressure using mineral oil as a PTM, with quasi-four probe Kelvin resistivity technique and visible 532 nm green light (details in the SI). Geometry optimizations and electronic structure calculations were performed with the Vienna Ab Initio Simulation Package (VASP), version 5.4.1.52,53 The atomic potentials were described using the PAW (projector augmented wave)54 method with the PBE55 exchange-correlation as implemented in the VASP code. The valence electron configuration used for the Pb, Br, C, N, and H atoms were 6s26p2, 4s24p5, 2s22p2, 3462


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The Journal of Physical Chemistry Letters 2s22p3 and 1s1, respectively. The k-point grids were generated using the Γ-centered Monkhorst−Pack scheme, and the number of divisions along each reciprocal lattice vector was chosen such that the product of this number with the real lattice constant was 20−40 Å in the geometry optimizations and electronic structure calculations. A plane wave energy cutoff of 400 eV was used. The MA unit does not occupy the exact Wyckoff positions in the two cubic Pm3̅m (I) and Im3̅ (II +III) phases. It only occupies the Wyckoff positions consistent with the symmetry of the Pnma (IV) structure. The computational methodology employed gave a lattice constant (averaged over four different orientations of the MA molecules) of 6.05 Å of the Pm3m ̅ phase at atmospheric pressures, which is within ∼2% of the experimental value (Figure 2a). FPMD simulations on the Pm3̅m, Im3̅, and Pnma phases have been carried out for pressures of 0.1/0.2/0.4 GPa, 0.6/ 1.0/1.4 GPa and 1.8/2.4/3.0 GPa, respectively, using DFT within PBE-GGA as implemented in VASP. The simulations were carried out in the canonical (NVT) ensemble. For each pressure (P) run, the system was equilibrated for 2 ps and simulated using a 0.50 fs ionic time-step for a total time of 12 ps. All FPMD simulations were carried out with 48-atom unit cell or supercells, a k-point grid consisting of only the Γ-point, and the same PAWs and energy cutoff as previously mentioned for the geometry optimizations and electronic structure calculations.

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Foundation - Earth Sciences (EAR-1128799) and Department of Energy-GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, 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. Use of the COMPRES-GSECARS gas loading system was supported by COMPRES under NSF Cooperative Agreement EAR 1157758 and by GSECARS through NSF grant EAR-1128799 and DOE grant DE-FG02-94ER14466. The work at the University of Utah for CZ and ZVV was supported by the Department of Energy Office of Science, grant DE-SC0014579. E.Z. acknowledges research support from the New York State Center of Excellence in Materials Informatics, a RENEW grant from the University at Buffalo, and the NSF (DMR-1505817). T.B. acknowledges financial support from the Department of Energy National Nuclear Security Administration under Award Number DE-NA0002006. We acknowledge the Center for Computational Research (CCR) at SUNY Buffalo for computational support.

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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.jpclett.7b01367. Details of photoconductivity measurement setup, synchrotron X-diffraction data, and PL and PC measurements and DFT calculations of MAPbBr3 under pressure (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: ezurek@buffalo.edu. ORCID

Weizhao Cai: 0000-0001-7805-2108 Z. Valy Verdeny: 0000-0002-2298-398X Eva Zurek: 0000-0003-0738-867X Shanti Deemyad: 0000-0001-5661-8801 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Drs. S. Tkachev, D. Zhang, and J. Smith for experimental support in APS. The High pressure x-ray diffraction data were collected at HPCAT (Sector 16) and GeoSoilEnviroCARS (The University of Chicago, Sector 13) of Advanced Photon Source (APS), Argonne National Laboratory. Beam time for experiments at HPCAT was provided by the Carnegie-DOE Alliance Center, which is supported by DOENNSA under grant number DE-NA-0002006. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974 and DOE-BES under Award No. DE-FG0299ER45775, with partial instrumentation funding by NSF. GeoSoilEnviroCARS is supported by the National Science 3463


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