Mechanism of Pressure-Induced Phase Transitions, Amorphization

Aug 19, 2016 - Our single-crystal X-ray diffraction study of methylammonium lead triiodide, MAPbI3, provides the first comprehensive structural inform...
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Mechanism of Pressure-Induced Phase Transitions, Amorphization, and Absorption-Edge Shift in Photovoltaic Methylammonium Lead Iodide Marek Szafrański*,† and Andrzej Katrusiak*,‡ †

Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznań, Poland Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznań, Poland

‡

S Supporting Information *

ABSTRACT: Our single-crystal X-ray diffraction study of methylammonium lead triiodide, MAPbI3, provides the first comprehensive structural information on the tetragonal phase II in the pressure range to 0.35 GPa, on the cubic phase IV stable between 0.35 and 2.5 GPa, and on the isostructural cubic phase V observed above 2.5 GPa, which undergoes a gradual amorphization. The optical absorption study confirms that up to 0.35 GPa, the absorption edge of MAPbI3 is red-shifted, allowing an extension of spectral absorption. The transitions to phases IV and V are associated with the abrupt blue shifts of the absorption edge. The strong increase of the energy gap in phase V result in a spectacular color change of the crystal from black to red around 3.5 GPa. The optical changes have been correlated with the pressure-induced strain of the MAPbI3 inorganic framework and its frustration, triggered by methylammonium cations trapped at random orientations in the squeezed voids.

T

efficiently generated by hydrostatic pressure, and we have used this stimuli for measuring the strain-dependent properties of this hybrid perovskite absorber. During the final stages of preparing our publication, we learned about the most recent papers related to the pressure effect on hybrid perovskites.19−24 However, we found considerable inconsistences between these reports as well as a number of unsolved issues, regarding the key properties and structural data of MAPbI3, which require clarification and explanation. Most of the high-pressure structural data in those studies were collected by powderdiffraction, the method not well suited for investigating pseudosymmetric soft materials like MAPbI3. Moreover, some of the results were collected without any hydrostatic medium20,22 and the inhomogeneous strain blurred the results and hindered their interpretation. In order to circumvent these difficulties, we have chosen the single-crystal diffraction studies despite a strong tendency of the crystals to form pseudomerohedral twins, misleading for the interpretation of crystal symmetry. Regarding the optical properties of MAPbI3 under pressure, in most of those papers, the photoluminescence spectra were used for estimating the energy gap. However, it can be overestimated as the photoluminescence bands originate from the near-band-edge transitions. Most importantly, the pressure effect on the energy gap in ref 23 suggests quite different tendency than those in refs 19−22.

he organic−inorganic lead halide perovskites of general formula MAPbX3 (MA = CH3NH3+, X = Cl, Br, I) have emerged as excellent absorbers for photovoltaic applications. The energy-conversion efficiency of solar cells based on these materials raised quickly from 3.8% to about 20%.1−9 This impressive progress was achieved within a few recent years as a result of intensive studies involving design of the photovoltaic cell architecture, fabrication of high-quality films, as well as modifications of optical and electronic characteristics through chemical substitution. Owing to the high optical absorption of these materials, the layers in typical hybrid perovskite solar devices can be ultrathin, even of a few hundred nanometers. The absorber, fabricated in the form of a homogeneous film or a thin layer with mesoporous scaffold, is sandwiched between two conductive layers. The strain generated at the interfaces can induce distortions of the perovskite lattice, affecting the optoelectronic properties of the absorber. This strain can be used for tuning the device performance. Such an approach was successfully applied for optimizing the magnitude of spontaneous polarization in ferroelectric perovskite superlattices.10 Moreover, the strain engineering is currently considered as one of the best methods for tuning electronic structure and energy gap of semiconductors.11 Calculations of the electronic structure of MAPbI3 showed that the organic MA+ cations do not contribute to the bands close to the band gap region, hence the energy gap, Eg, originates from the electronic states of the inorganic (PbI3)n framework.12−14 Consequently, the distortions of this framework are reflected in the optical and electronic properties of the material, as evidenced both experimentally15−18 and theoretically.13,14 Such distortions are © 2016 American Chemical Society

Received: July 26, 2016 Accepted: August 19, 2016 Published: August 19, 2016 3458

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The Journal of Physical Chemistry Letters In our paper, the first systematic study on truly single crystals of MAPbI3, performed as a function of pressure across all the crystal phases, is presented. Owing to the advantages of the single-crystal diffraction, we were able to derive precise structural parameters and correlated them with the bandgap determined from the pressure-induced changes in the absorption edge. At ambient pressure, MAPbI3 transforms at 327.4 K from the high-temperature cubic phase I (space group Pm3̅m) to tetragonal phase II (space group I4/mcm) and then at 162.2 K to the orthorhombic phase III (space group Pnma), all they retaining the (PbI3)n polyanionic framework with its voids occupied by MA+ cations.25−28 Calorimetric data showed that a pressure-induced phase IV appears above 0.3 GPa between phases II and III.29 We have determined the space-group symmetry of this phase IV as Im3̅, consistently with refs 21 and 22, but inconsistently with refs 19 and 20. Even more controversies concern the structure of phase V forming above 2.5 GPa. We show that the transition to this phase is isostructural in character and generates strong structural frustrations, profoundly affecting optical properties of the crystal. In accordance with the previously established p−T phase diagram,29 we have detected the transition to the pressureinduced phase IV at 0.35 GPa. The stepwise drop in the crystal volume and discontinuous changes in the lattice parameters, clearly seen in Figures 1a and 1b, are characteristic of a firstorder phase transition. Phase IV is stable at room temperature to about 2.5−2.7 GPa, where another volume drop marks a subsequent transition to phase V. These transitions induce discontinuous structural changes, illustrated for angles Pb−I− Pb in Figure 2a and for bonds Pb−I in Figure 2b. The spectacular color change from black to red of phase V around 3.5 GPa, shown in Figure 1a for the crystal squeezed from 2.5 to 3.8 GPa in a diamond-anvil cell (DAC), illustrates the extent of pressure effects on the optical properties of MAPbI3. By measuring the vis−NIR spectra of the sample compressed in the DAC, we determined the pressure dependence of the absorption edge. The absorption spectra, shown in Figure 3a, reveal huge shifts across the range of phases II, IV, and V. The optical energy gap, Eg, plotted as a function of pressure is shown in Figure 3b. The ambient-pressure value of Eg (∼1.5 eV) is close to the value of 1.51 eV determined earlier from the diffuse reflectance spectrum.2,13 It should be noted that the values of Eg determined by different methods are scattered between 1.5 and 1.63 eV.2,13,15,18,21,22,30 However, in this study the most important are the relative changes in Eg, which reflect the pressure effect on the optical properties of MAPbI3. As illustrated in Figure 3b, initially the bandgap of the crystal in phase II decreases from 1.50 to 1.475 eV, whereas at the transition to phase IV, at 0.35 GPa, Eg jumps to about 1.54 eV. In phase IV, the magnitude of Eg slightly increases up to about 1 GPa and then it decreases up to the transition pressure at 2.5 GPa. The transition to phase V abruptly widens the bandgap again. As a result of this initially sharp and then continuous steep increase of Eg in phase V, the crystal becomes transparent to red light above 3.5 GPa. We redetermined the crystal structure of MAPbI3 at ambient conditions without the DAC, in order to derive a reliable model for phase II in its pressure range to 0.35 GPa. We established that the symmetry of phase II is described by space group I4/ mcm, consistently with the previous single-crystal X-ray27 and powder neutron diffraction28 studies. The recently proposed37

Figure 1. Unit-cell dimensions of compressed MAPbI3. (a) Molecular volume (V/Z) in phases II, IV, and V. The insets show a single-crystal plate 38 μm thick, in a diamond-anvil cell (DAC) chamber, 0.35 mm in diameter, at 2.5 and 3.8 GPa. (b) Pressure dependence of lattice parameters. The vertical dashed lines separate the crystal phases; the data points at 2.51 GPa were obtained for the sample with coexisting domains of phases IV and V.

alternative polar space group I4cm was discarded by our structural results, and also as unsupported by any relevant physical properties of the crystal. Similarly, according to our measurements, there are no arguments supporting space-group symmetry Fmmm postulated recently for phase II.21 A careful analysis of the ambient-pressure data shows that the reciprocal lattice and systematic absences are fully consistent with space group I4/mcm. However, it should be noted that the crystals in phase II are most often twinned of three components in the manner presented in Figure 4a: the tetragonal axis c is directed along the diagonals [110] and [110̅ ] of the twin counterparts. This is a pseudomeroedric twinning and the twin reflections nearly ideally superimpose. Of our crystallizations most of the crystals were twinned in this way (Figure 4b) and it required 3459

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Figure 3. Absorption edge of MAPbI3 as a function of pressure. (a) Vis-NIR absorption spectra in phases II, IV, and V. (b) Pressure dependence of the optical energy gap.

Figure 2. Structural parameters of compressed MAPbI3. (a) Angles Pb−I−Pb; in phase II an independent Pb−I−Pb angle (not shown) between bonds aligned along axis z is restricted to 180° by symmetry. (b) Pb−I distances; in phase II, two independent Pb−I bonds are marked with red and black symbols.

to the pseudomeroedric (as opposed to meroedric) twinning, a subtle splitting can be observed for the higher-angle reflections. The MAPbI3 perovskite structure can be generally described as a (PbI3)n polyanionic framework built of the PbI6 octahedra sharing the vertices. In the prototypical high-temperature phase I, of cubic space group Pm3̅m, Z = 1, the PbI6 octahedra are Odsymmetric, and their Pb−I bonds lie exactly along the crystal axes. In phase II, the symmetry is lowered to tetragonal space group I4/mcm, and the octahedra are alternately rotated by ±8.3° (this tilt angle is further denoted as θ) about two Pb−I bonds remaining aligned along the crystal z axis (Figure 5a). The tilts of PbI6 octahedra are described by Glazer’s symbols31 a0a0a0 in phase I and a0a0c− in phase II; this notation was primarily designed for mineral perovskites built of monatomic spherical ions, and in the hybrid perovskite MAPbI3 the aspherical CH3NH3+ cations are disordered according to their site symmetries. The disorder of cations CH3NH3+ plays a central role for the properties of MAPbI3 and it will be discussed in detail later below. In phase II, the high-pressure

very scrupulous searches and preliminary examinations before small single crystals could be selected (Figure 4c). Except for these four single crystals, most of even very carefully selected samples showed some traces of twinning, which manifested in very weak reflections of intensities of about 3σI, at the places of systematic absences. This twinning effect can be misinterpreted as the lack of symmetry elements connected with the systematic absences (see ref 21). For example, reflection 103, which should be absent due to the glide plane c ⊥ y in space group I4/ mcm, appears to not be extinguished due to reflections 211 and 121, respectively of the second and third twin components, superimposed at the same position. Noteworthy, this twinning does not change the systematic absences of the Bravais lattices I of the twins, but only affects the absences of glide planes c. Due 3460

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Figure 4. Possible orientations of tetragonal I cells (red, green, blue) in twinned MAPbI3 phase II (a) and the reconstruction of layer h0l of the diffraction pattern of the twinned crystal (b) and of the single crystal (c). Small circles indicate nodes of the common pseudo cubic lattice F (thin black lines).

Figure 5. Crystal structures of MAPbI3 phases II, IV, and V. (a) Projection of phase II at 0.1 MPa, with cations CH3NH3+ disordered at the special positions of site symmetry D2d. (b) Projection of phase IV at 1.45 GPa, with cations CH3NH3+ disordered at D2h- and Tdsymmetric sites. (c) Average structural model of phase V. Mean atomic displacement ellipsoids of Pb and I atoms are shown at the 50% probability level.

significantly increases the tilt angle θ about axis z to ±9.5°. The octahedron tilt and angle Pb−I−Pb (denoted φ) are strictly interdependent according to the formula: θ = (180° − φ)/2. The cubic phase IV can be described as the structure of prototypic phase I distorted by the PbI6 octahedra rotated each about one of the space diagonals, e.g. [111], as illustrated in

Figure 5b (see also Figure S1). Glazer encoded it as the three equal component rotations about orthogonal axes in symbol a+a+a+.31 This distortion reduces the symmetry of phase IV to space group Im3̅, Z = 8, and the unit cell becomes 8 times larger, twice in each direction, compared to the prototypic phase I. Accordingly, the unit cell volume in phase IV is twice 3461

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The Journal of Physical Chemistry Letters Table 1. Selected Crystal Data for MAPbI3 Phases II, IV, and V at 293 K phase

II

II

IV

IV

V

pressure (GPa) crystal dimensions (mm3) crystal system space group a = b (Å) c (Å) Glazer’s symbol volume (Å3) Z Dx (g cm−3) μ (mm−1) Tmax/Tmin absorption correction theta range (deg) reflections collected/unique final R1 (I > 2σI)

ambient 0.095 × 0.032 × 0.024 tetragonal I4/mcm 8.8660(3) 12.6524(9) aoaoc− 994.55(9) 4 4.140 26.193 0.620/0.440 numeric 3.22 to 26.56 913/284 0.0368

0.30(2) 0.134 × 0.047 × 0.040 tetragonal I4/mcm 8.7945(8) 12.6096(13) aoaoc− 975.26(16) 4 4.222 26.711 0.428/0.318 numeric 3.28 to 27.25 1708/163 0.0645

0.45(2) 0.134 × 0.047 × 0.040 cubic Im3̅ 12.4067(5) 12.4067(5) a+a+a+ 1909.74(13) 8 4.312 27.281 0.428/0.318 numeric 3.28 to 27.39 3602/266 0.0691

2.34(2) 0.13 × 0.11 × 0.10 cubic Im3̅ 11.9846(4) 11.9846(4) a+a+a+ 1721.35(10) 8 4.784 30.267 0.152/0.121 numeric 3.40 to 27.71 3259/214 0.0822

2.71 (2) 0.13 × 0.11 × 0.10 cubic Im3̅ 11.7936(13) 11.7936(13) a+a+a+ 1640.4(3) 8 5.021 31.761 0.152/0.121 numeric 3.45 to 27.06 3124/206 0.1485

different types of voids in the structure of MAPbI3 phase IV is shown in Figure S3. The site symmetry D2h (mmm) additionally requires that the H atoms be disordered in two positions, so the resultant disorder is 4-fold at least. The disorder of H atoms corresponds to the cation rotations by 60° about the C−N bond, but we found that the rotation by 30° does not significantly affect the dimensions of hydrogen bonds NH+···I. So, it is likely that at both Wyckoff positions a and b, the free rotations, or weakly damped rotations, contribute to the disorder of the cations. The entropy change of the crystal between phases II and IV arising from the contribution of cations [CH3NH3]+ should include their disordering in both these phases. In phase II, the minimum disorder is 4-fold for all cations, but the experimental value of the entropy gain of R ln 9.8 (R is the gas constant), determined for the transition between the ordered phase III and disordered phase II, is much higher.26 This entropy gain implies that the cation is more than 8-fold disordered and that positional disorder or free rotations of H atoms about the C−N bond contribute to the disorder in phase II. In phase IV, of eight cations in the unit cell, the minimum disorder for two cations in Wyckoff position a is 8-fold and for six cations at the Wyckoff position b it is 4-fold; consequently, the combined multiplicity of these minimum disorders in phase IV is 8·2/8+4·6/8 = 5. This would be a marginal increase of entropy connected to the disorder of cations; however, it was shown above that there are also other possible disorder modes, which do not differ significantly in the cation-framework contacts. The alternative orientation of cations in Wyckoff a position along axes x, y, and z, resulting in the 12-fold disorder, increases the combined minimum disorder to 12·2/8 + 4·6/8 = 6. This analysis of the entropy changes shows that in phase IV the H atoms positions (evidently disordered in phase II) are partly stabilized, except for the H-disordering required by the symmetry, as discussed above. The disorder models and entropy changes are essential elements of the mechanism of the transition between phases IV and V. This transition is isostructural, so it retains the lattice type and space group symmetry Im3̅. Accordingly, characteristic features of isostructural phase transitions,35,36 i.e., the discontinuous volume change (Figure 1) and considerable pressure hysteresis of about 1 GPa, were observed. It should be noted that in ref 22, phase V is assigned the orthorhombic

larger than that in phase II (Table 1). This space-group symmetry was also assigned to phase IV in refs 21 and 22 and to the high-pressure phase of analogous MAPbBr3.32,33 Thus, an orthorhombic symmetry of phase IV, postulated in refs 19 and 20, should be excluded. In phase IV, the octahedra are located at Wyckoff position c of the site symmetry C3i (international symbol 3̅, at 1/41/41/4 etc.). It allows the rotation of the octahedra at 1/41/41/4 and 3/43/43/4 about direction [111] in the same sense, but in the opposite sense to the rotation of the octahedra at 1/43/41/4 and 3/41/43/4 about diagonal [11̅ 1]. Moreover, in phase IV, the voids containing cations [CH3NH3]+ are differentiated into two types. In each unit cell, two of 8 cations are located at the Wyckoff a positions (000 and 1/21/21/2) of site symmetry m3̅; and six cations are located at the Wyckoff b positions (01/ 21/2 etc.) of site symmetry mmm. These site symmetries are higher than the symmetry C3 (3m) of the cation, which implies its disorder. The disorder can involve different orientations of the cation and its displacements off the void center.28,34 We have analyzed possible positions of the cation by taking into account the number of orientations and distances to the I atoms forming the cage, and particularly the hydrogen bonds NH+···I. The detailed analysis is presented in the Supporting Information, but the following conclusions relevant to the entropy of the crystal can be listed. The minimum disorder in the void associated with Wyckoff position a is when the C−N bond is aligned (up and down) along four diagonal directions [111], [1̅11], [11̅1] and [111̅]; no additional disorder of the H atoms is required, so the cation is 8-fold disordered. An alternative type of disorder in this Wyckoff a position is for the cation aligned (up and down) along axes x, y, and z and the H atoms rotated by 60°, resulting in the minimum 12-fold disorder of the cation. The overall shape of the cations disordered in both these ways is very similar (see Figure S2), and the H-bond geometry is only slightly favored for the second type of the disorder. All this information suggests that both types of disorder (as well as also other types) can be present in Wyckoff position a. In Wyckoff position b, the minimum disorder is when the molecule is aligned (up and down) along one crystal direction, for example along axis x, clearly defined by the elongated shape of the void due to the confining positions of I atoms of the Pb− I−Pb bridges bent toward the void center. The shape of two 3462

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The Journal of Physical Chemistry Letters space group Immm, on the basis of splitting of 00l reflections in the X-ray powder-diffraction patterns. However, this conclusion was derived from the data collected for the powder sample compressed without a hydrostatic medium, which undoubtedly caused strong uniaxial strains in the sample grains. Meanwhile, in the powder-diffraction patterns presented in ref 21., where helium was applied as a hydrostatic fluid, no splitting of reflections was detected, consistently with our results. The transition between phases IV and V is associated with a strong strain (Figure 1) and a considerable pressure hysteresis. Therefore, special care had to be taken to avoid misinterpretations caused by reflections splitting due to the coexistence of phases IV and V (see the point at 2.51 GPa in Figure 1). On releasing pressure, phase V could be detected down to 1.3 GPa. We also observed that the phase equilibrium depends on time lapsed after inducing the pressure changes. By using this information, we were able to repeatedly transform single crystals into pure phase V, as exemplified in Figures 6a and 6b, respectively at 2.71 and 3.83 GPa. Layer hk2 was chosen to demonstrate the cubic symmetry of the crystal, because any distortion from this symmetry would show up by the splitting of reflections in this layer. Figure 6 shows that no splitting can be seen neither at 2.71 nor at 3.83 GPa, in phase V. The precise unit-cell dimensions and the systematic absences all testify to the space group Im3̅. The occurrence of phase V changes the crystal compression mode, which in phases II and IV was hardly affected by the presence of cations in the voids of the (PbI3)n framework. The compressed volume of phases II and IV is “absorbed” by bending angles Pb−I−Pb and shortening bonds Pb−I in almost continuous manner up to 2.5 GPa (Figure 2), when the internal pressure of the cations exerted on their cages halts this mode of compression. At the transition point, the Pb−I−Pb angle increases stepwise by about 7° and then continuously straightens up in phase V, while the crystal compression fully relies on the shortening of the Pb−I bonds. This transition is a consequence of the volume of voids gradually confined around the disordered cations until they tightly fill the cages in phase IV. At about 2.5 GPa the interactions between the iodine atoms with the methyl and ammonium groups are so strong that they damp the rotations of cations and hinder the tilting of PbI6 octahedra. The disorder of randomly trapped cations is transmitted to the (PbI3)n framework around, resulting in the partial disorder of the orientation and distortions of the PbI6 octahedra. Thus, in phase V the framework (PbI3)n is disordered. This disorder is seen by the increased magnitudes of atomic displacement parameters (ADP) of atoms Pb and I, plotted as a function of pressure and depicted in structural drawings in Figure 5c and in Figure 7a,b. The ADP values allow the estimation of the tilt disordering as ±16.5° at 3.5 GPa. In this averaged model of phase V, the H···I distances of the randomly trapped cations are of about 2.75 Å, i.e., 0.4 Å shorter than the summed van der Waals radii of H and I atoms. This H···I distance is similar to the length of NH···I hydrogen bonds in the ordered phase III,28 but the methyl groups disordered at the same sites as the ammonium groups cannot be reconciled with such short nonbonding H···I contacts (cf. Figure S4). Another consequence of the tightly confined space around the cations trapped in random positions is that the Pb−I−Pb bending is stopped in a process described recently as inflating the voids in a metal−organic framework.37 However, the most important feature in phase V is that the disorder of clamped cations is expanded into the glassy state of all lattice. The

Figure 6. Reconstruction of layer hk2 of the MAPbI3 diffraction pattern in phase V at 2.71 (a) and 3.83 GPa (b). The reciprocal vector h is directed down, and k is horizontally to the right.

frustrated (PbI3)n framework in phase V implies considerable distortions of PbI6 octahedra. The ADP magnitudes of atoms Pb and I, the strongest scatterers of X-rays, considerably increase with pressure, which reduces the intensity of Bragg reflections and hampers the structure determination. The increasing ADP values in the average model result in different tilts and deformations of the PbI 6 octahedra. These observations result in the shortening of long-range structural correlations and mark a partial amorphization of the crystal. A similar behavior was observed for MAPbBr3 above 2 GPa.32,33 The pressure-induced amorphization was discovered relatively recently for ice and other common minerals.38−41 Due to the frequent occurrence and technological importance of amorphization, it continues to be an intensively studied phenomenon in high-pressure physics. To our knowledge, the transition between MAPbI3 phases IV and V is the first example of an isostructural phase transition triggering the onset of amorphization. This demonstrates that the amorphization does not 3463

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site. Thus, the I atoms are differentiated by the configurations of −CH3 and −NH3 groups around. The strong repulsions CH···I and attractions NH+···I induce the distortions of the geometry of PbI6 octahedra, and modify the covalent/ionic character of Pb−I bonds, which are additionally affected by the surrounding H atoms either of the methyl or ammonium groups, and hydrogen bonds. The strong CH···I and NH+···I interactions and related distortions in the inorganic framework are responsible for profound changes in the electronic structure of MAPbI3 and for the hypsochromic shift of the absorption edge, which at about 3.5 GPa appears as the color change of the crystal. This conclusion is supported by the band structure calculations of MAPbI3 and related systems,42 which demonstrated that modification of spin−orbit coupling through lattice distortions and hydrogen bonds is a key factor for the absorption edge variation. In summary, our systematic high-pressure study on the crystal structure and optical properties of MAPbI3 reveals its properties relevant to the envisaged photovoltaic applications. The pressure-induced strain within phase II improves the photovoltaic performance, but at the subsequent phase transitions and through the continuously compressed phases IV and V, the hypsochromic shift of the absorption edge is observed. We have correlated this shift with the distortions of the (PbI3)n framework and its frustrations, induced by strongly interacting cations trapped at random orientations in the contracting space of their squeezed voids. Such pressureinduced structural distortions strongly affect the electronic states responsible for the crystal absorption edge, resulting in huge variations of the energy gap, much larger than those observed at ambient pressure as a function of temperature in crystal phases I, II, and III.15−18 We have evidenced that the gradual amorphization of phase V is triggered by an isostructural phase transition, abruptly altering the trends in structural changes associated with the crystal compression. The atomic-scale mechanism leading to the amorphization, involving the frustrations in the (PbI3)n framework induced by its strong interactions with disordered CH3NH3+ cations, has been described. To our knowledge, this is the first evidence that an isostructural phase transition marks the onset of the amorphization process and that it is not necessarily continuous in its initial stages. The local-symmetry breaking while the average crystal symmetry is maintained, nucleates the amorphization at the phase transition. This mechanism opens new paths for obtaining materials prone or resistant to amorphization, often required for making products in very different areas of applications, including photovoltaic cells and pharmaceutical ingredients.43,44 Finally, special care was taken for verifying and determining the symmetries and structures of MAPbI3 crystal phases II, IV, and V. This information is essential not only for the properties discussed in this paper, but for all intense research on this important photovoltaic material.

Figure 7. Amplitudes of mean square atomic displacement parameters (ADP) in the PbI3 framework plotted as a function of pressure. (a) The principal ADPs for iodine atom. (b) The principal ADPs for lead atom. The insets illustrate the changes in the mean atomic displacements within the PbI6 octahedron at 0.55, 2.34, and 2.71 GPa.

necessarily evolve continuously from the crystalline state, but it can occur suddenly at the transition point and then proceeds gradually with increasing pressure. Moreover, this process is fully reversible, although the samples display a considerable pressure hysteresis, as described above. It is apparent that in the MAPbI3 structure, these are the highly polar [CH3NH3]+ cations, trapped in random orientations, that entail the amorphization of the (PbI3)n framework. While in phase IV all the I atoms are equivalent and each surrounded by four disordered cations (one at the site Th and three at the sites D2h), in phase V the I atoms of the PbI6 octahedra become differentiated due to a number of possible configurations of the four neighboring cations: three of them at the D2h sites (each of them frozen in one of the two possible orientations) and one of the cations frozen among numerous disordered states at the Th

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EXPERIMENTAL SECTION The synthesis of CH3NH3PbI3 was performed in aqueous solution by the method described previously.18 The concentrated solution containing HI and CH3NH3I was heated to about 90 °C and then Pb(CH3CO2)2·3H2O dissolved in water was slowly added at constant stirring until saturation. Black single crystals of CH3NH3PbI3 with well-developed faces grew of the solution slowly cooled down to about 40 °C. Below this temperature, pale yellow needle-shaped crystals of the monohydrate, CH3NH3PbI3·H2O, were formed9,45 which in 3464

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the solution at room temperature slowly recrystallized to bulky crystals of the dihydrate, (CH3NH3)4 PbI6·2H2O.46 Carefully selected CH3NH3PbI3 crystals were used for X-ray experiments at ambient and high pressures. After examination under microscope the quality of the crystals was verified by preliminary diffraction experiments, which allowed us to discriminate the single crystals from the twinned samples. This was essential for unequivocally determining the symmetry and structures of phases II, IV and V. We managed to find four good-quality single crystals for ambient and high-pressure measurements, achieving a very good reproducibility of the structural data in the all crystal phases. A series of high-pressure experiments have been performed with a modified Merrill-Basset anvil cell (DAC)47 equipped with diamond anvils supported on steel discs. The anvils culets were 0.8 mm in diameter. The gasket was made of 0.3 mm thick tungsten foil with a spark-eroded hole of 0.35 mm in diameter. The gasket was preindented before loading the cell and then a single crystal of CH3NH3PbI3 together with two ruby chips were glued to the culet surface of DAC. The chips were placed inside the pressure cell as far as possible from the highly absorbing sample crystal, in order to avoid its damage by the laser light used for the pressure calibration by fluorescence method.48 Propanol was used as the hydrostatic fluid. The DAC was mounted on an Oxford Diffraction Gemini A Ultra diffractometer operating with graphite-monochromated MoKα radiation. The high-pressure chamber was centered by the gasket-shadowing method.49 The CrysAlisPro software50 was used for the data collection and processing. All the structures were solved with direct methods using Shelxs97 and refined with Shelxl97.51 The refinements of the structures were performed by full-matrix least-squares method on all intensity (F2’s) data with anisotropic temperature factors for lead and iodine atoms. We have used the CrysAlisPro unwrap procedure for generating the reciprocal layers based on our X-ray diffraction measurements. Crystallographic information files CCDC 1476010−1476025 have been deposited with the Cambridge Crystallographic Data Centre and can be obtained free of charge from www.ccdc.cam.ac.uk/data_request/cif. To reduce the forbiddingly high diffusion of light from the powders, we measured the vis−NIR spectra for the samples of small single-crystal plates approximately 50 μm thick, arranged on one diamond culet into a mosaic fully covering the aperture of the DAC chamber. The ruby chip for pressure calibration was mounted on the opposite culet. A precise ad hoc insertion optic for condensing the probing beam on the sample was constructed for a Jasco V-650 spectrometer. The absorbance was measured in the one-beam mode at continuous scan speed of 100 nm min−1, in the 600−900 nm range. The energy gap was derived from the Tauc plots.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Prof. Andrzej Maciejewski of the Faculty of Chemistry for granting access to the Jasco spectrometer and for valuable discussions.

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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.6b01648. Figures illustrating structural details, and discussion of the disorder of methylammonium cations (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail address: [email protected] (M.S.). *E-mail address: [email protected] (A.K.). 3465

DOI: 10.1021/acs.jpclett.6b01648 J. Phys. Chem. Lett. 2016, 7, 3458−3466

Letter

The Journal of Physical Chemistry Letters

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DOI: 10.1021/acs.jpclett.6b01648 J. Phys. Chem. Lett. 2016, 7, 3458−3466