Pressure-induced transformation of CH3NH3PbI3: the role of the noble-gas pressure transmitting media

A structural study of methylammonium lead triiodide [CH3NH3PbI3 (MAPbI3)], at high pressures up to 20 GPa using noble gases Ne and Ar as pressure-transmitting media is reported. It is found that both noble gases are chemically active at high pressures. In particular, Ne stabilizes the high-pressure structure of NexMAPbI3 and prevents amorphization up to 20 GPa. In contrast, Ar acts as a stabilizer only up to 2.4 GPa and accelerates irreversible amorphization upon further compression.


Introduction
MAPbI 3 is currently considered as one of the most promising compounds in photovoltaic technologies for making cheap and highly efficient solar cells (Green et al., 2014;Weber et al., 2015). One of the hurdles which reduces the enthusiasm for practical applications of MAPbI 3 is its content of lead which confers toxicity on this material (Benmessaoud et al., 2016). Therefore, it would be highly desirable to replace lead with a non-toxic element, while still preserving the high light conversion efficiency. The conditio sine qua non for achieving this goal is a better understanding of the microscopic origin of this high energy conversion efficiency. There are opinions that the rotation or other type of motion of methylammonium cations (MA) introduces a slightly indirect band gap in this material, which extends the lifetime of photoelectrons (Motta et al., 2015), helping them to flow out of the material as a current. Valuable information about the role of the cation can be obtained by studying its behaviour in the lattice under high pressure (Ou et al., 2016). The perovskite crystal structure of MAPbI 3 is very flexible due to a large interstitial space (of $8 Å in diameter), where the small ($1.5 Å ) linear cation is located. The PbI 6 octahedra and the cations interact via weak hydrogen bonds. The strength and the conformation of the hydrogen bonds can be influenced either by the presence of captured exogenous substances, such as water (Arakcheeva et al., 2016), or by external factors such as pressure or temperature, leading to changes in the symmetry of the unit cell. A recently published extended review (Lü et al., 2017) on the pressure-induced evolution of the structure and the physical properties of organic-inorganic halide perovskites demonstrates a poor reproducibility of the changes. This calls for more investigations to uncover the underlying mechanisms. Fig. 1 shows a schematic representation of the two major pressure-induced structural transformations in MAPbI 3 , a phase transition and amorphization, reported in the literature together with the ones obtained in the present study. At very low pressures (0.1-0.3 GPa), a first-order phase transition from the tetragonal body-centred to the body-centred (pseudo) cubic unit cell takes place (Fig. 1, blue to red lines). The amorphization of the compound occurs after the structural phase transition (Fig. 1, black line). However, in different experiments the onset of the amorphization has been observed under different pressures, and the pressure transmitting medium (PTM) could be considered as a factor affecting the pressure-induced transformation of MAPbI 3 (Fig. 1). Such an effect has already been observed in organic compounds (Zakharov et al., 2016) and also in some minerals (Ardit et al., 2014;Sato et al., 2013;Guń ka et al., 2015;Lobban et al., 2002). This provided the motivation for the present work on studying the influence of PTM on the structure of MAPbI 3 . We have chosen two noble gaseous media, Ne and Ar, which are among the most hydrostatic PTMs and have distinct atomic radii: Ne (0.38 Å ) and Ar (0.71 Å ). Single-crystal synchrotron X-ray diffraction (XRD) measurements provided a precise data set, which allowed structure determination as a function of pressure with the best precision. Surprisingly, we found that under high pressure both Ar and Ne were incorporated into the structure. Moreover, the pressure-induced phase of Ne 0.97 MAPbI 3 remained stable and highly crystalline after decompression.

Synthesis and crystallization
The preparation of MAPbI 3 single crystals has been reported elsewhere (Arakcheeva et al., 2016). Several crystals were initially tested for their crystal structure and crystal quality at ambient conditions. All of them showed the space group I422 crystal symmetry with the unit-cell parameters and the atomic coordinates identical to those reported by Arakcheeva et al. (2016). Preliminary diffraction measurements under high pressure showed as expected that the starting quality of the crystal (degree of mosaic and strains) has a significant effect on the high-pressure transformation (Fig. S1). Therefore, only the highest quality crystals have been selected for the final diffraction experiments.

Single-crystal synchrotron XRD experiments
Room-temperature (293 K) high-pressure (up to 20.27 GPa) XRD data were collected at the ID27 High Pressure Beamline of the European Synchrotron Radiation Facility (ESRF) in Grenoble. Diamond anvil cells were used with rhenium gaskets and Ne or Ar gases as the PTM to generate hydrostatic conditions. The X-ray wavelength was set to 0.3738 Å . The minimal linear size of the crystals was about 10 mm. Diffraction data were recorded with a Mar165 CCD detector and the pressure was measured using the ruby fluorescence technique (Syassen, 2008). After decompression, additional measurements were conducted on the Necontaining crystal to determine its structure under ambient conditions. CrysAlis PRO (Rigaku Oxford Diffraction, 2014) and JANA2006 (Petříček et al., 2014) software packages were used for data processing and structural refinement, respectively. Experimental details are listed in Tables S1 and S2 and are illustrated in Fig. 2 and also in Figs. S1, S2 and S3.
2.2.1. Structure determination. The general scheme of the structure determination is illustrated in Fig. 3     pressure was determined by testing the inorganic framework for all possible space groups in all possible crystallographic systems (cubic, tetragonal, trigonal and orthorhombic). Next, after the refinement of the atomic positions of Pb and I [ Fig. 3(a)], difference electron density maps were calculated for each case [ Fig. 3(b)]. The pristine crystal can be characterized by the maxima in the expected MA positions [dark-blue circles in Fig. 3(b)]. These maxima are also perfectly identified at 0.69 GPa and for the released crystal, whereas they are absent at 20.27 GPa. However, even stronger and more localized maxima [cyan circles in Fig. 3(b)] are clearly visible for each pressure, with the exception of the starting (pristine) case. These additional maxima are attributed to Ne atoms, giving a compatible electron density contribution [ Fig. 3(c)]. At 20.27 GPa, the usual MA positions show nearly zero electron density and only the Ne-attributed maxima are observable. We have to emphasize that these maxima could not be fitted with displaced MA cations. The schematic diagrams of the crystal structure for each pressure after including MA and/or Ne in the refined models are shown in Fig. 3(c). The residual electron density maps calculated after the refinements have values close to zero [Fig. 3(d)], this supports the models.
Refinement of all atomic parameters for MA and Ne helped to confirm or correct the space group selected previously for the framework at each pressure. A similar procedure for the structure determination was used for all applied pressures for both Ne and Ar as the PTM.
As expected, the inclusion of Ne and Ar in the structure has a varying effect on the symmetry and the bulk modulus at different pressures. The resulting structures made it possible to determine the interatomic distances, and consequently the nature and hierarchy of interactions between various components of each structure.
The basic crystallographic information for eight different pressures with Ne as the PTM and six different pressures with Ar as the PTM is provided in Tables 1 and 2, respectively, and is visualized in Fig. 4. Further details are listed in Tables S1 and S2. It should be noted that, despite the reflections-toparameters ratio which is 5.6 and 4.1 for the Ne-containing crystal at 16.43 and 20.27 GPa, respectively, the criteria R, wR, S = 0.051, 0.057, 1.44 for 16.43 GPa and 0.043, 0.064, 1.49 for 20.27 GPa, and the residual electron density max , min = 0.58, À0.60 e Å À3 for 16.43 GPa and 1.06, À1.25 e Å À3 for 20.27 GPa (column VI and VII in Table S1) show that the corresponding structure solutions are correct. These structures The XRD patterns illustrating the pressure-induced transformation of MAPbI 3 with (a) Ne and (b) Ar as PTM. The powder XRD patterns calculated from the single-crystal experiments are presented at the logarithmic scale. Sections of the reciprocal space are shown for l = 0. The axes h, k (H, K) specific only for the tetragonal (pseudo-cubic) modification are emphasized in blue (red). obtained at the highest pressures are essential for understanding the Ne intercalation, which is also confirmed by the presence of Ne in the released crystal under ambient conditions.

Results and discussion
3.1. The high-pressure NeMAPbI 3 compounds The crystal structure at room temperature (293 K) was determined for 0. 69, 1.5, 2.69, 4.56, 7.4, 16.43 and 20.27 GPa pressures and for ambient pressure after decompression (Table 1). Above 4 GPa an amorphous phase appears, coexisting with the crystalline one. The amorphous contribution slowly grows upon further increase of pressure and completely disappears after pressure is released (Fig. S1).
The XRD data obtained at 0.11 GPa indicate the coexistence of two phases: a tetragonal phase, characteristic of ambient pressure, and a pseudo-cubic phase, which is typical for higher pressures (Fig. S3). This is consistent with the firstorder phase transition, which was reported before between 0.3 GPa and 0.4 GPa (Capitani et al., 2016;Szafrań ski & Katrusiak, 2016;Francisco-Ló pez et al., 2018). However, for the pseudo-cubic phase, we found the space group R 3 3, which differs from the cubic Im 3 3 and orthorhombic Imm2 reported previously by Szafrań ski & Katrusiak (2016)  Determination of the MA and Ne positions in the crystal structure of MAPbI 3 at selected pressures. Projections of the relevant fragments of the structure onto the ac plane are in (a) and (c). Small dots indicate I atoms in the plane for which the difference electron density maps are shown in (b) and (d) with positive (red) and negative (blue) areas. (b) The maps were calculated using only Pb and I atoms. The maxima of the difference electron densities are identified with MA cations (dark-blue circles) and Ne atoms (cyan circles). The density varies: from À0.32 to 1.58 e Å À3 for the released crystal at P = 16.43 GPa. The unit cells and space groups corresponding to each phase are shown in Fig. 4(a)(i). We explain the difference in the symmetry, the presence of a third phase transition and the lack of amorphization up to 20 GPa by the intercalation of Ne, which we found for the high-pressure phases. The four neighbouring I atoms surround each Ne atom. The average occupancy of Ne sites starts at approximately one-third in the low-pressure phases and goes up with pressure, eventually approaching one when Ne atoms sit at each face. At 20.27 GPa, Ne sites are fully occupied for the faces of only four out of six possible orientations, and the remaining two are half occupied. One can therefore expect that even higher pressure is required in order to achieve the full occupancy. The corresponding composition, Ne 3 PbI 3 (MA), reflects the maximum possible Ne content in the structure.
The observation (ii) needs some extended explanation. A reversible transformation, which restores all the long-rangeordered positions of MA in the released crystal at ambient pressure (Fig. 3), indicates that this cation can be in two different states in the high-pressure crystal: the long-rangeordered state (i.e. periodic) and the randomly distributed one (i.e. non-periodic). In order to emphasize these two states of MA, we use the Ne x MA y PbI 3 (MA) 1-y designation for the chemical formula, where indices y and (1 À y) denote the relative quantities of the long-range-ordered and the randomly distributed MA, respectively. The presence of two  Table 1 Crystallographic data and composition for Ne-MAPbI 3 in the 0-20 GPa pressure range.    (2016). According to this assumption, a lack of long-range ordering of MA leads to destruction of the inorganic (Pb,I)-framework, which is kept by the I-MA periodic interactions. Indeed, a decrease in the amount for long-range-ordered MA leads to a huge increase in the atomic displacement parameters for I atoms (Fig. 6), which is a sign of the destruction of the inorganic framework.   Destruction of the inorganic framework leads to compound amorphization. However, we observed that with the Ne PTM, the periodic crystal structure still exists up to 20 GPa. The reasons of this can be understood from the analysis of the Ne-I, Ne-AM and I-MA interatomic distances found for the longrange-ordered MA cation [ Fig. 5(c)]. The short distances MA-Ne of $1.9 Å indicating the corresponding interaction at P < 3 GPa can be recognized as unstable with respect to pressure, since the content of the long-range-ordered MA rapidly reduces from 0.5 (4.56 GPa) to 0.25 (7.4 GPa) per chemical formula, and the long-range-ordered MA is no longer observed for P ! 16.3 GPa. If the expected rapid amorphization does not appear, it means that stabilization of the (Pb,I)-framework is kept by another stabilizer, which is different from MA; it is Ne atoms in the considered case. Indeed, for P > 4 GPa, the loss of the long-range order for more than 50% of MA switches the shortest interatomic distance (i.e. the strongest interaction of Ne) from Ne-MA to ]. Consequently, the Ne-I interactions can be considered as the cause of stabilization of the framework, preventing the amorphization.
The stabilizing role of Ne under high pressure is also confirmed by the distortions observed in the (Pb,I)-framework: the minimal distortions (the I-Pb-I angle ' 88.4-90.3 instead of the ideal 90 and the Pb-I-Pb angle ' 178-180 instead of the ideal 180 ) were found in tetragonal structures at the highest pressure, when the long-range order MA is lost and the content of Ne is maximal (Table S3).
Thus, the progressive intercalation of Ne is the general trend in the pressure evolution of the NeMAPbI 3 . The phenomenon is driven by the Ne-I interaction, which is substantially enhanced above 4 GPa.
Surprisingly, the occupation of Ne sites in all directions is preserved in the released structure under ambient pressure, but with a much smaller probability of approximately onethird [ Fig. 5(d)]. The mean Ne-MA and Ne-I distances in the released structure, 2.83 Å and 2.78 Å , respectively, are very similar [ Fig. 5(c)]. The minimum Ne-MA distance of approximately 2.3 Å in the released crystal can be responsible for this stabilization. However, this is the distance between the partially occupied Ne and MA sites, so it is problematic to say with certainty whether such separation is indeed realized.  In the 0-20 GPa pressure range the bulk modulus B 0 , calculated using the BirchÀMurnaghan model (Birch, 1947), varies from 5.3 GPa at the lowest to 10.6 GPa at the highest pressure. These numbers fall within the spread of values in the literature (Capitani et al., 2016;Jaffe et al., 2016) even for the crystalline phase at 20 GPa, which was absent for other PTMs.

The high-pressure ArMAPbI 3 compounds
The structure was determined at pressures of 0.18, 0.49, 0.98, 1.34, 2.1, and 2.39 GPa at room temperature (Table 2). Above 1.34 GPa an amorphous phase appears, coexisting with the crystalline one. The amorphous contribution grows rapidly with increasing pressure until no crystalline phase can be observed above 3.6 GPa (Fig. S2). This amorphization is irreversible in our experiments.
The tetragonal unit-cell parameters, characteristic for the ambient conditions, have been identified in the ArMAPbI 3 structure up to 1 GPa [ Table 2 and Fig. 4(b)(i)]. The space group P4 2 bc, which is new for the title compound, turned out to be the best fit in the 0.18-0.98 GPa pressure range. Only one transformation of this unit cell was detected for ArMAPbI 3 , happening between 1 GPa and 1.3 GPa. The pseudo-cubic orthorhombic unit cell [ Fig. 4(b)(i)] and the Immm space group are characteristic for the lattice for P > 1 GPa (Table 2). No distortion in the long-range order was detected for the MA cation. In comparison to NeMAPbI 3 , growth in the Ar content as a function of pressure is 5.5 times larger, reaching the composition Ar 1.4 MAPbI 3 at 1.34 GPa. In the case of Ne PTM, the nearly equivalent composition, Ne 1.41 MA 0.25 PbI 3 (MA) 0.75 , occurs at 7.4 GPa (Tables 1 and 2). This indicates a much stronger interaction of MAPbI 3 with Ar than with Ne.
The pressure-induced structural evolution of ArMAPbI 3 (Fig. 7) is associated with a gradual growth in the Ar content [ Fig. 7(b)], accompanied by the contraction of the Ar-I and Ar-MA distances [Fig. 7(c)]. For P < 1 GPa, the MA cations, Ar and I atoms closest to each other are arranged into chains, which polymerize in two dimensions upon further compression [Fig. 7(d)]. Influence of the lattice contraction on the chain geometry is illustrated in Fig. 8.
The shortest interatomic distances of $2 Å found for Ar-I and Ar-N (MA) at 2.1 and 2.39 GPa can be considered as a reasonable approximation if two arguments are taken into account. First, the value of 1.84 Å was theoretically predicted for the Ar-N separation for ambient pressure (Novak & Fortenberry, 2016). Second, large and prolate ADP ellipsoids of the I atoms imply an anharmonic character of the atomic displacements, which indicates that the actual Ar-I distances are longer.
Nevertheless, we consider the dramatic reduction of the minimum Ar-MA and Ar-I distances at P ! 2 GPa [circled by red line in Fig. 7(c)] as a trigger for the complete and irreversible structure amorphization. In fact, we recorded a crystalline state at the time of its destruction.
Thus, the pressure-induced formation of the stable I-Ar-MA-Ar-chains and their polymerization [Fig. 7(d)] is at the origin of the structural evolution and amorphization of ArMAPbI 3 .
Detailed characteristics of the progressive changes in the ArMAPbI 3 structure under pressure are also given in the supporting information (Figs. S4b, S6 and S7).

Different impact of Ne and Ar as a pressure transmitting medium
The present results show that both Ne and Ar interact with vanishing of the long-range ordering of MA. Ne serves as a structural stabilizer instead of MA for pressures higher than 4 GPa. Unlike Ne, Ar interacts with both I and MA even under low pressure of 0.18 GPa. These interactions accelerate the irreversible amorphization, which starts at P > 2 GPa. The difference between the influences of Ne and Ar is clearly related to the difference in their chemical activities, which are defined by their electronic configurations, i.e. their atomic radii being 0.38 Å for Ne and 0.71 Å for Ar.
The photo-luminescence (PL) microscopy images and steady-state PL spectra acquired under ambient pressure revealed that the PL properties were strongly dependent on the type of PTM and on the maximum pressure attained. Using the dual wavelength excitation technique (Mor et al., 2016) these observations, described in detail in Section S6, point to a markedly higher degree of amorphization for MAPbI 3 single crystals exposed to high pressure in Ar than in Ne. Thus, the optical characterization of MAPbI 3 supports the conclusions of the X-ray study that the large difference in the atomic radii of Ar and Ne leads to significantly different effects of high pressure on the crystallinity of MAPbI 3 after pressure release.

Influence of Ne and Ar on the methylammonium cation mobility
The role of the mobility of the MA cation is often discussed (Motta et al., 2015;Ou et al., 2016;Capitani et al., 2016;Szafrań ski & Katrusiak, 2016). The high-pressure amorphization is also directly linked to this phenomenon. According to Ou et al. (2016), Capitani et al. (2016) and Szafrań ski & Katrusiak (2016), the strong pressure-induced interaction between the MA cations and I atoms leads to the absence of the MA periodicity, which was considered as a template for the stability of the (Pb,I)-framework. Following this interpretation, our results support the following scenarios of the influence of Ne and Ar on the MA cation behaviour. In both cases, Ne-MA and Ar-MA interactions minimize the MA mobility by fixing the position of the MA dumbbell (the logrange-ordered form of MA). In particular, this explains the absence of cubic symmetry for the high-pressure phases.
In the case of Ne-PTM, up to $3 GPa, MA interacts with both Ne and I; that stabilizes its long-range ordering. For P > 3 GPa, intensifying interaction between MA and 12 neighbouring I atoms forces this cation to lose its long-range periodicity, first partially (for 3 GPa < P < 8 GPa) then completely (for P > 8 GPa). However, the Ne-I interaction becomes sufficiently strong to stabilize the (Pb,I)-framework. This prevents amorphization and maintains the crystalline state up to 20.27 GPa. These pressure-induced Ne-I interactions remain after decompression.
In the case of Ar PTM, MA cations interact with Ar to form -I-Ar-MA-Ar-chains as Ar enters the crystal. Formation of the chains prevents strong interaction between MA and I and, consequently, stabilizes the long-range periodicity of MA. At 1.34 GPa, some of the cations still exist outside of the chains. The uncovered cations stimulate additional absorption of Ar by the crystal, and all MA cations are involved in the polymerized chains at 2.39 GPa, but some of the Ar-MA and Ar-I distances are too short. On the one hand, it points to strong MA-Ar interactions, which benefit from the competition with the MA-I ones. On the other hand, the I-Ar-MA-Ar-chains get crowded in the restricted space between the PbI 6 octahedra and they crush the structure. This means collapse of the structure. Despite somewhat speculative character, this scenario can explain both the small observed contribution of the crystalline phase at 2.39 GPa and the rapid irreversible amorphization upon further pressure increase.

Conclusions
Using single-crystal synchrotron diffraction data, the crystal structure of CH 3 NH 3 PbI 3 (MAPbI 3 ) was studied under different pressures with noble gases, Ne and Ar, as pressure transmitting media creating hydrostatic conditions. The following main conclusions have been drawn from the study.
(i) In the case of MAPbI 3 compression up to 20.3 GPa, the noble gas atoms of the pressure transmitting media are not inert, but rather they form NeMAPbI 3 and ArMAPbI 3 highpressure-induced compounds.
(ii) Ne mainly interacts with I atoms, preventing amorphization and stabilizing the high-pressure crystalline state up to 20.27 GPa, despite the migration of MA cations to non-periodic positions. This means a loss of the long-range ordering of MA within the crystal lattice. The high-pressure transformation is reversible and the Ne 0.97 MAPbI 3 compound is stable at ambient conditions after decompression.
(iii) Ar interacts with both MA and I, thus forming chains and driving their pressure-induced polymerization up to P = 2.39 GPa. Compression of the (Pb, I)-framework destroys the polymerized structure and, consequently, the framework itself, initiating the irreversible and rapid amorphization of the compound.
(iv) The difference between the pressure-induced impacts of Ne and Ar is related to the difference in their atomic radii and, consequently, their propensity towards interatomic interactions in the restricted space between the PbI 6 octahedra.
We believe that the findings presented can encourage the research community to conduct deeper experimental and theoretical studies of plausible chemical reactions of noble gases in the interstitial compartments of other compounds under high pressure.