Transition from the Tetragonal to Cubic Phase of Organohalide

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Letter

Transition from Tetragonal to Cubic Phase of Organohalide Perovskite: The Role of Chlorine in Crystal Formation of CH3NH3PbI3 on TiO2 Substrates Qiong Wang, Miaoqiang Lyu, Meng Zhang, Jung-Ho Yun, Hongjun Chen, and Lianzhou Wang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b01682 • Publication Date (Web): 29 Sep 2015 Downloaded from http://pubs.acs.org on October 1, 2015

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The Journal of Physical Chemistry Letters

Transition from Tetragonal to Cubic Phase of Organohalide Perovskite: The Role of Chlorine in Crystal Formation of CH3NH3PbI3 on TiO2 Substrates Qiong Wang, Miaoqiang Lyu, Meng Zhang, Jung-Ho Yun, Hongjun Chen, and Lianzhou Wang*

Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology, the University of Queensland, Brisbane, QLD, 4072, Australia. Email: [email protected]

ACS Paragon Plus Environment


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ABSTRACT: The role of chlorine in the superior electronic property and photovoltaic performance of CH3NH3PbI(3-x)Clx perovskite has attracted recent research attention. Here we study the impact of chlorine in the perspective of crystal structure of perovskite layer, which can provide important understanding on its excellent charge mobility and extended lifetimes. In particular, we find that in the presence of chlorine (PbCl2 or CH3NH3Cl), when CH3NH3PbI3 films are deposited on TiO2 mesoporous layer instead of planar TiO2 substrate, stable cubic phase rather than commonly observed tetragonal phase is formed in CH3NH3PbI3 perovskite at room temperature. The relative peak intensity of two major facets of cubic CH3NH3PbI3 crystals, (100)C and (200)C facets, can also be easily tuned, depending on the film thickness. Furthermore, compared with pristine CH3NH3PbI3 perovskite films, in the presence of chlorine, CH3NH3PbI3 crystals grown on planar substrates exhibit strong preferred orientations on (110)T and (220)T facets.

TOC GRAPHICS

KEYWORDS: phase transition, tetragonal/cubic phase, CH3NH3PbI3 perovskite


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Organohalide lead perovskite solar cells (PSCs) have seen significant progresses in boosting the power conversion efficiency in the past five years,1-2 which are primarily attributed to the outstanding photophysical properties of organic-inorganic lead perovskite, including high absorption coefficient,3 extraordinary long charge diffusion lengths/lifetimes,4-5 high carrier mobilities,6 low exciton binding energy7-8 and strong photoluminescence efficiency.9-10 In addition, the solution processability makes PSCs one of the most promising new generation solar cells that could compete with silicon based solar cells, with power conversion efficiency (PCE) surging from 9% in 201211 to around 20% in 2015.12 In order to further improve the performance of PSCs, extensive works have been made to investigate a number of challenging issues including the working mechanism and stability of PSCs,13-14 the cause of hysteresis in PSCs,15-16 the influence of preparation techniques on film morphology and crystal formation of the perovskite layer.17-18 In particular, the influence of chlorine in the precursor solution of perovskites has drawn recent attention due to its significant impact on the morphology and electronic property of the perovskite layer and thus on the photovoltaic performance of PSCs.19-21 For polycrystalline perovskite, CH3NH3PbI3, prepared from spin-coating or dip-coating method, the electron diffusion length is on order of or slightly shorter than the absorption depth (～100 nm),5 which indicates an efficient mesoporous electron-transport layer is generally required.22 However, for polycrystalline perovskite, CH3NH3PbI3-xClx deposited from chlorine-contained solutions using one-step spin-coating method, surprisingly longer rang electron and hole diffusion lengths of around one micrometer can be obtained,4 which implies an electron-selective layer is not necessitated for CH3NH3PbI3-xClx perovskite.23 Although single crystal CH3NH3PbI3 perovskite of large grain sizes prepared by advanced techniques can also exhibit very long charge diffusion lengths ranging from a few micrometers to one hundred micrometers,24-25 it is still of high importance to study the role of chlorine on the crystal formation of perovskite film prepared from simple one-step spin-coating method.26-27 In particular, it is recently reported that perovskite films prepared from precursor solutions with and without chlorine share the same crystal structure and are



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actually the same material as chlorine would be evaporated as CH3NH3Cl during the annealing process.28 Hence, it is suggested that the expression of CH3NH3PbI3-xClx is indeed not accurate because techniques such as X-ray photoelectron spectroscopy (XPS) or energy dispersive spectroscopy (EDS) have been used to demonstrate zero or trace amount of chlorine on the surface of the final perovskite layer.29-30 However, Starr et al.,31 demonstrated an inhomogeneous chlorine distribution in CH3NH3PbI3-xClx layers as they observed a higher average concentration of chlorine throughout the perovskite layer than on the surface. In their work, they also discussed the possible electronic consequences of the physical location of chlorine in the perovskite layer, which contributed to superior charge mobility and lifetimes compared with chlorine-free, CH3NH3PbI3 perovskite layer. Colella et al.,27 conducted a combined angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and first-principles DFT modeling in investigation of CH3NH3PbI3-xClx/TiO2 interface and revealed that chloride was preferentially located in close proximity to the perovskite/TiO2 interface due to an increased chloride-TiO2 surface affinity. In light of the arguments on the role of chlorine in determining some important properties including charge mobility and lifetimes of halide perovskite films,32-33 it is of importance to further investigate the influence of chlorine on the crystal structure of perovskite materials, which is expected to provide better understanding on the structure-property relation of the material system. In this study, we focus on TiO2 substrate, because TiO2 has been widely used as an electron-selective layer in conventional PSCs,26 and good understanding including theoretical calculations on the interaction between halide and TiO2 have been reported.34-35 For instance, Mosconi et al.,36 used first-principles electronic structure calculations, suggesting that the presence of chlorine improved the preferred orientation for perovskite film grown on TiO2 due to enhanced binding energy between the substrate and perovskite, thereby leading to enhanced stability. Here, we adopt two chloride precursors, PbCl2 and CH3NH3Cl, and three substrates, planar TiO2 compact layer and TiO2 mesoporous layers with two different film thicknesses, to study the role of chlorine in crystal structure of CH3NH3PbI3 perovskite. By


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comparing the crystal structure of CH3NH3PbI3 perovskite deposited on planar substrate and TiO2 mesoporous layer, we find that the resulted perovskite experiences transition from tetragonal to cubic phase as the film thickness of TiO2 mesoporous layer increases. Under the same mesoporous layer, the relative peak intensity of two major facets of cubic CH3NH3PbI3 crystals, (100)C and (200)C facets, can be tuned. Compared with pristine CH3NH3PbI3 perovskite film, chlorine involved CH3NH3PbI3 perovskite film exhibits significantly enhanced peak intensity of (110)T and (220)T facets for TiO2 planar substrates. The finding here provides new fundamental understanding on the crystal structural control of the organicinorganic hybrid perovskite that could be associated with desirable optoelectronic properties. Pristine CH3NH3PbI3 perovskite films were prepared by spin-coating a solution containing equal molar ratio of CH3NH3I and PbI2 at a concentration of 40 wt% in N,N-dimethylformamide (DMF) solvent onto certain substrates. Samples prepared under this condition are referred as S1. Then sample S2 were prepared by dissolving additional amount of CH3NH3Cl in S1 under same molar ratio of CH3NH3I and PbI2. Sample S3 were prepared by dissolving CH3NH3I and PbCl2 at a molar ratio of 3:1 with a total mass weight of 40 wt% in DMF solvent. Detailed experimental conditions are given in supporting information. Transition between tetragonal and cubic crystal phases has been observed in bromide substituted lead triiodide perovskite.37 It is reported that as the concentration of bromide in CH3NH3PbI3-xBrx increases, (004) and (220) facets of tetragonal I4/mcm phase located at 2θ range of 28°to 29°are gradually ഥm phase, indicating increased merged into one single peak corresponding to (200) facet in cubic Pm૜ symmetry. As a result, X-ray diffraction (XRD) patterns of CH3NH3PbI3 perovskite films prepared from S1, S2, and S3 precursors, deposited on planar TiO2 compact layer were measured and the results are given in Figure 1a. It is clear that all samples have the (004) facet that sits closely to the (220) facet, indicating tetragonal phase of perovskite crystal.38 Meanwhile, it is also noted that the relative peak intensity between (004)T facet and (220)T facet is quite different in three samples. Compared with pristine CH3NH3PbI3 perovskite prepared from S1 precursor, CH3NH3PbI3 perovskite prepared from CH3NH3Cl



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contained precursor (S2) presents (004)T facet as the preferential facet. However, the one prepared from PbCl2 contained precursor (S3) exhibits the preferred facet of (220)T facet. Interestingly, when the perovskite film deposited on TiO2 mesoporous layer with a thickness of around 500 nm, the (004)T facet is disappeared and merged into (200)C facet for CH3NH3PbI3 perovskite deposited from S2 and S3 precursors, Figure 1b. This implies that CH3NH3PbI3 crystals have transited from tetragonal phase to cubic phase.39-40 Calculation of XRD pattern of S3 in Figure 1b gives the lattice parameter of around 6.265 Å, which matches well with the computer modulation data of cubic CH3NH3PbI3.41 The influence of TiO2 mesoporous layer thickness on crystal structure of CH3NH3PbI3 perovskite is presented in Figure 1c. It shows that as the film thickness of TiO2 mesoporous layer increases from 0 nm (planar substrate) to around 300 nm, the peak intensity of (004)T facet is greatly decreased while the (220)T facet is kept intact. Further increase in the film thickness of photoanodes leads to the disappearance of (004)T facet and prominent enhancement in (200)C facet. Eventually, as the film thickness of TiO2 mesoporous layer increases, the perovskite layer deposited from CH3NH3Cl contained precursor (S2) gradually transforms from tetragonal phase to cubic phase. a)

S1-TiO2 PS S2-TiO2 PS

(004)T

b)

S1-TiO2 MS S2-TiO2 MS

(004)T

S3-TiO2 PS (220)T

(200)C (200)C

S3-TiO2 MS S2-TiO2 PS

(220)T

c)

(004)T

S2-TiO2 PS S2-TiO2 MS-1 S2-TiO2 MS-2 (200)C (220)T

28.0 28.1 28.2 28.3 28.4 28.5 28.6 28.7 28.8 28.9 29.0 28.0 28.1 28.2 28.3 28.4 28.5 28.6 28.7 28.8 28.9 29.0 28.0 28.1 28.2 28.3 28.4 28.5 28.6 28.7 28.8 28.9 29.0 2 theta (degree)

2 theta (degree)

2 theta (degree)

Figure 1. X-ray diffraction (XRD) patterns of CH3NH3PbI3 films deposited from precursors S1, S2 and S3 on planar TiO2 substrates (TiO2-PS) a) and on TiO2 mesoporous substrates (TiO2-MS, ～500 nm) b), and c) CH3NH3PbI3 films deposited from S2 precursor on planar and mesoporous TiO2 substrates with different film thicknesses: TiO2-MS-1 for 300 nm; TiO2-MS-2 for 500 nm. (Thicker TiO2 mesoporous substrates were prepared by depositing twice TiO2 solution via spin-coating method, see supporting


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information. XRD measurement of anatase TiO2 compact layer and S1 and S3 on TiO2 substrates of different film thicknesses are given in S-Figure 3 and S-Figure 4, respectively.)

Scheme 1. Illustration of CH3NH3PbI3 perovskite deposited from S2 precursor on different substrates: a) TiO2 planar, b) mesoporous layer of around 300 nm thickness, and c) 500 nm thickness.

a1

b1

10 µm

10 µm

a2

c1

b2

1 µm

10 µm

c2

1 µm

1 µm

Figure 2. Scanning electron microscopy (SEM) images of surface morphology of CH3NH3PbI3 perovskite films deposited from precursor S1 (a1, a2), S2 (b1, b2), and S3 (c1, c2) on planar substrates. It is known that when planar substrate or thin TiO2 mesoporous layer is used as a photoanode, CH3NH3PbI3 perovskite tends to form a capping layer, and when the photoanode is composed of a thick TiO2 mesoporous layer, it forms a CH3NH3PbI3 perovskite sensitized structure, as illustrated in Scheme



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1.42 (see supporting information for cross-sectional images, S-Figure 1) On planar substrates, it is noted that two chlorine resources, CH3NH3Cl and PbCl2 can affect the (004)T and (220)T facets of CH3NH3PbI3 perovskite differently, as judged from XRD data in Figure 1. SEM images of CH3NH3PbI3 perovskite films prepared from S1, S2 and S3 precursors on planar substrates are given in Figure 2. It can be seen that jigsaw-like crystals with grain edges are formed for samples prepared from S2. However, bulk crystals with clearly ordered plates on the surface are characterised for samples prepared from S3. Compared with pristine CH3NH3PbI3 perovskite, it implies that in the presence of CH3NH3Cl and PbCl2, the nucleation and growth of CH3NH3PbI3 crystals are modified. Recent reports suggest that these two chlorine resources have different impacts on the nucleation process and crystal growth process, where the former slows down the perovskite formation process and thus promotes the growth of crystal domains during annealing due to an initial introduction of a CH3NH3+ rich environment,28 while the later changes the nucleation dynamic and alters the growth kinetic and crystallite sizes upon chloride inclusion.21 Although the existence of chlorine in the final perovskite film is still under debate (our XPS measurement of perovskite prepared from chlorine-contained precursors given in the supporting information shows no existence of chlorine in the final product, S-Figure 2),31, 43 our study shows that CH3NH3PbI3 perovskite prepared from chlorine-contained precursors experiences transition from tetragonal to cubic phase when it is deposited on thick TiO2 mesoporous layer. It has been reported that cubic phase of CH3NH3PbI3 perovskite transits to tetragonal phase at around 54 °C, and CH3NH3PbI3 perovskite should present a tetragonal structure at room temperature.44-45 Therefore, it is very interesting to observe the formation of cubic CH3NH3PbI3 perovskite as the final product in our study. More explanation on this finding will be given in the following section.


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Figure 3. XRD patterns of CH3NH3PbI3 films deposited from precursors S1, S2 and S3 on ～300 nm mesoporous TiO2 substrates. (Note: the y scales of peak intensity of three samples are different.) Figure 3 presents the XRD patterns of CH3NH3PbI3 films deposited from precursors S1, S2 and S3 on ～300 nm mesoporous TiO2 substrates. (Absorbance spectra of these three samples are given in the supporting information, S-Figure 5) It shows that the sample deposited from S1 exhibits weak peaks as indexed according to references.41 In this sample, the peak intensity of (110)T and (220)T facets are as weak as that of other facets of CH3NH3PbI3 crystals. However, these two facets are greatly enhanced in samples prepared from S2 source. The clear split of two facets (004) and (220) at 2θ ranging from 28° to 29 ° signifies the tetragonal phase of CH3NH3PbI3 perovskite. The sample spin-coated from S3 precursor exhibits one strong peak at 2θ of around 28.5°, corresponding to the (200) facet of cubic CH3NH3PbI3 perovskite. Therefore, compared with pristine CH3NH3PbI3 perovskite, the crystallinity and/or the quantity of two facets at 2θ in range of 14°to 15°and 28°to 29°that are exposed at the surface are significantly improved for CH3NH3PbI3 perovskite deposited from chlorine contained precursors, while other facets are barely influenced. It has been theoretically calculated that both CH3NH3PbI3 and CH3NH3PbI(3-X)Clx would exhibit preferred orientation on (110) facet due to the binding of perovskite halide atoms to under-coordinated Ti(Ⅳ) atoms of the anatase TiO2.36 However, in our experiment, we find that chlorine-free CH3NH3PbI3



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perovskite exhibits no any clear preference of (110) facet. As shown in Figure 3, (202) and (114) facets exhibit comparable peak intensity to (110) and (220) facets. On the other hand, based on their work, the binding energy of Cl-Ti (-27.3 eV) is slightly increased compared with I-Ti bond (-24.2 eV), which cannot be the dominant reason for the significantly enhanced preferential (110) and (220) facets in chlorine-involved perovskite. Meanwhile, they calculated that the (110) facet of tetragonal perovskite showed a higher (ca. 7 eV) binding energy to anatase TiO2 surface than that of the (001) facet of pseudocubic phase.36 We also used anatase TiO2 as substrates (S-Figure 3) but as observed in Figure 1, perovskite films prepared from chlorine precursors (S2 and S3) exhibit preferred cubic phase to tetragonal phase for mesoporous substrates. As a result, it is difficult to explain these differences using their theory, or this is because the interfacial chlorine atoms at interface between perovskite and TiO2 are not substantial in our situation.36, 46 Recently, several works have been published, trying to reveal the fundamental understanding of the crystal growth of perovskite from the perspective of the path that a system traverses. Up to now, it has been widely accepted that in an un-optimized one-step solution deposition, the formation of intermediates seems to kinetically dominate the initial nucleation, causing poor crystallinity and short-rang order.21 Jeon et al.,47 found that a mixed solvent (γ-butyrolactone and dimethylsulphoxide, DMSO) could be used to tune the growth kinetics of perovskite by creating a new intermediate phase, CH3NH3I-PbI2-DMSO, which ultimately produced a much more crystalline CH3NH3PbI3 film. Alternatively, chloride addition opens up another route to circumvent perovskite nucleation from an amorphous phase through creating the kinetically accessible and structurally coherent PbCl2 and CH3NH3PbCl3 intermediates.48 Tidhar et al.,49 found that the presence of PbCl2 nanocrystals could act as heterogeneous nucleation sites for the formation of perovskite crystals in solution. Colella et al. suggested that the formation of CH3NH3PbCl3 intermediate phase largely guided perovskite nucleation and growth in chloride containing systems 19. More encouragingly, Pistor et al. monitored the evolution of different crystalline phases during thin film


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growth by co-evaporating CH3NH3I and PbCl2 using in-situ XRD, and found that all XRD peaks of films deposited under low PbCl2 fluxes and measured at room temperature could be indexed to cubic CH3NH3PbI350. They speculated that the incorporation of chlorine into CH3NH3PbI3-xClx perovskite could effectively lower the cubic-tetragonal transition temperature and stabilize the cubic modification at room temperature. However, more detailed data on the CH3NH3PbI3-CH3NH3PbCl3 system and its phases are desirable to elucidate this point.51 (100)C

(200)C

S4-TiO2 MS

S3-TiO2 MS

S3-Glass

10

15

20

25

30

35

40

2 theta (degree)

Figure 4. XRD patterns of CH3NH3PbI3 films deposited from S3 and S4 precursors on top of nonconductive side of FTO and ～300 nm mesoporous TiO2 substrates. It is worth noting that the relative peak intensity between (100)C and (200)C facets can be tuned. Precursor S4 containing CH3NH3I and PbCl2 at a molar ratio of 3:1 and a mass weight concentration of 60 wt% is used to deposit CH3NH3PbI3 perovskite films on photoanodes composed of TiO2 mesoporous layer of around 300 nm. XRD patterns of CH3NH3PbI3 perovskite films spin-coated from S3 and S4 precursors on conductive glass and mesoporous TiO2 substrates are given in Figure 4. The peak intensity ratios between two major facets of CH3NH3PbI3 perovskite prepared under above conditions are summarized in Table 1. It can be seen that under the same concentration of CH3NH3PbI3 precursor (S3), the peak intensity ratio of (100)C facet to (200)C facet is decreased from 1.05 to 0.95 when the substrates are changed from conductive glass to mesoporous TiO2 layer. With the same film thickness of TiO2 mesoporous layer of around 300 nm, higher concentration of CH3NH3I:PbCl2 in DMF solvent (S4) leads



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the peak intensity ratio of (100)C facet to (200)C facet dropping down to around 0.77. Therefore, for CH3NH3PbI3 perovskite prepared from PbCl2 involved precursor, choice of substrates can subtly affect crystal orientations of CH3NH3PbI3 crystals.52 Comparing the crystal structure of CH3NH3PbI3 perovskite deposited from S3 with S4 precursor, CH3NH3PbI3 crystals exhibit preferred orientation of (200)C facet over (100)C facet for more concentrated precursor. Table 1. The relative peak intensity of (100)C and (200)C facets of CH3NH3PbI3 crystals given in Figure 4. Samples

S3-Glass

S3-TiO2 MS

S4-TiO2 MS

Peak intensity ratio of (100)C to (200)C facet

1.05

0.95

0.77

As a result, in the presence of chlorine, CH3NH3PbI3 crystals have more exposure of (110)T and (220)T facets (planar substrate) or (100)C and (200)C facets (mesoporous substrate). For many inorganic semiconductors, such as TiO2, crystal facet is highly related to surface electronic structure and thus determines charge mobility.53-54 Particularly, it has been demonstrated in photoelectrochemical studies that nanowires or nanotubes of inorganic semiconductors generally show superiority in electron transportation along the oriented facet.55 Therefore, in the future, the effect of crystal orientation of CH3NH3PbI3 crystals on the charge lifetime and mobility of CH3NH3PbI3 perovskite film needs to be studied to provide guidance for optimizing the photovoltaic performance of PSCs.30 In summary, we studied the influence of chlorine on crystal structure of CH3NH3PbI3 perovskite using two different chlorine sources, PbCl2 and CH3NH3Cl, deposited on three different substrates. It is found that for the photoanodes composed of thick TiO2 mesoporous layer (～500 nm), under the involvement of chlorine, CH3NH3PbI3 crystals tend to form cubic structure rather than tetragonal structure, which is very different from what is generally observed for CH3NH3PbI3 perovskite at room temperature. It is speculated that the existence of chlorine can significantly reduce the cubic to tetragonal transition


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temperature, thereby leading to the stable presence of cubic CH3NH3PbI3 perovskite at room temperature. Moreover, it is demonstrated that with the presence of chlorine, the preferred orientation of CH3NH3PbI3 crystals on TiO2 substrates can be shifted from (110)T to (220)T facet (planar substrate) or (100)C to (200)C facet (mesoporous substrate), confirming the influence of substrates on preferential orientation of perovskite. The findings reported herein provide new understanding on crystal formation of the perovskite layer and the interaction at perovskite/TiO2 interface under the influence of chlorine, which could provide further guidance on the design of better PSCs. Future work on the effect of crystal orientation of CH3NH3PbI3 perovskite on its electronic properties, such as charge separation/transport, charge carrier mobilities and trap states/densities needs to be conducted under the assistance of techniques, such as transient absorption spectroscopy, time-resolved photoluminescence or newly-applied confocal fluorescence microscopy33.



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ASSOCIATED CONTENT Supporting Information Experimental details, SEM, XPS, XRD, UV-Vis data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was financially supported by the Australian Research Council (ARC) through its Discovery and Future Fellowship schemes. This work was performed in part at the QLD node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and micro-fabrication facilities for Australia’s researchers. Q.W. acknowledges the support from Chinese Scholarship Council (CSC).


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Transition from the Tetragonal to Cubic Phase of Organohalide

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