**3. Synchrotron X-ray diffraction studies**

Synchrotron X-ray diffraction (SXRD) technique provides an extreme angular resolution of the patterns, useful to define the symmetry of the different phases and to determine its evolution below room temperature (RT). The crystal structures at RT are cubic, and they are well defined in the space group *Pm*¯ <sup>3</sup>*m*. In this model, the lead and bromine atoms are placed in 1*a* (0,0,0) and 3*d* (1/2,0,0) Wyckoff sites, respectively; and the organic unit is positioned in 1*b* (1/2,1/2,1/2). **Figure 2a** shows selected reflections of the Rietveld refinements corresponding to x = 0 in comparison with x = 0.33, 0.5, 0.67, and 1 members at RT. **Figure 2b** and **c** plot the unit-cell parameter variation and the anisotropic atomic displacement parameters (ADPs) of X site, respectively. The unit-cell parameters exhibit an expected reduction as the amount of Cl increases, but this change is not linear. As shown in the **Figure 2c**, the disks perpendicular to the Pb–X–Pb bonds exhibit an oblate shape, meaning the ADPs of X atoms are considerably anisotropic. Since the thermal vibrations in this direction are allowed in perovskites, this behavior is not surprising. However, for the intermediate mixed halide phases (x = 0.33, 0.5, and 0.67), the ADPs show a nonmonotonic variation compared to both end members (x = 0 and 1), although the difference does not overcome two times the standard deviations and is less significant.

These anomalies were assigned to the structural disorder introduced by the mixture of halides, for x = 0.33, 0.5, and 0.67. The ADPs should account for the structural

#### **Figure 2.**

*(a) SXRD profiles for CH3NH3PbBr3 at RT, after a pattern matching showing the characteristic perovskite peaks and the absence of impurities. Red circles are the experimental points, the black full line is the calculated profile. The green vertical marks represent the allowed Bragg positions in the Pm ¯ 3*m *space group. The Cl-doped patterns are added in this plot to compare with the bromide parent. (b) Unit-cell parameters evolution and (c) variation of the anisotropic atomic displacement parameters (ADP) of X site with the Cl contents.*

**97**

**Figure 4.**

**Figure 3.**

*and 1).*

*Structural Phase Transitions of Hybrid Perovskites CH3NH3PbX3 (X = Br, Cl) from Synchrotron…*

disorder, which can also generate a perturbation in the interactions between the inorganic PbX6 skeleton and the methyl-ammonium units. This perturbation is absent in MAPbBr3 and MAPbCl3, containing single halide ions. In **Figure 3**, a statistical probability distribution of the halide environment of MA groups is plotted, showing this behavior for the different samples (CH3NH3Pb(Cl1−xBrx)3, x = 0, 0.33, 0.5, 0.67, and 1). It is remarkable that for MAPbBr3 and MAPbCl3 the probability is 100%, because all of their MA units are coordinated to 12 Br or Cl atoms; this state contrasts with the mixed halide situations (x = 0.33, 0.5, and 0.67). These distributions reveal the high structural disorder given in mixed situations, in contrast to both end members. These probably induce tensions in the lattice preventing a linear behavior between the pure bromine and chlorine compounds. The inserts in **Figure 3** include illustrative schemes of the extreme situations in comparison with an intermediate case where the MA is coordi-

*Statistical probability distribution of the halide environment in CH3NH3Pb(Cl1−xBrx)3 (x = 0, 0.33, 0.5, 0.67,* 

The thermal variation of the crystallographic structures was followed between 120 K and RT. **Figure 4** shows the temperature evolution of selected diffraction

*Thermal evolution of selected diffraction lines in which the phase transitions are evidenced, from SXRD data,* 

*collected at MSPD diffractometer at ALBA synchrotron (Spain) [32–34].*

*DOI: http://dx.doi.org/10.5772/intechopen.91421*

nated to eight chlorides and four bromides, y = 8.

*Structural Phase Transitions of Hybrid Perovskites CH3NH3PbX3 (X = Br, Cl) from Synchrotron… DOI: http://dx.doi.org/10.5772/intechopen.91421*

**Figure 3.** *Statistical probability distribution of the halide environment in CH3NH3Pb(Cl1−xBrx)3 (x = 0, 0.33, 0.5, 0.67, and 1).*

disorder, which can also generate a perturbation in the interactions between the inorganic PbX6 skeleton and the methyl-ammonium units. This perturbation is absent in MAPbBr3 and MAPbCl3, containing single halide ions. In **Figure 3**, a statistical probability distribution of the halide environment of MA groups is plotted, showing this behavior for the different samples (CH3NH3Pb(Cl1−xBrx)3, x = 0, 0.33, 0.5, 0.67, and 1). It is remarkable that for MAPbBr3 and MAPbCl3 the probability is 100%, because all of their MA units are coordinated to 12 Br or Cl atoms; this state contrasts with the mixed halide situations (x = 0.33, 0.5, and 0.67). These distributions reveal the high structural disorder given in mixed situations, in contrast to both end members. These probably induce tensions in the lattice preventing a linear behavior between the pure bromine and chlorine compounds. The inserts in **Figure 3** include illustrative schemes of the extreme situations in comparison with an intermediate case where the MA is coordinated to eight chlorides and four bromides, y = 8.

The thermal variation of the crystallographic structures was followed between 120 K and RT. **Figure 4** shows the temperature evolution of selected diffraction

#### **Figure 4.**

*Thermal evolution of selected diffraction lines in which the phase transitions are evidenced, from SXRD data, collected at MSPD diffractometer at ALBA synchrotron (Spain) [32–34].*

*Perovskite and Piezoelectric Materials*

**3. Synchrotron X-ray diffraction studies**

crystals was observed in SEM images as is also shown in **Figure 1**. In all cases, the obtained perovskites show cuboid-type microcrystals. The content of chloride

Synchrotron X-ray diffraction (SXRD) technique provides an extreme angular resolution of the patterns, useful to define the symmetry of the different phases and to determine its evolution below room temperature (RT). The crystal structures at

lead and bromine atoms are placed in 1*a* (0,0,0) and 3*d* (1/2,0,0) Wyckoff sites, respectively; and the organic unit is positioned in 1*b* (1/2,1/2,1/2). **Figure 2a** shows selected reflections of the Rietveld refinements corresponding to x = 0 in comparison with x = 0.33, 0.5, 0.67, and 1 members at RT. **Figure 2b** and **c** plot the unit-cell parameter variation and the anisotropic atomic displacement parameters (ADPs) of X site, respectively. The unit-cell parameters exhibit an expected reduction as the amount of Cl increases, but this change is not linear. As shown in the **Figure 2c**, the disks perpendicular to the Pb–X–Pb bonds exhibit an oblate shape, meaning the ADPs of X atoms are considerably anisotropic. Since the thermal vibrations in this direction are allowed in perovskites, this behavior is not surprising. However, for the intermediate mixed halide phases (x = 0.33, 0.5, and 0.67), the ADPs show a nonmonotonic variation compared to both end members (x = 0 and 1), although the difference does not overcome two times the standard deviations and is less significant. These anomalies were assigned to the structural disorder introduced by the mixture

of halides, for x = 0.33, 0.5, and 0.67. The ADPs should account for the structural

*(a) SXRD profiles for CH3NH3PbBr3 at RT, after a pattern matching showing the characteristic perovskite peaks and the absence of impurities. Red circles are the experimental points, the black full line is the calculated profile. The green vertical marks represent the allowed Bragg positions in the Pm ¯ 3*m *space group. The Cl-doped patterns are added in this plot to compare with the bromide parent. (b) Unit-cell parameters evolution and (c) variation of the anisotropic atomic displacement parameters (ADP) of X site with the Cl* 

<sup>3</sup>*m*. In this model, the

induces a decrease in the size of the crystals of the mixed perovskites.

RT are cubic, and they are well defined in the space group *Pm*¯

**96**

**Figure 2.**

*contents.*

lines for all members of the series. In this temperature range, only both end members of the series (x = 0 and 1) exhibit phase transitions; in contrast, for the mixed halide compositions, no phase transitions have been detected, observing cubic structures (*Pm*¯ <sup>3</sup>*m*) either at RT or down to 120 K, as illustrated in **Figure 4**.

Previous works also report on the polymorphic evolution of Br and Cl phases. Swainson et al. report on two phase transitions for MAPbBr3: *Pnma* ← (≈150 K) → *I*4/*mcm* ← (≈223 K) → *Pm*¯ <sup>3</sup>*m* [25]. These phases were also described by other authors from single crystal data [26, 27]. It was reported by Poglitsch et al*.* that the MAPbCl3 perovskite goes through two phase transitions: *P*2221 ← (173 K) → *P*4/*mmm* ← (179 K) → *Pm*¯ <sup>3</sup>*m* [35]. Afterward, Chi et al*.* stated that the orthorhombic polymorph corresponds to the *Pnma* space group, but with a unit-cell twice the size of the cubic aristotype (a ≈ b ≈ c ≈ 2ap) [36].

Our work on CH3NH3PbBr3 shows that the SXRD patterns collected at RT, 270 and 240 K correspond to cubic symmetry, defined in the *Pm*¯ <sup>3</sup>*m* space group; at 210 and 180 K to tetragonal symmetry in the *I*4/*mcm* space group; and at 120 K to orthorhombic symmetry, defined in the *Pnma* space group [32]. On the other hand, CH3NH3PbCl3 remains stable as cubic down to 180 K; however, the SXRD patterns at 120 and 150 K exhibit a conspicuous splitting of some reflections [33]. According to the model proposed by Chi et al. [36]; this splitting (at 120 and 150 K) is possible. Nevertheless, some extra lines during the preliminary refinements seem to indicate that there is another phase, orthorhombic (*Pnma*), resembling the one observed in MAPbBr3 (*a* ≈ √2*a*p; *b* ≈ 2*a*p; *c* ≈ √2*a*p). If the coexistence of both phases mentioned is taken into consideration, a satisfactory refinement at 150 and 120 K can be completed, with only slight differences in the peak width; being wider for the case of the conventional *Pnma* phase compared to the doubled *Pnma* structure. A possible explanation could be that microstructural features cause this phase mixture.

For MAPbCl3, the mechanism or transient state from cubic to orthorhombic symmetry has been until now only partially known. As indicated previously, the tetragonal phase (*P*4/*mmm*) was observed in a very narrow range of temperature, 172.9–178.8 K. Up to now, no new reports have appeared on this transient tetragonal phase. Several patterns were collected sequentially [33] in this narrow temperature range, with intervals of 2.5 K, as illustrated in **Figure 5a**, where a different phase is evidenced between 169 and 164 K. This phase, corresponding to the pattern at 167.2 K, was initially fitted to the tetragonal model; however, additional diffraction lines were observed evidencing an orthorhombic symmetry. Matching the patterns observed at 120, 150, and 160 K, *Pnma* (*a* ≈ √2*a*p; *b* ≈ 2*a*p; *c* ≈ √2*a*p) provided a satisfactory fit at 167.2 K. In this last case, at low temperatures the unit-cell

#### **Figure 5.**

*(a) Selected angular region of the SXRD patterns of MAPbCl3 at increasing temperatures (from 150 to 180 K), illustrating the evolution and splitting of some characteristic reflections. (b) Thermal evolution of the unit-cell parameters of the MAPbCl3 perovskite in the same temperature range [33].*

**99**

displayed in **Figure 6**.

**4. Neutron diffraction studies**

range.

**Figure 6.**

*range [33].*

*Structural Phase Transitions of Hybrid Perovskites CH3NH3PbX3 (X = Br, Cl) from Synchrotron…*

parameters tend to be considerably less split. In **Figure 5b**, the unit-cell parameters variation as a function of temperature is displayed in this narrow temperature

*Thermal evolution of the unit-cell parameters of MAPb(Br1−xClx)3 perovskites in the 120–300 K temperature* 

From these facts, it is clear that MAPbCl3 goes through a more complex transition compared to the ones already reported. To summarize, there are three phases: cubic, orthorhombic (*a* ≈ √2*a*p; *b* ≈ 2*a*p; *c* ≈ √2*a*p), and a second orthorhombic (*a* ≈ 2*a*p; *b* ≈ 2*a*p; *c* ≈ 2*a*p), as indicated in **Figure 5b** as C, O1, and O2, respectively. Moreover, the O1 phase can be separated into two states with different distorted degrees: O1HT and O1LT. In the first case, the unit-cell parameters splitting with respect to cubic *a*p is substantially lower than in the second case. Moreover, an inversion between *b*/2 and *c*/√2 values is observed in **Figure 5b**. Additionally, a high tetragonality is observed in O1HT where the following relationship between the unit-cell parameters is observed: *a*/√2 < *b*/2 ≈ *c*/√2. This fact can explain the previ-

ous description in the tetragonal symmetry in this short temperature range. As reported in [32, 33], MAPbCl3 and MAPbBr3 are stable above 179 and 237 K respectively; therefore, it is relevant to mention the lack of any other phase transition down to 120 K in mixed halide perovskites. The anion disorder in the mixed-halide phases could explain this surprising behavior. As it was previously mentioned, the interactions between the organic and inorganic parts differ from one sample to another within the present series (see **Figure 3**). The phase transitions are prevented due to the halide disorder, because it does not allow the rearrangements that are required in terms of octahedral tilting. Finally, SXRD measurements and their analysis (Rietveld refinements) provide new evidence that allows to complete the polymorphic evolution in the MAPb(Br1−xClx)3 family, as

The neutron powder diffraction (NPD) investigation is essential to obtain a detailed

description of these hybrid materials. The NPD data were collected at the D2B diffractometer (ILL, France) [32, 34]. In particular, the distribution of MA groups can be elucidated taking advantage of the high contrast in the coherent scattering lengths of Pb, Br, Cl, C, N, and H, of 9.405, 6.795, 9.577, 6.646, 9.36, and −3.739 fm, respectively. However, there is an issue in the resolution of the structure that needs to be considered. The incoherent background in a powder experiment is considerably large due to the presence of hydrogen, which has an important inelastic component

*DOI: http://dx.doi.org/10.5772/intechopen.91421*

*Structural Phase Transitions of Hybrid Perovskites CH3NH3PbX3 (X = Br, Cl) from Synchrotron… DOI: http://dx.doi.org/10.5772/intechopen.91421*

**Figure 6.**

*Perovskite and Piezoelectric Materials*

← (≈150 K) → *I*4/*mcm* ← (≈223 K) → *Pm*¯

*P*2221 ← (173 K) → *P*4/*mmm* ← (179 K) → *Pm*¯

structures (*Pm*¯

lines for all members of the series. In this temperature range, only both end members of the series (x = 0 and 1) exhibit phase transitions; in contrast, for the mixed halide compositions, no phase transitions have been detected, observing cubic

Previous works also report on the polymorphic evolution of Br and Cl phases. Swainson et al. report on two phase transitions for MAPbBr3: *Pnma*

described by other authors from single crystal data [26, 27]. It was reported by Poglitsch et al*.* that the MAPbCl3 perovskite goes through two phase transitions:

unit-cell twice the size of the cubic aristotype (a ≈ b ≈ c ≈ 2ap) [36].

and 240 K correspond to cubic symmetry, defined in the *Pm*¯

that the orthorhombic polymorph corresponds to the *Pnma* space group, but with a

Our work on CH3NH3PbBr3 shows that the SXRD patterns collected at RT, 270

210 and 180 K to tetragonal symmetry in the *I*4/*mcm* space group; and at 120 K to orthorhombic symmetry, defined in the *Pnma* space group [32]. On the other hand, CH3NH3PbCl3 remains stable as cubic down to 180 K; however, the SXRD patterns at 120 and 150 K exhibit a conspicuous splitting of some reflections [33]. According to the model proposed by Chi et al. [36]; this splitting (at 120 and 150 K) is possible. Nevertheless, some extra lines during the preliminary refinements seem to indicate that there is another phase, orthorhombic (*Pnma*), resembling the one observed in MAPbBr3 (*a* ≈ √2*a*p; *b* ≈ 2*a*p; *c* ≈ √2*a*p). If the coexistence of both phases mentioned is taken into consideration, a satisfactory refinement at 150 and 120 K can be completed, with only slight differences in the peak width; being wider for the case of the conventional *Pnma* phase compared to the doubled *Pnma* structure. A possible explanation could be that microstructural features cause this phase mixture. For MAPbCl3, the mechanism or transient state from cubic to orthorhombic symmetry has been until now only partially known. As indicated previously, the tetragonal phase (*P*4/*mmm*) was observed in a very narrow range of temperature, 172.9–178.8 K. Up to now, no new reports have appeared on this transient tetragonal phase. Several patterns were collected sequentially [33] in this narrow temperature range, with intervals of 2.5 K, as illustrated in **Figure 5a**, where a different phase is evidenced between 169 and 164 K. This phase, corresponding to the pattern at 167.2 K, was initially fitted to the tetragonal model; however, additional diffraction lines were observed evidencing an orthorhombic symmetry. Matching the patterns observed at 120, 150, and 160 K, *Pnma* (*a* ≈ √2*a*p; *b* ≈ 2*a*p; *c* ≈ √2*a*p) provided a satisfactory fit at 167.2 K. In this last case, at low temperatures the unit-cell

*(a) Selected angular region of the SXRD patterns of MAPbCl3 at increasing temperatures (from 150 to 180 K), illustrating the evolution and splitting of some characteristic reflections. (b) Thermal evolution of the unit-cell* 

*parameters of the MAPbCl3 perovskite in the same temperature range [33].*

<sup>3</sup>*m*) either at RT or down to 120 K, as illustrated in **Figure 4**.

<sup>3</sup>*m* [25]. These phases were also

<sup>3</sup>*m* [35]. Afterward, Chi et al*.* stated

<sup>3</sup>*m* space group; at

**98**

**Figure 5.**

*Thermal evolution of the unit-cell parameters of MAPb(Br1−xClx)3 perovskites in the 120–300 K temperature range [33].*

parameters tend to be considerably less split. In **Figure 5b**, the unit-cell parameters variation as a function of temperature is displayed in this narrow temperature range.

From these facts, it is clear that MAPbCl3 goes through a more complex transition compared to the ones already reported. To summarize, there are three phases: cubic, orthorhombic (*a* ≈ √2*a*p; *b* ≈ 2*a*p; *c* ≈ √2*a*p), and a second orthorhombic (*a* ≈ 2*a*p; *b* ≈ 2*a*p; *c* ≈ 2*a*p), as indicated in **Figure 5b** as C, O1, and O2, respectively. Moreover, the O1 phase can be separated into two states with different distorted degrees: O1HT and O1LT. In the first case, the unit-cell parameters splitting with respect to cubic *a*p is substantially lower than in the second case. Moreover, an inversion between *b*/2 and *c*/√2 values is observed in **Figure 5b**. Additionally, a high tetragonality is observed in O1HT where the following relationship between the unit-cell parameters is observed: *a*/√2 < *b*/2 ≈ *c*/√2. This fact can explain the previous description in the tetragonal symmetry in this short temperature range.

As reported in [32, 33], MAPbCl3 and MAPbBr3 are stable above 179 and 237 K respectively; therefore, it is relevant to mention the lack of any other phase transition down to 120 K in mixed halide perovskites. The anion disorder in the mixed-halide phases could explain this surprising behavior. As it was previously mentioned, the interactions between the organic and inorganic parts differ from one sample to another within the present series (see **Figure 3**). The phase transitions are prevented due to the halide disorder, because it does not allow the rearrangements that are required in terms of octahedral tilting. Finally, SXRD measurements and their analysis (Rietveld refinements) provide new evidence that allows to complete the polymorphic evolution in the MAPb(Br1−xClx)3 family, as displayed in **Figure 6**.

### **4. Neutron diffraction studies**

The neutron powder diffraction (NPD) investigation is essential to obtain a detailed description of these hybrid materials. The NPD data were collected at the D2B diffractometer (ILL, France) [32, 34]. In particular, the distribution of MA groups can be elucidated taking advantage of the high contrast in the coherent scattering lengths of Pb, Br, Cl, C, N, and H, of 9.405, 6.795, 9.577, 6.646, 9.36, and −3.739 fm, respectively.

However, there is an issue in the resolution of the structure that needs to be considered. The incoherent background in a powder experiment is considerably large due to the presence of hydrogen, which has an important inelastic component (25,274 barns) that gives rise to a significant incoherent background. This is not an obstacle for the crystallographic Rietveld refinement if sufficient statistic is achieved during the measurement.

On the other hand, due to the negative contribution of hydrogen to the scattering, some strategies are necessary to find the adequate distribution of MA in the inorganic PbX6 framework. First, the instrumental and the unit-cell parameters were refined using the Le Bail method. The structure of the octahedral PbX6 framework was eventually considered in the model, placing the lead atoms at 1*a* (0,0,0) sites and bromine/chlorine atoms at 3*d* (1/2,0,0) positions in the *Pm*¯ <sup>3</sup>*m* space group. Later on, Difference Fourier Maps (DFM) were acquired from the observed and calculated patterns, unveiling the missing negative and positive nuclear density (in scattering length) around A site of the perovskite structure (1*b* (1/2,1/2,1/2)).

This analysis in MAPbBr3 [32] yields the positive (represented in yellow) and negative (represented in blue) isosurfaces shown in **Figure 7**a, which correspond to the C/N and H positions, respectively. The negative scattering regions are due to the negative scattering of protons while the positive zone corresponds to C/N atoms. These nuclear densities support that the MA units are delocalized in the A site of the perovskite. Considering the positive density, the C/N atoms are located at 24*i* (1/2,*y*,*y*) positions. Then, the H positions can be elucidated from the geometric shape of methylammonium group. The observed geometry can be satisfied with two hydrogen atoms located at 24 *l* (1/2,*y*,*z*) and 24 *m* (*x*,*x*,*z*) Wyckoff sites. This analysis in MAPbBr3 reveals that the MA group is delocalized at room temperature along the [110] direction, involving six possible orientations.

In contrast, the DFM for MAPbBr2Cl show that the positive and negative densities match exactly with those expected for the MA cations oriented along [111] directions. It is possible to deduce that the C/N and H atoms are placed at 8 *g* (*x*, *x*, *x*) and 24 *m* (*x*, *x*, *z*) Wyckoff positions, respectively (**Figure 7**b). MAPbBr1.5Cl1.5 exhibits a peculiar negative distribution (see **Figure 7c**), where there are four negative zones along the [001] directions, suggesting that the MA units are along this direction. However, along [111] directions, there also appears a non-negligible density, which is unrealistic. This dichotomy is resolved by analyzing the C/N density from the positive surface; it unveils that the C/N atoms are indeed delocalized along the [100]

#### **Figure 7.**

*DFM isosurfaces of MAPb(Br1−xClx)3 series for x = 0 (a), 0.33 (b), 0.5 (c), 0.67 (d), and 1 (e), from NPD data [32, 34].*

**101**

**Figure 8.**

*Structural Phase Transitions of Hybrid Perovskites CH3NH3PbX3 (X = Br, Cl) from Synchrotron…*

directions. The same procedure in MAPbBrCl2 and MAPbCl3, see **Figure 7d** and **e**, shows that the MA groups are oriented along [100] directions, in the same way as MAPbBr1.5Cl1.5. From this result, which only can be obtained from NPD data, a clear restriction in the MA delocalization is unveiled, evolving from [110] to [111] and finally to [100] directions, while X is progressively enriched in Cl<sup>−</sup> anions.

From the Fourier synthesis maps, a crystallographic model can be built with the MA configurations, to start the structural Rietveld refinements from the NPD data. Additionally, the MA displacement toward –NH3 group also can be considered, as it was proposed from theoretical calculations [37, 38]. This last approach corresponds to the plausible chemical interactions between the organic cation and inorganic framework; the greater electronegativity of nitrogen produces H-bond interactions shorter than C–H⋯X. Considering this last fact and the DFM results, the Rietveld refinement of NPD data was made. Minor amounts of MACl were identified and included in the refinements as second phase in the chlorine phase. The Rietveld fits for MAPbBr3 and MAPbCl3 are illustrated in **Figure 8**. The crystal structure data after these refinements are listed in **Table 1** for all the compositions. This table shows the three different combinations of C/N and H positions considering three possibilities for the MA delocalization in the PbX6 framework observed in the MAPb(Br1−xClx)3 series. The variation of the unit-cell parameter with the Cl-doping is illustrated in **Figure 9**; here, a change of slope is observed in the perovskite with an equimolar amount of Br and Cl atoms, according to the anomaly obtained from

For x = 0.5, 0.67, and 1, the unit cell experiences a contraction, keeping a constant slope as Cl is introduced, with the MA oriented in the same direction. On the other hand, for x = 0 and 0.33, the unit cell parameters do not follow this trend,

A feature to highlight is the difference in negative density (arising from H positions) observed between [111] and [100] orientations. While along the [111] direction,

+

From the negative density displayed in **Figure 7c–e**, it is noticeable that four H atoms are comprised in the (100) plane, suggesting that the MA units occupy equivalent positions, so that each H points to each of the four halides in the edges of the perovskite unit cell, thus manifesting the importance of H⋯X hydrogen bonds. This bonded H is located in H11 site and the other two H of the amine group are located in the H12 site. Similarly, the methyl group is formed by C2, H21, and H22 atoms.

units are fixed, as it is shown in **Figure 7b**.

*DOI: http://dx.doi.org/10.5772/intechopen.91421*

synchrotron data.

the MA orientation is different.

the three terminal H atoms of the H3C-NH3

*Rietveld plots of a) MAPbBr3 and b) PbPbCl3 from NPD data, at RT.*

directions. The same procedure in MAPbBrCl2 and MAPbCl3, see **Figure 7d** and **e**, shows that the MA groups are oriented along [100] directions, in the same way as MAPbBr1.5Cl1.5. From this result, which only can be obtained from NPD data, a clear restriction in the MA delocalization is unveiled, evolving from [110] to [111] and finally to [100] directions, while X is progressively enriched in Cl<sup>−</sup> anions.

From the Fourier synthesis maps, a crystallographic model can be built with the MA configurations, to start the structural Rietveld refinements from the NPD data. Additionally, the MA displacement toward –NH3 group also can be considered, as it was proposed from theoretical calculations [37, 38]. This last approach corresponds to the plausible chemical interactions between the organic cation and inorganic framework; the greater electronegativity of nitrogen produces H-bond interactions shorter than C–H⋯X. Considering this last fact and the DFM results, the Rietveld refinement of NPD data was made. Minor amounts of MACl were identified and included in the refinements as second phase in the chlorine phase. The Rietveld fits for MAPbBr3 and MAPbCl3 are illustrated in **Figure 8**. The crystal structure data after these refinements are listed in **Table 1** for all the compositions. This table shows the three different combinations of C/N and H positions considering three possibilities for the MA delocalization in the PbX6 framework observed in the MAPb(Br1−xClx)3 series. The variation of the unit-cell parameter with the Cl-doping is illustrated in **Figure 9**; here, a change of slope is observed in the perovskite with an equimolar amount of Br and Cl atoms, according to the anomaly obtained from synchrotron data.

For x = 0.5, 0.67, and 1, the unit cell experiences a contraction, keeping a constant slope as Cl is introduced, with the MA oriented in the same direction. On the other hand, for x = 0 and 0.33, the unit cell parameters do not follow this trend, the MA orientation is different.

A feature to highlight is the difference in negative density (arising from H positions) observed between [111] and [100] orientations. While along the [111] direction, the three terminal H atoms of the H3C-NH3 + units are fixed, as it is shown in **Figure 7b**. From the negative density displayed in **Figure 7c–e**, it is noticeable that four H atoms are comprised in the (100) plane, suggesting that the MA units occupy equivalent positions, so that each H points to each of the four halides in the edges of the perovskite unit cell, thus manifesting the importance of H⋯X hydrogen bonds. This bonded H is located in H11 site and the other two H of the amine group are located in the H12 site. Similarly, the methyl group is formed by C2, H21, and H22 atoms.

**Figure 8.** *Rietveld plots of a) MAPbBr3 and b) PbPbCl3 from NPD data, at RT.*

*Perovskite and Piezoelectric Materials*

achieved during the measurement.

(25,274 barns) that gives rise to a significant incoherent background. This is not an obstacle for the crystallographic Rietveld refinement if sufficient statistic is

Later on, Difference Fourier Maps (DFM) were acquired from the observed and calculated patterns, unveiling the missing negative and positive nuclear density (in scattering length) around A site of the perovskite structure (1*b* (1/2,1/2,1/2)). This analysis in MAPbBr3 [32] yields the positive (represented in yellow) and negative (represented in blue) isosurfaces shown in **Figure 7**a, which correspond to the C/N and H positions, respectively. The negative scattering regions are due to the negative scattering of protons while the positive zone corresponds to C/N atoms. These nuclear densities support that the MA units are delocalized in the A site of the perovskite. Considering the positive density, the C/N atoms are located at 24*i* (1/2,*y*,*y*) positions. Then, the H positions can be elucidated from the geometric shape of methylammonium group. The observed geometry can be satisfied with two hydrogen atoms located at 24 *l* (1/2,*y*,*z*) and 24 *m* (*x*,*x*,*z*) Wyckoff sites. This analysis in MAPbBr3 reveals that the MA group is delocalized at room temperature

In contrast, the DFM for MAPbBr2Cl show that the positive and negative densities match exactly with those expected for the MA cations oriented along [111] directions. It is possible to deduce that the C/N and H atoms are placed at 8 *g* (*x*, *x*, *x*) and 24 *m* (*x*, *x*, *z*) Wyckoff positions, respectively (**Figure 7**b). MAPbBr1.5Cl1.5 exhibits a peculiar negative distribution (see **Figure 7c**), where there are four negative zones along the [001] directions, suggesting that the MA units are along this direction. However, along [111] directions, there also appears a non-negligible density, which is unrealistic. This dichotomy is resolved by analyzing the C/N density from the positive surface; it unveils that the C/N atoms are indeed delocalized along the [100]

*DFM isosurfaces of MAPb(Br1−xClx)3 series for x = 0 (a), 0.33 (b), 0.5 (c), 0.67 (d), and 1 (e), from NPD data* 

sites and bromine/chlorine atoms at 3*d* (1/2,0,0) positions in the *Pm*¯

along the [110] direction, involving six possible orientations.

On the other hand, due to the negative contribution of hydrogen to the scattering, some strategies are necessary to find the adequate distribution of MA in the inorganic PbX6 framework. First, the instrumental and the unit-cell parameters were refined using the Le Bail method. The structure of the octahedral PbX6 framework was eventually considered in the model, placing the lead atoms at 1*a* (0,0,0)

<sup>3</sup>*m* space group.

**100**

**Figure 7.**

*[32, 34].*


**Table 1.**

*Crystallographic data for MAPb(Br1−xClx)3 with x = 0, 0.33, 0.5, 0.67, and 1 at room temperature from NPD.*

Looking back at the unit-cell variation, the contraction of the PbX6 octahedron as Cl is introduced leads to a localization of the MA units, limiting their orientation. This decrease has been deeply studied below room temperature, as the crystal goes through the phase transitions: cubic-tetragonal-orthorhombic [37, 39, 40]; however, in this case, CH3NH3Pb(Br1−xClx)3, the behavior is observed in the same crystal system at RT [34]. This finding shows that the MA freedom degree reduction can occur either by a decrease in the symmetry or by size reduction (keeping the symmetry) in the PbX6 network. These discoveries renew the question whether the inorganic framework deformation occurs and then the MA units accommodate in the preferred directions, or the reduction in the MA freedom degree enables the

**103**

dichotomy.

**Figure 9.**

*Structural Phase Transitions of Hybrid Perovskites CH3NH3PbX3 (X = Br, Cl) from Synchrotron…*

inorganic framework to adopt a low symmetry. This dichotomy is as the *chicken and egg* paradox, which has previously been analyzed from DFT calculations, concluding that phase transitions occur as a synergic effect between both organic and inorganic components [38]. In the present case, the MA restrictions occur without symmetry reduction in the inorganic framework, but as a consequence of a contraction in the unit-cell size, so it is possible to think that the PbX6 lattice drives the MA freedom degree. However, it is only an appreciation and it is not enough to resolve this

*Unit-cell parameters variation for MAPb(Br1−xClx)3 from NPD data, at RT.*

On the other hand, the role of H⋯X interactions in the MA conformations is not unknown [41, 42] but shows some controversy among the different results. Theoretical studies from DFT indicate that the energy difference between the low index [100], [110], and [111] orientations of the MA units is similar, but with a subtle preference for the [100] orientation in the tetragonal MAPbI3 [40]. However, the ab initio simulations performed by Shimamura et al. [43] unveiled that [110] was the preferred orientation due to the H⋯X interactions in MAPbI3. Differently, Li et al. identify [111] and [100] for the MAPbI3 and MAPbCl3 perovskites as the favorable orientations [38]. Afterward, Varadwaj et al. reinforced this last idea by also stating that these orientations were the most energetically favorable [37]. Although there is significant theoretical work regarding the MA units orientation in cubic symmetry, experimental evidence is lacking [25, 27, 36]. It is the case of the Baikie et al. [27] report that focuses on the three structures for X = I, Br, and Cl from X-ray and neutron diffraction data. In all the cases, the MA components were refined in the [110] direction. In this context of theoretical results, the considerations made in the CH3NH3Pb(Br1−xClx)3 refinements offer new possibilities to achieve a detailed analysis of the crystal studies in comparison with these theoretical works. Once the atomic positions are refined, a "deconvolution" of the MA units orientation can be done for the directions [111] and [100]. The H⋯X interactions

The MA units within the cubic unit cell are presented in **Figure 10**, the three alignment possibilities ([110], [111], and [100]) are shown. The MA in MAPbBr3 is aligned along [110] and, as can be seen in **Figure 10a** and **d**, in this orientation the N–H bonds point toward face centers or cube corners forming smalls angles N–H⋯Br; hence, this configuration is not suitable for hydrogen bonding. On the other hand, for x = 0.33 (along [111] direction), the H–C and H–N interact with a

obtained can be compared with those predicted theoretically.

*DOI: http://dx.doi.org/10.5772/intechopen.91421*

*Structural Phase Transitions of Hybrid Perovskites CH3NH3PbX3 (X = Br, Cl) from Synchrotron… DOI: http://dx.doi.org/10.5772/intechopen.91421*

**Figure 9.** *Unit-cell parameters variation for MAPb(Br1−xClx)3 from NPD data, at RT.*

inorganic framework to adopt a low symmetry. This dichotomy is as the *chicken and egg* paradox, which has previously been analyzed from DFT calculations, concluding that phase transitions occur as a synergic effect between both organic and inorganic components [38]. In the present case, the MA restrictions occur without symmetry reduction in the inorganic framework, but as a consequence of a contraction in the unit-cell size, so it is possible to think that the PbX6 lattice drives the MA freedom degree. However, it is only an appreciation and it is not enough to resolve this dichotomy.

On the other hand, the role of H⋯X interactions in the MA conformations is not unknown [41, 42] but shows some controversy among the different results. Theoretical studies from DFT indicate that the energy difference between the low index [100], [110], and [111] orientations of the MA units is similar, but with a subtle preference for the [100] orientation in the tetragonal MAPbI3 [40]. However, the ab initio simulations performed by Shimamura et al. [43] unveiled that [110] was the preferred orientation due to the H⋯X interactions in MAPbI3. Differently, Li et al. identify [111] and [100] for the MAPbI3 and MAPbCl3 perovskites as the favorable orientations [38]. Afterward, Varadwaj et al. reinforced this last idea by also stating that these orientations were the most energetically favorable [37]. Although there is significant theoretical work regarding the MA units orientation in cubic symmetry, experimental evidence is lacking [25, 27, 36]. It is the case of the Baikie et al. [27] report that focuses on the three structures for X = I, Br, and Cl from X-ray and neutron diffraction data. In all the cases, the MA components were refined in the [110] direction. In this context of theoretical results, the considerations made in the CH3NH3Pb(Br1−xClx)3 refinements offer new possibilities to achieve a detailed analysis of the crystal studies in comparison with these theoretical works. Once the atomic positions are refined, a "deconvolution" of the MA units orientation can be done for the directions [111] and [100]. The H⋯X interactions obtained can be compared with those predicted theoretically.

The MA units within the cubic unit cell are presented in **Figure 10**, the three alignment possibilities ([110], [111], and [100]) are shown. The MA in MAPbBr3 is aligned along [110] and, as can be seen in **Figure 10a** and **d**, in this orientation the N–H bonds point toward face centers or cube corners forming smalls angles N–H⋯Br; hence, this configuration is not suitable for hydrogen bonding. On the other hand, for x = 0.33 (along [111] direction), the H–C and H–N interact with a

*Perovskite and Piezoelectric Materials*

Pb (1*a*) (0,0,0)

Br1−xClx (3*d*) (0.5,0,0)

N/C C/N

H H1(0.5,*y*,*x*)

(*0.5*,*y*,*y*) 0.407(2)

0.414(9) 0.247(9)

H2(*x*,*x*,*z*) 0.326(3) 0.448(4)

**MAPb(Br1−xClx)3 x = 0 [32] x = 0.33** 

**[34]**

N1(*x*,*x*,*x*) 0.5809(5)

C2(*x*,*x*,*x*) 0.4378(5)

H11(*x*,*y*,*x*) 0.5839(6) 0.7363(9)

H12(*x*,*y*,*x*) 0.4416(7) 0.2841(8)

*a* (Å) 5.9259(4) 5.8454(8) 5.80459(6) 5.76955(7) 5.6813(5)

U11 = U22 = U33 0.025(2) 0.048(3) 0.0313(3) 0.031(2) 0.0239(4)

*x* 0 0.38(3) 0.55(5) 0.67(8) 1 U11 0.018(5) 0.028(5) 0.0147(5) 0.026(4) 0.0471(8) U22 = U33 0.138(5) 0.130(5) 0.1318(6) 0.104(4) 0.1151(7)

Uiso\*/Ueq 0.039(7) 0.061(1)\* 0.051\* 0.051\* 0.039(1)\*

N1(0.5,*0.5*,*z*) 0.6290(4)

C2(0.5,*0.5*,*z*) 0.3777(6)

H11(0.5,*y*,*z*) 0.664(1) 0.702(3)

H12(*x*,*y*,*z*) 0.640(1) 0.419(1) 0.702(3)

H21(0.5,*y*,*z*) 0.331(1) 0.305(3)

H22(*x*,*y*,*z*) 0.357(1) 0.583(1) 0.305(3)

**x = 0.5 [34] x = 0.67 [34] x = 1 [34]**

N1(0.5,*0.5*,*z*) 0.6294(5)

C2(0.5,*0.5*,*z*) 0.3782(6)

H11(0.5,*y*,*z*) 0.665(1) 0.702(3)

H12(*x*,*y*,*z*) 0.641(1) 0.418(1) 0.702(3)

H21(0.5,*y*,*z*) 0.331(1) 0.306(1)

H22(*x*,*y*,*z*) 0.356(1) 0.583(1) 0.306(3)

N1(0.5,*0.5*,*z*) 0.6594(5)

C2(0.5,*0.5*,*z*) 0.4113(5)

H11(0.5,*y*,*z*) 0.670(1) 0.720(2)

H12(*x*,*y*,*z*) 0.6445(9) 0.4166(9) 0.720(2)

H21(0.5,*y*,*z*) 0.326(1) 0.351(2)

H22(*x*,*y*,*z*) 0.354(1) 0.585(1) 0.351(2)

**102**

**Table 1.**

Looking back at the unit-cell variation, the contraction of the PbX6 octahedron as Cl is introduced leads to a localization of the MA units, limiting their orientation. This decrease has been deeply studied below room temperature, as the crystal goes through the phase transitions: cubic-tetragonal-orthorhombic [37, 39, 40]; however, in this case, CH3NH3Pb(Br1−xClx)3, the behavior is observed in the same crystal system at RT [34]. This finding shows that the MA freedom degree reduction can occur either by a decrease in the symmetry or by size reduction (keeping the symmetry) in the PbX6 network. These discoveries renew the question whether the inorganic framework deformation occurs and then the MA units accommodate in the preferred directions, or the reduction in the MA freedom degree enables the

*Crystallographic data for MAPb(Br1−xClx)3 with x = 0, 0.33, 0.5, 0.67, and 1 at room temperature from NPD.*

Uiso\*/Ueq 0.06(1) 0.063\* 0.063\* 0.063\* 0.059(2)\* Rp(%) 1.3 1.3 1.0 1.2 0.8 Rwp(%) 1.6 1.7 1.3 1.5 0.9 χ<sup>2</sup> 1.1 1.1 1.3 1.1 1.2 RBragg(%) 3.9 10.8 6.5 6.5 6.4

**Figure 10.**

*Methyl-ammonium conformations in the MAPbBr3 (a) and (d), MAPbBr2Cl (b) and (e) and MAPbCl3 (c) and (f) perovskites, from NPD data at RT. The lowest figures (d), (e) and (f) are shown along the MA direction.*

unique halide (X = Cl0.33Br0.67) with a distance of 2.921(5) and 3.088(5) Å, respectively (**Figure 10b** and **e**). As mentioned, the MA conformation is slightly displaced, the (C)H⋯X distances are larger than (N)H⋯X, which could be explained by the bigger electronegativity of N compared to C.

Varadwaj et al. reported on DFT calculations for the MAPbBr3 perovskite [37]; the distances were determined, being ≈2.44 and ≈3.47 Å for (N)H⋯X and (C) H⋯X, respectively, in agreement with the NPD experimental results. Carrying out the same procedure for x = 0.5, 0.67 and 1, we can determine the orientation of the MA units, which resulted to be [100]. The MA units can be rotated in four positions along the three [100] directions giving 12 possible conformations. With the help of DFT calculations, the orientation of the MA unit for bromine and chlorine phases can be estimated, with the additional fact that the (N)H⋯X distances are shorter that the (C)H⋯X ones [37, 38]. **Figure 10c** and **f** illustrate a MA unit from the obtained C, N, and H atomic positions for MAPbCl3; two different types of H-bonds can be distinguished: normal (N1–H11⋯X) and bifurcated (N1–H12⋯X). Similarly, the methyl group shows the same interactions through H21 and H22 atoms. The obtained distances for x= 1 phase are 2.459(8) and 2.615(8) Å for N1–H11∙∙∙X and N1–H12∙∙∙X, respectively [38].

The structural refinement from NPD data provide, for the first time, experimental evidence of the MA alignments, confirming the theoretical results reported up to now.

### **5. Optoelectronic properties**

The optoelectronic properties are characterized using different techniques such as diffuse reflectance UV-vis spectra and building a photodetector device where the current-voltage (IV) curves are measured under illuminations. This device was fabricated by drop-casting the perovskite solution onto Au/Cr pre-patterned electrodes.

**105**

**Figure 12.**

*Structural Phase Transitions of Hybrid Perovskites CH3NH3PbX3 (X = Br, Cl) from Synchrotron…*

The UV-vis spectra, illustrated in **Figure 11**, are used to calculate the optical

*UV-vis absorption spectra for MAPb(Br1 − xClx)3. The colored line corresponds to the band gap energy.*

where R is the reflectance (%). The optical band gap is determined from the extrapolation of the linear part of the transformed Kubelka-Munk spectrum with the hν axis. Absorbance vs. wavelength of the incident radiation for MAPb(Br1<sup>−</sup> xClx)3 series is illustrated in **Figure 11**. In this plot, the included color chart allows

The absorption edge presents a gradual evolution, which is consistent with the continuous structural changes, mainly with the unit-cell parameter evolution. The energy of the gap experiences an increment as Br is progressively replaced with Cl. This is, in principle, expected as a consequence of the smaller covalency of the leadhalide bonds within the PbX6 (X = Cl, Br) octahedra [31]. Recent studies by density functional theory (DFT) show that the edges of the valence band and conduction band of CH3NH3X3 perovskites are principally made up of *p*X and *p*Pb states, respectively [44]. It is shown that near the gap, the predominant contribution is brought about by Pb-Pb transitions, involving *s*-type orbitals. The characteristic absorption band above 3 eV is fundamentally driven by *s*Pb-*p*Pb interactions, tuned by the unit-cell

*Comparison between the band gap energy and unit-cell parameter in MAPb(Br1−xClx)3, as a function of the* 

*Cl content, x. The color in the circles corresponds to the band gap energy.*

F(R) = α = (1 − R)2/ (2R) (3)

absorption coefficient (α) according to the Kubelka-Munk equation:

accounting for the sample color shown in **Figure 1**.

*DOI: http://dx.doi.org/10.5772/intechopen.91421*

**Figure 11.**

*Structural Phase Transitions of Hybrid Perovskites CH3NH3PbX3 (X = Br, Cl) from Synchrotron… DOI: http://dx.doi.org/10.5772/intechopen.91421*

**Figure 11.**

*Perovskite and Piezoelectric Materials*

unique halide (X = Cl0.33Br0.67) with a distance of 2.921(5) and 3.088(5) Å, respectively (**Figure 10b** and **e**). As mentioned, the MA conformation is slightly displaced, the (C)H⋯X distances are larger than (N)H⋯X, which could be explained

*Methyl-ammonium conformations in the MAPbBr3 (a) and (d), MAPbBr2Cl (b) and (e) and MAPbCl3 (c) and (f) perovskites, from NPD data at RT. The lowest figures (d), (e) and (f) are shown along the MA* 

Varadwaj et al. reported on DFT calculations for the MAPbBr3 perovskite [37]; the distances were determined, being ≈2.44 and ≈3.47 Å for (N)H⋯X and (C) H⋯X, respectively, in agreement with the NPD experimental results. Carrying out the same procedure for x = 0.5, 0.67 and 1, we can determine the orientation of the MA units, which resulted to be [100]. The MA units can be rotated in four positions along the three [100] directions giving 12 possible conformations. With the help of DFT calculations, the orientation of the MA unit for bromine and chlorine phases can be estimated, with the additional fact that the (N)H⋯X distances are shorter that the (C)H⋯X ones [37, 38]. **Figure 10c** and **f** illustrate a MA unit from the obtained C, N, and H atomic positions for MAPbCl3; two different types of H-bonds can be distinguished: normal (N1–H11⋯X) and bifurcated (N1–H12⋯X). Similarly, the methyl group shows the same interactions through H21 and H22 atoms. The obtained distances for x= 1 phase are 2.459(8) and 2.615(8) Å for N1–H11∙∙∙X and

The structural refinement from NPD data provide, for the first time, experimental evidence of the MA alignments, confirming the theoretical results reported up to now.

The optoelectronic properties are characterized using different techniques such as diffuse reflectance UV-vis spectra and building a photodetector device where the current-voltage (IV) curves are measured under illuminations. This device was fabricated by drop-casting the perovskite solution onto Au/Cr pre-patterned

by the bigger electronegativity of N compared to C.

N1–H12∙∙∙X, respectively [38].

**5. Optoelectronic properties**

**104**

electrodes.

**Figure 10.**

*direction.*

*UV-vis absorption spectra for MAPb(Br1 − xClx)3. The colored line corresponds to the band gap energy.*

The UV-vis spectra, illustrated in **Figure 11**, are used to calculate the optical absorption coefficient (α) according to the Kubelka-Munk equation:

$$\mathbf{F(R)} = \mathbf{a} = (\mathbf{1} - \mathbf{R})\mathbf{2}/(\mathbf{2R})\tag{3}$$

where R is the reflectance (%). The optical band gap is determined from the extrapolation of the linear part of the transformed Kubelka-Munk spectrum with the hν axis. Absorbance vs. wavelength of the incident radiation for MAPb(Br1<sup>−</sup> xClx)3 series is illustrated in **Figure 11**. In this plot, the included color chart allows accounting for the sample color shown in **Figure 1**.

The absorption edge presents a gradual evolution, which is consistent with the continuous structural changes, mainly with the unit-cell parameter evolution. The energy of the gap experiences an increment as Br is progressively replaced with Cl. This is, in principle, expected as a consequence of the smaller covalency of the leadhalide bonds within the PbX6 (X = Cl, Br) octahedra [31]. Recent studies by density functional theory (DFT) show that the edges of the valence band and conduction band of CH3NH3X3 perovskites are principally made up of *p*X and *p*Pb states, respectively [44]. It is shown that near the gap, the predominant contribution is brought about by Pb-Pb transitions, involving *s*-type orbitals. The characteristic absorption band above 3 eV is fundamentally driven by *s*Pb-*p*Pb interactions, tuned by the unit-cell

#### **Figure 12.**

*Comparison between the band gap energy and unit-cell parameter in MAPb(Br1−xClx)3, as a function of the Cl content, x. The color in the circles corresponds to the band gap energy.*

size, with a lower contribution by *p*Pb-*p*X and *s*X-*p*X transitions. These latter contributions decrease from iodine to bromine and then to chlorine, that is, when increasing the electronegativity of the halide, which accounts for the variation of the gap with the chemical nature of X, as shown in **Figures 11** and **12**. Endres et al. [45] found a remarkably low DOS at the valence band of MAPbX3, from either ultraviolet and inverse photoemission spectroscopies or from theoretical densities of states (DOS), calculated via density functional theory. They found a strong band dispersion at the valence band due to the coupling between halide *p* and Pb *s* antibonding orbitals, observing the absence of significant densities of states tailing into the perovskite gaps.

As already mentioned, an interesting feature is that the compositional evolution of the band gap energy (Eg) does not possesses a linear behavior; it seems that the deviation from linearity is reminiscent of that unveiled for the variation of the unit-cell parameters at RT (see **Figure 12**). By additionally contrasting the Eg value with the Pb–X bond lengths, a non-linear behavior is apparent, suggesting that the evolution of Eg is not only related to the inorganic features. From this point of view, it is interesting to consider that, although all these studies are based on the nominally simple cubic perovskite structure, these compounds are in fact very complex. For example, in (CH3NH3)PbI3, the dynamics associated with the (CH3NH3)<sup>+</sup> ions are still not fully understood, although ab initio calculations show [46] that at room and higher temperature, the rotation of CH3NH3 molecules can be viewed as effectively giving local structures that are cubic- and tetragonal-like from the point of view of the PbI3 framework, though

#### **Figure 13.**

*(a) View of the photodetector device, including a crystal of MAPbBr3 grown between two pre-patterned electrodes, separated apart by 10 μm. (b) Current-voltage (I-V) curves obtained with 505-nm illumination, by changing the incident optical power densities. (c) Spectrum displaying the responsivity for different wavelengths. (d) Generated photocurrent vs. time, when a modulated illumination is applied with a fixed voltage [32].*

**107**

**6. Conclusions**

**Acknowledgements**

and PROICO 2-2016), Argentine.

*Structural Phase Transitions of Hybrid Perovskites CH3NH3PbX3 (X = Br, Cl) from Synchrotron…*

in fact having lower symmetry. These arrangements are locally polar, with sizable

tural transitions are thus analogous to the transitions between two ferroelectric structures, where there is strong screening of charged defects that can lead to

This observation seems to confirm that the increase in the Eg with the amount of Cl<sup>−</sup> is closely correlated with the effect of the MA and its freedom degree in the inorganic framework. The change in the MA delocalization upon the incorporation of smaller Cl<sup>−</sup> anions is an additional parameter to be considered in the tuning of

Furthermore, the potential of these materials in optoelectronic applications can be exemplified for MAPbBr3 measuring the current-voltage (I-V) in a photodetector device that is illustrated in **Figure 13a** [32]. The effect of the optical power densities under illumination at 505 nm in the I-V curves is plotted in **Figure 13b**. This device shows a photoresponse among the best ones reported for other perovskite-based photodetectors, even considering different geometries: 2D-MAPbI3 [47, 48], thin films [49, 50], nanowires [51, 52], and networks [53]. Starting from I-V measurements using different wavelengths from alternative light sources, always keeping

responsivity spectrum of the photodetector, by using the next formula:

comparable with values previously reported [47, 48, 54, 55].

transitions below RT, and the conformation of the CH3NH3

halide perovskites with tunable properties for solar cell technologies.

with R being the responsivity, Iph the generated photocurrent, and P the illumination power on the device. These data are illustrated in **Figure 13c**, they show a maximum in the responsivity (0.26 A/W) for light with wavelength of 505 nm, which is near the wavelength with maximum solar irradiance. **Figure 13d** plots the current through the device as a function of time with a fixed voltage under pulsed illumination. This illumination mode allows to characterize the response time of the photodetector, in this device <100 ms (limited by the experimental setup), which is

This chapter reviews recent structural results on the CH3NH3(Br1−xClx)3 series. The combination of SXRD and NPD has permitted to address issues related to phase

ganic cages, formed by corner-sharing PBX6 octahedra. Interestingly, it was observed that the progressive localization of the organic units is not only achieved through a reduction in symmetry (from cubic to tetragonal, and finally to orthorhombic), but also simply as a consequence of the contraction of the cubic unit cells as the Cl contents increase. This can be interpreted as an evolution of the X—H hydrogen bonds, as previously observed from DFT calculations. In summary, we have contributed with novel experimental evidence that facilitates a more reasonable design of hybrid

This work was supported by the Spanish MINECO for funding MAT2017- 84496-R. The authors thank ALBA and ILL for making the facilities available. CAL acknowledges ANPCyT and UNSL for financial support (projects PICT2017-1842

due to the dipoles on the organic part [46]. The struc-

, it is possible to calculate the wavelength

R = Iph/P (4)

+

units within the inor-

*DOI: http://dx.doi.org/10.5772/intechopen.91421*

enhanced mobility and charge collection.

the optical properties of these hybrid materials.

the same illumination power of 1 mW/cm2

polarization, ~10 μC/cm<sup>2</sup>

*Structural Phase Transitions of Hybrid Perovskites CH3NH3PbX3 (X = Br, Cl) from Synchrotron… DOI: http://dx.doi.org/10.5772/intechopen.91421*

in fact having lower symmetry. These arrangements are locally polar, with sizable polarization, ~10 μC/cm<sup>2</sup> due to the dipoles on the organic part [46]. The structural transitions are thus analogous to the transitions between two ferroelectric structures, where there is strong screening of charged defects that can lead to enhanced mobility and charge collection.

This observation seems to confirm that the increase in the Eg with the amount of Cl<sup>−</sup> is closely correlated with the effect of the MA and its freedom degree in the inorganic framework. The change in the MA delocalization upon the incorporation of smaller Cl<sup>−</sup> anions is an additional parameter to be considered in the tuning of the optical properties of these hybrid materials.

Furthermore, the potential of these materials in optoelectronic applications can be exemplified for MAPbBr3 measuring the current-voltage (I-V) in a photodetector device that is illustrated in **Figure 13a** [32]. The effect of the optical power densities under illumination at 505 nm in the I-V curves is plotted in **Figure 13b**. This device shows a photoresponse among the best ones reported for other perovskite-based photodetectors, even considering different geometries: 2D-MAPbI3 [47, 48], thin films [49, 50], nanowires [51, 52], and networks [53]. Starting from I-V measurements using different wavelengths from alternative light sources, always keeping the same illumination power of 1 mW/cm2 , it is possible to calculate the wavelength responsivity spectrum of the photodetector, by using the next formula:

$$\mathbf{R} = \mathbf{I}\_{\text{ph}} / \mathbf{P} \tag{4}$$

with R being the responsivity, Iph the generated photocurrent, and P the illumination power on the device. These data are illustrated in **Figure 13c**, they show a maximum in the responsivity (0.26 A/W) for light with wavelength of 505 nm, which is near the wavelength with maximum solar irradiance. **Figure 13d** plots the current through the device as a function of time with a fixed voltage under pulsed illumination. This illumination mode allows to characterize the response time of the photodetector, in this device <100 ms (limited by the experimental setup), which is comparable with values previously reported [47, 48, 54, 55].

### **6. Conclusions**

*Perovskite and Piezoelectric Materials*

ated with the (CH3NH3)<sup>+</sup>

size, with a lower contribution by *p*Pb-*p*X and *s*X-*p*X transitions. These latter contributions decrease from iodine to bromine and then to chlorine, that is, when increasing the electronegativity of the halide, which accounts for the variation of the gap with the chemical nature of X, as shown in **Figures 11** and **12**. Endres et al. [45] found a remarkably low DOS at the valence band of MAPbX3, from either ultraviolet and inverse photoemission spectroscopies or from theoretical densities of states (DOS), calculated via density functional theory. They found a strong band dispersion at the valence band due to the coupling between halide *p* and Pb *s* antibonding orbitals, observing the absence of significant densities of states tailing into the perovskite gaps. As already mentioned, an interesting feature is that the compositional evolution of the band gap energy (Eg) does not possesses a linear behavior; it seems that the deviation from linearity is reminiscent of that unveiled for the variation of the unit-cell parameters at RT (see **Figure 12**). By additionally contrasting the Eg value with the Pb–X bond lengths, a non-linear behavior is apparent, suggesting that the evolution of Eg is not only related to the inorganic features. From this point of view, it is interesting to consider that, although all these studies are based on the nominally simple cubic perovskite structure, these compounds are in fact very complex. For example, in (CH3NH3)PbI3, the dynamics associ-

calculations show [46] that at room and higher temperature, the rotation of CH3NH3 molecules can be viewed as effectively giving local structures that are cubic- and tetragonal-like from the point of view of the PbI3 framework, though

*(a) View of the photodetector device, including a crystal of MAPbBr3 grown between two pre-patterned electrodes, separated apart by 10 μm. (b) Current-voltage (I-V) curves obtained with 505-nm illumination, by changing the incident optical power densities. (c) Spectrum displaying the responsivity for different wavelengths. (d) Generated photocurrent vs. time, when a modulated illumination is applied with a fixed* 

ions are still not fully understood, although ab initio

**106**

**Figure 13.**

*voltage [32].*

This chapter reviews recent structural results on the CH3NH3(Br1−xClx)3 series. The combination of SXRD and NPD has permitted to address issues related to phase transitions below RT, and the conformation of the CH3NH3 + units within the inorganic cages, formed by corner-sharing PBX6 octahedra. Interestingly, it was observed that the progressive localization of the organic units is not only achieved through a reduction in symmetry (from cubic to tetragonal, and finally to orthorhombic), but also simply as a consequence of the contraction of the cubic unit cells as the Cl contents increase. This can be interpreted as an evolution of the X—H hydrogen bonds, as previously observed from DFT calculations. In summary, we have contributed with novel experimental evidence that facilitates a more reasonable design of hybrid halide perovskites with tunable properties for solar cell technologies.

### **Acknowledgements**

This work was supported by the Spanish MINECO for funding MAT2017- 84496-R. The authors thank ALBA and ILL for making the facilities available. CAL acknowledges ANPCyT and UNSL for financial support (projects PICT2017-1842 and PROICO 2-2016), Argentine.

We acknowledge support for the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).
