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A Critical Comparison of FAPbX and MAPbX (X = Br and Cl): How Do They Differ? 3

Sharada Govinda, Bhushan P Kore, Diptikanta Swain, Akmal Hossain, Chandan De, Tayur N. Guru Row, and D. D. Sarma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00602 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018

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The Journal of Physical Chemistry

A Critical Comparison of FAPbX3 and MAPbX3 (X = Br and Cl): How Do They Differ? Sharada Govinda,† Bhushan P. Kore, † Diptikanta Swain, † Akmal Hossain, † Chandan De, ‡ Tayur N. Guru Row† and D. D. Sarma† * † Solid

State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru-560012, India.

‡ Jawaharlal

Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru-560064, India.

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ABSTRACT: Dielectric measurements on formamidinium lead halide perovskites, FAPbCl3 and FAPbBr3, compared to those of MAPbCl3 and earlier reported MAPbBr3, reveal strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140 – 300 K. Although the behavior of dielectric constants of FA compounds for temperatures < 140 K resemble that of the MAPbX3 system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature dependent single crystal XRD results.

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INTRODUCTION Organic-inorganic hybrid perovskites have sparked an enormous excitement among researchers in the last few years, with a promise to deliver low-cost, solution-processable solar cells1-8 with the best efficiency till-date crossing 22%. A unique feature of these hybrid perovskites is the presence of a molecular organic cation at the A site of the ABX3 perovskite structure, usually with a permanent dipole moment. The most celebrated in this class of compounds are the methylammonium lead halides, with the organic methylammonium (MA) cation occupying the center of the lead-halide cage. All-inorganic analogues of this series, cesium lead halides have also attracted attention of the community9 for their enhanced stability over their hybrid counterparts in photovoltaic applications. Among organic ions, formamidinium ion, (CH(NH2)2)+, is one of the few that are known to fit into the lead halide cage, retaining the perovskite structure10. Recently, formamidinium lead halides (FAPbX3) are being increasingly investigated for their photovoltaic efficiencies and optical properties11-13, since they show similar band gaps as their methylammonium counterparts (MAPbX3), and have been used along with MA ions to achieve higher efficiencies and enhanced stabilities of photovoltaic cells, these two aspects being perceived as the most important factors in determining the future applicability of these materials. MAPbX3 compounds are known to exhibit successive phase transitions14, typically going through cubictetragonal-orthorhombic crystal phases with a decrease in temperature. Subsequent to early controversies, particularly related to MAPbI3, concerning the effective crystal structure being polar or nonpolar in these phases, it is now generally believed that the structure is nonpolar in all three phases15. Centrosymmetric nonpolar structure in the orthorhombic phase arises from an antiferroelectric ordering of the asymmetric MA units with permanent dipoles. However, in the high temperature tetragonal and cubic phases, the MA ions are dynamically disordered with a time-scale of rotation in the range of a few picoseconds, as estimated by various experiments14,

16-24

and calculations24-31, leading to the average

nonpolar structure. The presence of permanent dipole moments on the MA units and their rotational time-scale, shorter than the excitonic life-time32-33, have naturally raised interesting questions regarding possible roles of these fast rotating dipoles in giving rise to spectacular properties of these systems24, 28, 30

. With an increasing focus on systems with the organic FA substituting the MA units, it is reasonable

to expect that FAPbX3 systems would also be investigated to understand their crystal structures and the dynamical behavior of the organic units as a function of the temperature and in comparison to those of the corresponding MAPbX3 analogues. Thus, it is surprising to find a near absence of such contrasting 3



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investigation of MAPbX3 and FAPbX3, with only a few sporadic reports34-40 on FAPbI3 alone. In the present work, we report our detailed investigations of structures and dielectric properties of FAPbCl3, FAPbBr3, and MAPbCl3, together with those of earlier reported24 MAPbBr3 in order to present a critical comparison between FA and MA analogues of these hybrid perovskite compounds. In passing, we also compare properties of these hybrid perovskites with those reported24 for the all-inorganic analogue, CsPbBr3, as it helps to define contributions of the PbBr3 cage to the temperature dependent dielectric constant in absence of any permanent dipole at the A site in contrast to the case of the hybrid compounds.24,

41

Our results reveal qualitative differences in the behaviors of these two organic

moieties, MA and FA, in these two series of compounds. We note that the differing behaviors of the dipoles on MA and FA will have important consequences on relevant physical properties that control the ultimate photovoltaic properties of these materials; for example, the presence of the dipoles at the microscopic level and their contributions to determine the dielectric constant at the macroscopic level of the material will influence the excitonic binding energies as well as charge transports, as suggested by experiments.42 EXPERIMENTAL METHODS (a) Synthesis of samples. The samples were synthesized by drop-casting an equimolar solution containing PbX2 and MAX or FAX precursors, from procedures slightly modified from the methods described in Refs 12, 43-45 as described below. Synthesis of precursors: Methylammonium halide (MACl): Equimolar amount of methylamine (40% in water) and concentrated HCl (35% in water) were mixed and stirred for two hours at 0 °C. The resultant mixture was then distilled at about 40-50 °C to evaporate the solvent and a white precipitate was obtained. The precipitate was washed with diethyl ether several (7-8) times before drying in vacuum for 2-3 hours. The white solid (MACl) was stored in vacuum desiccator or nitrogen-filled glove box. Formamidinium halide (FAX): For the synthesis of FAX, equimolar amount of concentrated acid HX was reacted with formamidine acetate with continuous stirring for two hours at 0 °C. Concentrated HCl (35% in water) and HBr (48% in water) were used for the synthesis of FACl and FABr, respectively. The resultant mixture was distilled at 40-50 °C and the white precipitate obtained was washed several

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times with diethyl ether. The obtained product was dried in vacuum and stored in vacuum desiccator or nitrogen filled glove box. Synthesis of hybrid halide perovskite samples: Synthesis of MAPbCl3: MAPbCl3 was prepared from an equimolar solution of lead chloride and methylammonium chloride. 0.5 M solution of PbCl2 and MACl was prepared in dimethyl sulfoxide (DMSO) at room temperature. This solution was then drop-cast on glass slides at 100 °C. After the solvent evaporated in 8-10 minutes, white powder or crystals were formed. The powder and crystals were collected and stored in vacuum. Synthesis of FAPbCl3: Formamidinium chloride and lead chloride were used as precursors to synthesize FAPbCl3. FACl and PbCl2 were dissolved in 1:1 ratio in N, N-dimethyl formamide (DMF) to make a 0.5 M solution. This solution was drop-cast on glass slides kept at 100 °C and heated at the same temperature for 10 minutes in order to dry the solvent. White powder and crystals formed were collected and stored. The whole procedure was carried out inside a nitrogen-filled glove box. Small crystals obtained thus were carefully chosen for single-crystal XRD measurements. Synthesis of FAPbBr3: FAPbBr3 was prepared from equimolar solution containing lead bromide and formamidinium bromide. PbBr2 and FABr were dissolved in DMF at room temperature, to get a 1 M solution. This solution was drop-cast on glass slides kept at 165 °C inside a glove box. The substrates were heated for 20 minutes and then slowly cooled to room temperature. Small crystals and powder were collected from the glass slides. Single crystals of FAPbBr3 were grown from a 1 M solution containing 1:1 PbBr2 and FABr in 1: 1 DMF: GBL (γ-butyrolactone), similar to the procedure described in Ref 44. The solution was heated slowly from 40 °C to 55 °C and left for about three hours and orange crystals of FAPbBr3 were obtained. These were washed with n-hexane and dried in vacuum before storing in vacuum desiccators. The ground powder of the respective samples was pressed into pellets of 6 mm diameter in a hydraulic press with a pressure of about 0.15 kN/cm2. The pellets were annealed at 85-95 °C in argon or nitrogen atmosphere for one hour and were then used for P-E loop and dielectric measurements. Silver paint or paste was used to make the electrodes on either face of the pellets.

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(b) Powder X-Ray Diffraction. Powder XRD pattern for all the synthesized samples were recorded to check the phase purity. Phase purity was also checked after annealing of the pellets, to check for degradation of these samples or any signs of decomposition. Powder XRD pattern was recorded in PANalytical X-Ray Diffractometer with Cu Kα source. This was compared with the XRD pattern reported in literature, whenever available. (c) Differential Scanning Calorimetry (DSC). DSC measurements were performed on polycrystalline samples of MAPbCl3 and FAPbX3 to check for phase transitions and their corresponding temperatures. DSC measurements were performed using Mettler Toledo DSC823 system. About 10 mg of the sample and a reference were cooled from room temperature to 133 K and then heated to 300 K, at a rate of 3˚C per minute in nitrogen atmosphere to record DSC curves. (d) Temperature Dependent Single-Crystal XRD. Single crystal XRD experiments were performed using Oxford Xcalibur diffractometer with Mo Kα radiation (λ = 0.71073 Å) which was operated at 50 kV and 1 mA on FAPbBr3 at 298 K, 185 K and 100 K and on FAPbCl3 at 298 K, 200 K and 100 K. The single crystals were mounted on a Hampton cryo-loop. Care was taken to avoid exposure of the crystals to atmosphere by smearing them with paratone-N oil. The sample was maintained at the desired temperature under a stream of nitrogen gas from an Oxford cryosystems open-flow cryostat using liquid nitrogen (COBRA).

Data collection, data reduction and numerical absorption corrections were

performed using the programs in the CrysAlisPro software suite. (e) Spontaneous polarization (P-E loop). P-E loop was measured on polycrystalline pellets (prepared as described earlier in the synthesis procedure) of FAPbCl3 and FAPbBr3 (of thickness ~ 0.4 – 0.5 mm) using Radiant Technology Precision Multiferroic II Ferroelectric Test System by pulses of 1 kHz. P-E loop was measured at room temperature by immersing the sample in silicone oil and that at 80 K, by immersing the sample in liquid nitrogen. (f) Dielectric measurements. Pellets of FAPbBr3, FAPbCl3 and MAPbCl3 about 0.5-0.7 mm thick, prepared as described in the synthesis procedure were used for dielectric measurements. Dielectric measurements were performed in a closed cycle helium cryostat (Cryo Industries Inc) using Keysight E4990A Impedance Analyzer. Dielectric measurements were performed in the frequency range 1 kHz-1 MHz by cooling the sample from room temperature to 10 K, in steps of 5 K, and in steps of 2 K near phase transitions. The sample temperature was stabilized for sufficiently long before measuring, at each 6


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temperature and a ramp rate of 0.5 K per minute was used for cooling the sample between two temperatures. Measurements were also performed in the heating cycle in order to check for reproducibility and the measured dielectric constants overlapped with that in the cooling cycle, except for some hysteresis of a few K near the phase transitions, indicative of the first-order nature of the structural transition responsible for changes in the dielectric constants. Measurements were also carried out with applied DC voltage at several temperatures in the range 10 K – 300 K in order to check for space charge effects arising from external contributions as in electrodes. No effect from space charge polarization was found in any sample, since all dielectric data overlapped with the data at zero applied voltage within the relevant frequency range.

RESULTS AND DISCUSSION MAPbX3 with X = I, Br and Cl are known14 to have several crystallographic phase transitions as a function of temperature, typically spanning through cubic-tetragonal-orthorhombic phases with decreasing temperature. In order to identify the transition temperatures and relevant temperature ranges for different crystallographic phases, we performed Differential Scanning Calorimetry (DSC) measurements on FAPbBr3 and FAPbCl3 together with that on MAPbCl3 with the results shown in Figures S2 and S3 of Supporting Information (SI). These curves show peaks both in the cooling and heating cycles, corresponding to reversible phase transitions. Guided by the DSC data of FAPbBr3 (Figure S3(a)), three temperatures were selected for structure determination from single crystal XRD, namely, room temperature, 185 K and 100 K. While 100 K measurement provides information on the low temperature crystallographic phase of FAPbBr3, there is no phase transition indicated by the DSC data between room temperature and 185 K. However, structure determination using data collected at 185 K was carried out since disorder due to thermal effects will be reduced. The room temperature structure of FAPbBr3, shown in Figure 1(a), could be refined in a cubic structure with space group Pm-3m. The lattice parameters obtained are given in Table 1. Schueller et. al., have recently reported46 the temperature dependent crystal structure of FAPbBr3 using powder X-ray diffraction. Our room temperature structure agrees with the cubic room temperature structure reported there and also with other reports of room temperature cubic phase 13, 44. The room temperature structure 7



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of the cubic perovskite phase of FAPbI3 obtained by neutron diffraction34 suggests that FA ion is disordered over 12 equivalent positions. This model was used to refine the single crystal X-ray diffraction data for FAPbBr3; results obtained from this refinement suggests similarly disordered structure for FAPbBr3 at the room temperature, as shown in Figure 1(a), for carbon and nitrogen atoms, however, hydrogen atoms could not be located as can be expected. Interestingly, the thermal motion of the bromine atoms is more in the direction perpendicular to the Pb-Br bond which is similar to that observed for iodine in FAPbI3 in the literature38. The PbBr6 octahedra are undistorted (Br-Pb-Br angle =180) with a Pb-Br distance 3.007Å.

(a) 298 K

(b) 100 K

Figure 1. Crystal structure of FAPbBr3 at (a) room temperature and (b) 100 K obtained by single crystal XRD.

The single crystal XRD data collected at 185 K also refined in a cubic structure with the same space group Pm-3m, as for the room temperature structure. The crystallographic parameters are detailed in Table 1. The nitrogen atoms are disordered in 24 positions and the carbon in 6 positions, akin to that observed at room temperature. Here we note that MAPbBr3 also crystallizes in the cubic Pm-3m structure at the room temperature, with the methylammonium ions disordered17, 47. The lattice parameter in the case of cubic FAPbBr3 is slightly larger than that of MAPbBr3, which can be expected due to the larger size of the FA ion compared to that of MA10. As the temperature is lowered, MAPbBr3 undergoes

8


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phase transitions14, 18 into tetragonal phases where the disorder in the MA ions is reduced with fewer equivalent positions of the C and N atoms18. Table 1: Single crystal XRD data of FAPbBr3 Formula Formula weight Color Crystal form Crystal size (mm)

CH(NH2)2PbBr3 491.99 Orange Cuboid 0.150.110.09



Temperature (K)

295K

185K

100 K

Radiation Wavelength (Å) Crystal system Space group a (Å) b (Å) c (Å) Volume (Å3) Z  (mm-1) No. Unique Reflns. No. of parameters

Mo K 0.7107 Cubic Pm-3m 6.0134(3) 6.0134(3) 6.0134(3) 217.450(19) 1 33.090 91 9

Mo K 0.7107 Cubic Pm-3m 5.9582(2) 5.9582(2) 5.9582(2) 211.517(12) 1 34.018 90 9

Mo K 0.7107 Trigonal R-3 8.3470(5) 8.3470(5) 10.2180(7) 616.53(7) 3 35.012 345 15

CH(NH2)2PbBr3 491.99 Orange Cuboid 0.150.110.09 295K (Heating from 100K) Mo K 0.7107 Cubic Pm-3m 5.9916(3) 5.9916(3) 5.9916(3) 215.094(19) 1 33.452 90 9

R[F2>2σ(F2)], wR2(F2 )

0.0273, 0.0614

0.0197, 0.0473

min, max (eÅ–3) GooF (S)

-0.899, 0.0623 1.183

-0.647, 0.869 1.100

0.0369, 0.0842 -1.812, 2.074 1.109

0.0336, 0.0684 -1.051,1.369 1.175

Based on powder X-ray diffraction data, Schueller et. al., have recently reported46 a cubic-tetragonal phase transition between 275-250 K and an orthorhombic Pnma structure at 100 K in FAPbBr3. In contrast, we observe no phase transition from the room temperature cubic phase down to 185 K, also consistent with our DSC results. Moreover, our single crystal diffraction data at 100 K could be refined in R-3 space group (hexagonal setting), establishing this to be the low temperature crystal structure below the phase transition at about 162 K as suggested by DSC data (Figure S3(a)). Attempts to index the single crystal reciprocal lattice points in either a tetragonal or an orthorhombic phase were unsuccessful, as many reflections in our data remained unaccounted for (see Figure S4). The basic 9



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perovskite motif with corner-shared PbBr6 octahedra is intact with a reduction of Pb-Br distance (2.951Å). The structure obtained at 100 K is shown in Figure 1(b). Even at 100 K, the formamidinium ions were found to be disordered, however associated with only six equivalent positions for the nitrogen atoms and two for the carbon atoms as shown in Figure 1(b) with the lattice parameters and details of refinement shown in Table 1. Thus, there are distinct differences between the low temperature R-3 structure of FAPbBr3 with disordered FA ions and the low temperature orthorhombic phase of MAPbBr3, with ordered MA ions48, arranged in an antiferroelectric fashion. Guided by the DSC result shown in Figure S3(b), single crystal XRD of FAPbCl3 was measured at 298, 200 and 100 K. The room temperature structure of FAPbCl3 is refined in a cubic crystal system with a centrosymmetric space group Pm-3m (see Figure 2(a), Table 2), same as in the room temperature phase of FAPbBr3 discussed above. The organic cation (CH(NH2)2)+ is found to be disordered with carbon and nitrogen positions distributed in 6 and 24 equivalent positions, respectively. The Pb-Cl-Pb angle is 180o and Pb-Cl distance is 2.869Å in the Pm-3m structure. It is important to note that the thermal motion of Cl ion is highly anisotropic, being perpendicular to the Pb-Cl direction; similar observations have been made earlier in the literature38, 46 for FAPbI3 and FAPbBr3, and also discussed for FAPbBr3 in this work.

(a) 295 K

(b) 200 K

Figure 2. Crystal structure of FAPbCl3 at (a) room temperature and (b) 200 K. 10


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Table 2: Single crystal XRD data of FAPbCl3 Formula Formula weight Color Crystal form Crystal size (mm)

CH(NH2)2PbCl3 358.61 Colorless Cuboid 0.050.040.025



Temperature (K)

295K

200K

100K

Radiation Wavelength (Å) Crystal system Space group a (Å) b (Å) c (Å) Volume (Å3) Z  (mm-1) No. Unique Reflns. No. of parameters R[F2>2σ(F2)], wR2(F2 ) min, max (eÅ–3) GooF (S)

Mo K 0.7107 Cubic Pm-3m 5.7379(3) 5.7379(3) 5.7379(3) 188.91(2) 1 23.285 85 9

Mo K 0.7107 Orthorhombic Cmcm 8.7940(3) 7.3690(4) 11.4231(5) 740.25(6) 4 23.767 550 22

Mo K 0.7107 Orthorhombic Cmcm 8.8606(8) 7.2570(4) 11.4162(9) 734.08(9) 4 23.967 394 22

CH(NH2)2PbCl3 358.61 Colorless Cuboid 0.050.040.025 295K (After heating from 100K) Mo K 0.7107 Cubic Pm-3m 5.7360(5) 5.7360(5) 5.7360(5) 188.72(3) 1 23.308 82 9

0.0220, 0.0401

0.0593, 0.1020

0.0951, 0.2323

0.0216, 0.0380

-0.582, 0.652 1.122

-3.284, 2.624 0.990

-2.237, 3.630 1.013

-0.520, 0.472 1.195

A structural phase transition was observed on in situ cooling of the crystal to 200K (consistent with the DSC measurements (Figure S3b)) and the indexed space group is Cmcm.

The possibility of the

corresponding non-centrosymmetric space group (Cmc21) was negated based on the P-E loop measurements at liquid nitrogen temperature discussed in the next section. It is important to note that multiple twin domains are observed as the crystal underwent transition from cubic to orthorhombic phase and the diffraction data were processed by using the twin option provided in the software CrysAlisPro. The asymmetric unit of the orthorhombic phase contains one Pb, two Cl, one C and one N atoms. The organic cation (CH(NH2)2)+ is again found to be disordered with fewer distribution of carbon and nitrogen positions compared to the room temperature cubic phase. It is observed that the N-N axis of (CH(NH2)2)+ is along the a-direction of the crystal and the carbon atom positions are twofold related 11



about this axis as given in figure 2(b). The reduction in symmetry induces the PbCl6 octahedra to be distorted with three different Pb-Cl distances (2.728, 2.864 and 3.046 Å) and two different Cl-Pb-Cl angles (171.2, 166.9) (Figure 2b). Further, on cooling the sample to 100 K, multiple twin domains increase; however, the structure could still be solved in Cmcm space group with R1=0.0951. The cubic, room temperature structure of FAPbCl3, is similar to that of MAPbCl3, where the organic ion is disordered in the middle of the (PbCl3)- cage17-18,

47.

Lattice parameter in the case of FAPbCl3 is

observed to be slightly larger than that of MAPbCl3, because of larger FA ions. MAPbCl3 undergoes a phase transition into a tetragonal phase below 179 K where the MA ion is still disordered, but to a lower extent compared to the cubic phase18. The MA ions, however, order49 in an antiferroelectric arrangement in the low temperature orthorhombic phase below 172 K. In contrast, the FA ions are observed to be disordered even in the low temperature orthorhombic phase. Spontaneous electric polarization was measured on a polycrystalline pellet of FAPbCl3 in the orthorhombic phase at ~ 80 K. Since there is no phase transition observed between 200 and 100 K (see Figure S3(b)), P-E loop measurement carried out at 80 K would also represent the nature of the sample at 200 K. Measured polarization versus applied electric field is shown in Figure 3. It can be seen that the

Measured Polarization (C/cm2)

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0.6

FAPbCl3 at 80 K

0.4 0.2 0.0 -0.2 -0.4 -0.6

1 kHz

-100000

-50000

0

50000

100000

Applied electic field (V/cm)

Figure 3: Polarization versus electric field measured on polycrystalline pellet of FAPbCl3 at 80 K.

P-E loop is approximately linear, with no spontaneous polarization and also no evidence of a saturation polarization. The existence of a slight loop in the P-E measurement (Figure 3) arises from dielectric losses in the sample. However, the P-E loop exhibits only a very narrow width and the closure of the loop is nearly complete, evidencing that the dielectric loss in the sample is low. This loop is clearly 12


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different from the P-E loop of any ferroelectric material, which shows a hysteresis and a saturation polarization. Thus, we conclude that FAPbCl3 is not a ferroelectric in the low temperature orthorhombic phase, consistent with the assignment of the centrosymmetric, nonpolar space group of Cmcm. We also measured electric polarization of FAPbBr3 in terms of such P-E loop measurements and the result, obtained at 80 K for the lowest temperature phase, is shown in Figure S5 (see Supporting Information). Hysteresis, characteristic of a ferroelectric material is not observed in this P-E loop, although the loop is wider compared to FAPbCl3, due to a higher dielectric loss in the sample. A comparison of dielectric constant versus temperature measured on MAPbBr3, FAPbBr3 and CsPbBr3 is shown in Figure 4(a), for a particular AC frequency of 1.08 MHz. We have specifically chosen the high frequency of about 1 MHz, as extrinsic contributions are known to be suppressed at increasingly higher frequencies24, 50, thereby revealing the intrinsic properties. The dielectric data for MAPbBr3 and CsPbBr3 are taken from our earlier publication24 for the purpose of comparison, while the data for FAPbBr3 is new here. As already shown in Ref 24 the rapidly increasing ε′ of MAPbBr3 with a decrease in temperature from the room temperature, roughly varying as 1/T, is primarily contributed by the nearly freely rotating MA+ dipoles within the PbBr3 cage and can be described well as freely rotating dipoles in a dielectric medium. This behavior, seen down to 155 K in MAPbBr3, is totally absent in CsPbBr3, consistent with the absence of any permanent rotating dipole in this case, as already discussed in Ref 24. The nearly temperature independent ε′ of CsPbBr3, is quite similar to the low temperature behavior of ε′ in MAPbBr3 below ~ 120 K, where MAPbBr3 is in the orthorhombic phase. This similarity and the relatively temperature independent behavior of ε′ has been attributed to loss of the dynamic rotation of MA+ dipoles in this low temperature phase, due to an ordering of the dipoles in an antiferroelectric arrangement, leading to no net dipole moment; thus, it is not surprising to find similar ε′ behavior, presumably dominated by contributions from the (PbBr3)- cage, in both MAPbBr3 and CsPbBr3, with an ordered arrangement of MA+ units cancelling the dipolar contribution and also removing the ability of the dipoles to rotate freely in MAPbBr3. The comparison of ε′ of FAPbBr3 with MAPbBr3 and CsPbBr3 in Figure 4(a) makes it at once clear that the behavior of the FAPbBr3 is similar to that of CsPbBr3 and that of MAPbBr3 in the low temperature phase, where the dipoles have lost their ability to rotate freely and consequently, also to respond to applied electric fields as readily as in the higher temperature phase of MAPbBr3 with free dipoles, thereby suppressing the dielectric constant as well as its temperature dependence. The most spectacular 13



(a) 80

40

MAPbBr3 FAPbBr3

(b) FAPbBr3

38

CsPbBr3

60 36

'

'

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40

34

20

10.4 kHz 22.3 kHz 101 kHz 1.08 MHz

32 1.08 MHz

0 50

100

150

200

250

300

50

Temperature (K)

100

150

200

250

300

Temperature (K)

Figure 4. (a) Comparison of dielectric constant versus temperature measured on MAPbBr3, FAPbBr3 and CsPbBr3 at a probe AC frequency of 1.08 MHz. The data for MAPbBr3 and CsPbBr3 are taken from Ref 24. (b)

Dielectric constant as a function of temperature, measured on FAPbBr3, shown for specified frequencies as mentioned in the figure. difference between ε′ of FAPbBr3 and MAPbBr3 is the complete absence of the rapidly increasing ε′ with decreasing temperature observed in the higher temperature regime of MAPbBr3. This is a clear evidence that, unlike MA+ dipoles, FA+ dipoles are not rotating freely within the probe time-scale (~ 1 μs) even up to the room temperature. Considering the similar temperature independence and magnitude of ε′ of FAPbBr3 compared to CsPbBr3 and within the low temperature phase of MAPbBr3, it appears that the dipoles on FA+ are not free to rotate. We note that the single crystal XRD of FAPbBr3, presented above, suggested a disordered arrangement of FA ions, unlike the antiferroelectric ordered arrangement of MA ions in MAPbBr3 in the low temperature phase. These evidences of a disordered arrangement of the FA ions and its inability to rotate freely in FAPbBr3 suggest that the FA ions are randomly frozen in different equivalent orientations, giving rise to a glassy state. It also appears that a glassy, deep-frozen state of the dipoles with only very slow dynamics as in FAPbBr3 and an ordered antiferroelectric state of the dipoles as in MAPbBr3, behave similarly in terms of minor and relatively temperature independent contributions to the dielectric constants of their respective systems. We now turn to discussion of ε′(T) for FAPbBr3 in detail. Since such details are suppressed in the plot of Figure 4(a), covering a wide range of ε′ values, we plot ε′ of FAPbBr3 in an expanded scale in Figure 4(b). Figure 4(b) shows several features in the dielectric constant of FAPbBr3, not obvious in Figure 14


Page 15 of 24

4(a). ε′ exhibits an overall decreasing trend in most of the temperature range, decreasing continuously as the sample is cooled from 300 K to 163 K. Between 163 K and 157 K, the dielectric constant increases abruptly and then continues to decrease again down to 142 K, where it shows an abrupt decrease on further cooling. Both these abrupt changes, at about 162 K and 142 K, are correlated with peaks shown in DSC (Figure S3(a)), suggesting these 1.5 and 3.7% changes in ε′ are due to the first order transitions that modify the (PbBr3)- cage of FAPbBr3. Below 138 K, ε′ of FAPbBr3 continues the decreasing trend down to 12 K. ε′ shows relatively little frequency dependence except above 260 K. Between 260 and 300 K, ε′ shows a clear trend of increasing with a decreasing frequency, strongly suggestive of extrinsic contributions24, 50.

80 60

10.4 kHz 10.4 kHz fit

80

(b) FAPbCl3

(a)

30

10.4 kHz 1.08 MHz 60

25

1.08 MHz 1.08 MHz fit

200

240 T (K)

10.4 kHz 22.3 kHz 101 kHz 1.08 MHz

280

MAPbCl3

'

100

'

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

40 15

20

1.08 MHz

FAPbCl3

0 50

100

150

10

200

250

Temperature (K)

300

50

100

150

200

250

300

Temperature (K)

Figure 5. (a) Comparison of dielectric constant versus temperature, ε′(T) measured on MAPbCl3 and FAPbCl3 measured at an AC frequency of 1.08 MHz. The plot also shows ε′(T) for MAPbCl3 measured at 10.4 kHz and the inset shows the fit to dielectric data in the cubic phase of MAPbCl3 using equation 1. (b) ε′(T) measured on FAPbCl3 shown for specified frequencies as mentioned in the figure. Similar to the case of the bromide in Figure 4(a), we show the comparison of dielectric constant versus temperature measured on MAPbCl3 and FAPbCl3 in Figure 5(a) with a probing frequency of 1.08 MHz. ε′ of MAPbCl3 shows negligible frequency dependence, as illustrated by also plotting ε′ measured at 10.4 kHz in the same figure. The temperature dependence of ε′ in MAPbCl3 is very similar to that of MAPbBr3 (see Figure 4(a)) and MAPbI3, reported earlier24. Specifically, the dielectric constant of MAPbCl3 increases as it is cooled from 300 K to 178 K. Below 178 K, the dielectric constant falls sharply and this temperature matches with the cubic-tetragonal transition temperature obtained from DSC (178 K). The dielectric constant decreases further in this tetragonal phase and falls abruptly again 15



Page 16 of 24

as the sample is cooled below 162 K, below the tetragonal-orthorhombic transition temperature. In the orthorhombic phase, the dielectric constant shows very little temperature dependence. The observed dielectric constant as a function of temperature agrees well with the reported trends and values51-52. As discussed in our previous work24, the nearly temperature independent dielectric constant of MAPbBr3 below 120 K and of MAPbI3 below 134 K in the respective orthorhombic phases, where the dipoles are ordered antiferroelectrically giving rise to no net dipole moment, is similar to that of CsPbBr3 with no dipole at all. A very similar trend is observed also in MAPbCl3 below 150 K, where it is in the orthorhombic phase, suggesting the same antiferroelectric ordered arrangement of the dipoles on MA+ ions. In MAPbCl3, a strong temperature dependence of the dielectric constant is seen above 162 K, which can be attributed to the contribution from freely rotating dipoles in close analogy to the cases of MAPbBr3 and MAPbI3 in their tetragonal and cubic phases. Since an analysis of MAPbBr3 and MAPbI3 in their high temperature phases in terms of freely rotating dipoles in a dielectric medium was successful qualitatively and quantitatively in describing ε′(T, ω), we present below an analogous analysis of ε′ of MAPbCl3. We have used the same model of Debye relaxation of dipoles, similar to our earlier work24, considering the relaxation time to be very small compared to the frequency of the applied electric field in order to explain the Curie-Weiss like (1/(T-T*)) behavior of ε′ of MAPbCl3 in the cubic phase. Since the data for MAPbCl3 in its tetragonal phase does not show a (1/(T-T*)) like behavior, it is likely that the MA+ dipoles are severely constrained from their higher temperature state in the cubic phase, unlike the case of high temperature MAPbBr3 and MAPbI3 tetragonal phases. The slight frequency dependence is explained by Maxwell-Wagner type relaxation, which accounts for an extrinsic contribution to the dielectric constant. The near absence of any frequency dependence of ε′, shown in Figure 5(a) for MAPbCl3, suggests negligible extrinsic contributions to the dielectric constant in our sample. Following our previous analysis24, we fit the dielectric data of MAPbCl3 within the cubic phase using equation 1 below.  '   

( ext   ext ) C sinh[  ln( ext )]   ext  s (1  ) T T * 2 cosh[  ln( ext )]  cos(  / 2)

………………..(1)

The fits to dielectric data at the lowest and highest frequencies analyzed are shown in the inset in Figure 5(a). From such fitting of the experimental results, the Debye and the Maxwell-Wagner contribution to the dielectric constant has been estimated to be a maximum of only ~ 0.9%, while the corresponding

16


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extrinsic contribution to MAPbBr3 in our earlier work was found to be 6%. The intrinsic parameters obtained for MAPbCl3 and MAPbBr3 are given in Table 3. Table 3: Intrinsic parameters obtained from fitting the dielectric constant versus temperature in the

cubic/ tetragonal phases of MAPbX3. MAPbCl3

MAPbBr3

C (K)

6450 ± 59

7218 ± 76

T* (K)

90.5 ± 3

48.2 ± 2.6

μ (Cm)

(7.33 ± 0.04) x 10-30

(8.60 ± 0.05) x 10-30

In the present sample, the dipole moment values was found to be in the range of 10-30 Cm, the slight variation (7-9)*10-30 Cm could be due to slightly different choices of C and T* values (See Supporting Information for details). Having analyzed the ε′ data of MAPbCl3 quantitatively, we now turn to compare critically ε′ of FAPbCl3 and MAPbCl3, shown in the same scale in Figure 5(a). The most evident difference between the two is obviously the complete absence of the 1/T type dependence of ε′ in the case of FAPbCl3, while it is the dominant high temperature behavior for MAPbCl3. Since this behavior of ε′ in MAPbCl3 arises solely from the nearly-freely rotating MA+ dipoles, it is self-evident that the FA+ dipoles do not rotate within the time scale of the probe, just as also concluded for FA+ dipoles in FAPbBr3 (see Figure 4(a)). Combining with the observation of the disordered FA+ ions from single crystal XRD of FAPbCl3, the comparison of ε′ data presented in Figure 5(a) suggests a frozen disordered or a glassy arrangement of these dipoles. The overall behavior of ε′ of FAPbCl3 as a function of temperature is very similar to that of MAPbCl3 in the low temperature orthorhombic phase. As already commented upon, MA+ dipoles in MAPbCl3 form an ordered antiferroelectric arrangement with little dynamics and no net dipole moment in this low temperature phase. Thus, it appears that the absence of any short time-scale rotation and a net zero dipole moment of the dipoles are the basic ingredients required to obtain the relatively flat ε′ with temperature, contributed dominantly by the (PbCl3)- cage.

17



Page 18 of 24

The dielectric constant of FAPbCl3 as a function of temperature is shown on an expanded scale in Figure 5(b). In this scale, the various features of ε′(T) are easily visible. For example, there is a clear discontinuous change in ε′ at ~ 270 K, corresponding to the phase transition in FAPbCl3 (see Figure S3(b)) between the high temperature cubic phase and the lower temperature phase. Another small discontinuous change in ε′ can be barely observed at about 264 K signaling the second transition evidenced also by the DSC data in Figure S3(b). Below this temperature, ε′ shows a smoothly decreasing trend with decreasing temperature. ε′ exhibits a slight frequency dependence near room temperature, arising most probably from a nearly-negligible extrinsic component. Over the remaining temperature range, we could not find any frequency dependent dispersion of ε′, as would normally be expected from a glassy state of the dipoles as concluded here. This relatively frequency independent ε′ in presence of a glassy state may be explained in two ways. First, if ε′ is dominated by contributions from the (PbCl3)- cage with relatively minor contributions from the disordered, frozen dipoles, indeed we would not expect any significant frequency dependence of ε′ unless ε′ contributed by the (PbCl3)- cage is itself frequency-dependent; however that does not appear to be the case since ε′ of MAPbCl3 in the low temperature limit, contributed largely by the (PbCl3)- cage, is essentially frequency independent. Second, if the glassy dynamics of the disordered dipoles is on a very slow time-scale, compared to the probing time-scale, as would happen in a deep frozen glassy state, the frequency dependence of ε′ may not be visible.

CONCLUSION Using temperature dependent single crystal XRD, we have determined the temperature dependent structure of FAPbX3 for X = Cl and Br. These results show that FA ions are disordered in the room temperature phase as well as at temperatures as low as 100 K for both samples. This is in contrast to the MA counterparts that have been reported earlier to have ordered MA ions with dipoles on these, arranged antiferroelectrically in the low temperature orthorhombic phase. Measurements on FAPbCl3 and FAPbBr3 show relatively less temperature dependence of dielectric constants in the low temperature range (below ~138 K) similar to that in the low temperature orthorhombic phases of MAPbX3 for X = Cl and Br. Dielectric constants of all MAPbX3 compounds are dominated by a strong temperature dependence within the room temperature phases that is described well by a 1/(T-T*) form, representing freely rotating dipoles on the organic MA+ ion; the most distinctive feature of ε′ of the corresponding FAPbX3 compounds is a complete absence of this behavior, evidencing absence of any rotational 18


Page 19 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dynamics of the dipoles on FA+ ions. Combined with XRD results and details of the ε′ in each case, we conclude that FAPbX3 (X = Br and Cl) have the dipoles in a deeply frozen glassy state at all temperatures below 300 K, in contrast to MAPbX3 compounds, which have nearly freely rotating dipoles in the high and room temperature phases with a largely ordered antiferroelectric structure of the dipoles in the low temperature phases.

ASSOCIATED CONTENT Supporting Information. Includes powder XRD patterns, DSC curves and details of dielectric data analysis. CIF files. AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Also at Jawaharlal Nehru Centre for Advanced Scientific Research, Bengaluru. ACKNOWLEDGMENT The authors thank Department of Science and Technology, Government of India for support, Mr. I. S. Jarali and Ms. Pooja Nayak for DSC measurements and Prof. A. Sundaresan for giving access to his P-E loop measurement sret-up. SG thanks Dr. Sumanta Mukherjee for useful discussions and acknowledges CSIR for a student fellowship. BPK acknowledge UGC, India for a D.S. Kothari Postdoctoral Fellowship. DDS thanks Jamsetji Tata Trust for support. REFERENCES 1.

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