Structural dynamics of CH3NH3+ and PbBr3− in tetragonal and cubic phases of CH3NH3PbBr3 hybrid perovskite by nuclear magnetic resonance

Understanding the structural dynamics of lead-halide perovskites is essential for their advanced use as photovoltaics. Here, the structural dynamics of the CH3NH3 cation and PbBr6 octahedra in the perovskite CH3NH3PbBr3 were studied via nuclear magnetic resonance (NMR) to determine the mechanism of the transition from the tetragonal to cubic phase. The chemical shifts were obtained by 1H, 13C, and 207Pb magic angle spinning NMR and 14N static NMR. The chemical shifts of the 1H nuclei in CH3 and NH3 remained constant with increasing temperature, whereas those of the 13C and 207Pb nuclei varied near the phase transition temperature (TC = 236 K), indicating that the structural environments of 13C and 207Pb change near TC. The spin–lattice relaxation time T1ρ values for 1H, 13C, and 207Pb nuclei increased with increasing temperature and did not exhibit an abrupt change near TC. In addition, the two lines in the 14N NMR spectra superposed into one line near TC, indicating the occurrence of a phase transition to a cubic phase with higher symmetry than tetragonal. Consequently, the main factor causing the phase transition from the tetragonal to cubic phase near TC is a change in the surroundings of the 207Pb nuclei in the PbBr6 octahedra and of the C–N groups in the CH3NH3 cations.

www.nature.com/scientificreports/ from the tetragonal II phase to the tetragonal III phase at 154 K, the full width at half maximum of the Raman ν 6 band shows an abrupt increase 8 . At lower temperatures, CH 3 NH 3 PbBr 3 undergoes a first-order structural phase transition from the tetragonal III (I4/mmm) phase to the orthorhombic IV (Pnma) phase 9 .
In a previous nuclear magnetic resonance (NMR) investigation, the temperature dependence of 81 Br nuclear quadrupole resonance frequencies and 1 H spin-lattice relaxation times in the laboratory frame T 1 for CH 3 NH 3 PbBr 3 were discussed by Xu et al. 23 According to their results, the two 81 Br NQR lines in phase II were reduced to one line in phase I. The discontinuity of the NQR line at this transition point implied a first-order transition. 1 H T 1 varied continuously, and no discernible change in the free induction decay was observed during the I-II transition. The phase transition had no significant effect on the motional state of the CH 3 NH 3 + ions. Furthermore, Baikie et al. 13 reported that the 1 H magic angle spinning (MAS) NMR spectra showed two clear peaks corresponding to the CH 3 and NH 3 environments in the high-temperature phase, and the 1 H and 13 C NMR spectra of CH 3 NH 3 PbBr 3 showed that the CH 3 NH 3 + units undergo dynamic reorientation. Measuring the spin-lattice relaxation time in the rotating frame T 1ρ by MAS NMR allows for the probing of molecular motion in the kHz range, whereas the spin-lattice relaxation time in the laboratory frame T 1 measured by static NMR reflects motion in the MHz range. Although the 1 H T 1 of CH 3 NH 3 PbBr 3 has been examined by a few research groups, the corresponding phenomena by 1 H, 13 C, and 207 Pb MAS NMR spectra and T 1ρ have not been fully studied. In addition, information regarding 14 N in the CH 3 NH 3 cation has not yet been discussed.
In the present study, the structural dynamics of the CH 3 NH 3 cation and PbBr 6 octahedra in CH 3 NH 3 PbBr 3 were studied in detail by NMR to resolve the phase transition mechanisms from the tetragonal phase to the higher-temperature cubic phase. The temperature dependences of the chemical shifts and spin-lattice relaxation time in the rotating frame T 1ρ were measured using 1 H MAS NMR, 13 C cross-polarization (CP)/MAS NMR, and 207 Pb MAS NMR with emphasis on the role of the CH 3 NH 3 cation and PbBr 6 octahedra in CH 3 NH 3 PbBr 3 . In addition, the 14 N static NMR spectra of CH 3 NH 3 PbBr 3 in the laboratory frame were acquired near the phase transition temperature. The abovementioned results help in understanding the thermal stability and the structural dynamics based on the phase transition mechanism, towards the practical application of this material. experimental CH 3 NH 3 Br and PbBr 2 were dissolved in a dimethylformamide solution and heated the mixed suspension on a hot plate to obtain a transparent solution. Detailed methods for the crystal growth are given elsewhere 21,24,25 . The CH 3 NH 3 PbBr 3 single crystals obtained here were orange in colour with a square shape.
Differential scanning calorimetry (DSC) (TA, DSC 25) was conducted at a heating rate of 10 °C/min over a temperature range from 190 to 525 K under nitrogen gas. Thermogravimetric analysis (TGA) was performed on a thermogravimetric analyser (TA Instrument) in an interval from 300 to 780 K at a heating rate of 10 °C/min. Approximately 11.15 mg of CH 3 NH 3 PbBr 3 was used in each experiment.
NMR measurements were carried out at 9.4 T using a Bruker 400 MHz Avance II + spectrometer at the Korea Basic Science Institute, Western Seoul Center. The 1 H, 13 C, and 207 Pb NMR frequencies were 400.13, 100.61, and 83.75 MHz, respectively. Powdered samples were packed in zirconia MAS rotors with Macor caps, and the MAS rate was set to 10 kHz for the 1 H MAS, 13 C MAS, and 207 Pb MAS NMR measurements to minimise spinning sideband overlap. The spin-lattice relaxation time in the rotating frame T 1ρ was measured using an inversion recovery pulse sequence, which employs compensating pulses. The 13 C T 1ρ values were measured by varying the duration of the 13 C spin-locking pulse applied after the CP preparation period. The width of the π/2 pulse used for measuring T 1ρ of 1 H and 13 C was 3.45 µs, and that for measuring T 1ρ of 207 Pb was 3.5 µs. In addition, Temperature-dependent NMR spectra were recorded over 180 to 430 K; the NMR spectra and relaxation times could not be measured outside this temperature range because of the limitations of the spectrometer. Sample temperatures were held constant within ± 0.5 K by controlling the nitrogen gas flow and heating current. Figure 2 shows the DSC and TGA curves obtained under a nitrogen atmosphere. DSC analysis was used to determine the phase transition temperature; only one endothermic peak related to a phase transition was observed at 236 K, which is consistent with previously reported T C values. 13,14 The thermal stability of CH 3 NH 3 PbBr 3 was examined by TGA. The first occurrence of mass loss began at approximately 530 K, which represents the onset of partial thermal decomposition. The mass sharply decreased between 550 and 650 K, with a corresponding mass loss of 22% near 650 K. Optical polarizing microscopy experiments were also conducted to further understand the thermal stability at high temperatures. The colour of the crystal was orange at room temperature, as shown in the inset in Fig. 2. As the temperature increased, the state of the crystal remained the same from 400 to 500 K. Above 550 K, a slight opacity occurred at the bottom of the crystal, and at approximately 600 K, the crystal was nearly opaque.

experimental results
The 1 H NMR spectrum of CH 3 NH 3 PbBr 3 was recorded by MAS NMR at a frequency of 400.13 MHz. Figure 3 shows the 1 H MAS NMR spectrum at 300 K, where the spinning sidebands are marked with open circles and asterisks. The two peaks in the 1 H spectrum correspond to CH 3 and NH 3 environments, with the chemical shifts at δ = 3.27 and 6.36 ppm assigned to 1 H in CH 3 and NH 3 , respectively. The chemical shifts remained quasi-constant with increasing temperature, indicating that the structural environments of 1 H in the CH 3 and  www.nature.com/scientificreports/ NH 3 groups were unchanged (see the Supplementary Information). Additionally, the line width (full-width at half-maximum) of the 1 H MAS NMR signal at 300 K is approximately 1.62 ppm, which also remained nearly constant with temperature change. The 1 H inversion-recovery curves for both CH 3 and NH 3 at each temperature were fitted to exponentials to extract T 1ρ . The data were well fitted a single exponential, indicating that there is one dominant relaxation mechanism acting per environment. Thus, T 1ρ was determined by fitting the decay plots with the equation below 26,27 . where P(t) is the magnetisation, t is the spin-locking pulse duration, and P 0 is the total nuclear magnetisation of 1 H at thermal equilibrium. The recovery curves of 1 H in CH 3 NH 3 PbBr 3 were measured for various delay times at each temperature. Figure 4 (inset) shows the recovery traces for 1 H measured for delay times ranging from 1 to 200 ms at 300 K. The intensity of the recovery traces differed with delay time. The T 1ρ values obtained from the intensity versus delay time and shown in Fig. 4 reveal that T 1ρ increased with temperature because proton hopping was accelerated. This is in agreement with Xu et al. 23 , who reported that the 1 H T 1 increased smoothly with increasing temperature through the high-temperature phase transition. The T 1ρ values of 1 H in CH 3 and NH 3 in the CH 3 NH 3 + cation show similar trends with temperature and are nearly the same within the error range. The T 1ρ values show no change near the phase transition temperature (T C = 236 K). T 1ρ increased with increasing temperature, reaching the maximum values of 592 ms and 456 ms for CH 3 and NH 3 , respectively, near 330 K above the phase transition temperature, and then decreased with increasing temperature. Although the structural environment of 1 H in the CH 3 NH 3 groups does not change with temperature, their molecular motion increases at high temperatures, as indicated by the T 1ρ values. Above T C , the 1 H T 1ρ value for CH 3 slightly exceed that for NH 3 .
Structural analysis of the 13 C and 207 Pb nuclei in CH 3 NH 3 PbBr 3 was performed by MAS NMR, and the corresponding spectra at 300 K are shown as insets in Fig. 5. At room temperature, the 13 C and 207 Pb MAS NMR spectra show one signal each at chemical shifts of δ = 30.66 and 89 ppm with respect to tetramethylsilane and PbNO 3 , respectively. Here, the line width for 13 C at 300 K is narrow at 2.77 ppm, whereas that for 207 Pb is quite broad at 206.24 ppm. Figure 5 shows the 13 C and 207 Pb chemical shifts of CH 3 NH 3 PbBr 3 measured as a function of temperature, illustrating that the 13 C and 207 Pb peak positions moved to higher chemical shifts upon heating. The chemical shifts near T C changed, in contrast to the 1 H chemical shifts. The chemical shifts of the 13 C and 207 Pb signals relative to the reference signal are sensitive to the electronic environment of the nucleus. In particular, the 207 Pb chemical shift changed more rapidly than that of 13 C near T C . From these results, the phase transition from the tetragonal to cubic phase is thought to arise from a change in the PbBr 6 octahedra.
To determine the T 1ρ values of 13 C and 207 Pb in the rotating frame, the nuclear magnetisation was measured as a function of delay time. The signal intensities of the nuclear magnetisation recovery curves could be described by the single exponential function in Eq. (1), and the signal intensity followed this single exponential decay at all temperatures. From these results, the T 1ρ values were obtained for 13 C and 207 Pb in CH 3 NH 3 PbBr 3 as a function of inverse temperature, as shown in Fig. 6. The 13 C and 207 Pb T 1ρ values for CH 3 and PbBr 3 seem to follow a similar trend with temperature to that of the 1 H T 1ρ , where the values increase with increasing temperature and are approximately continuous near T C . In addition, the 207 Pb T 1ρ values are much lower than 13 C T 1ρ .
To obtain information concerning possible changes in the surroundings of the 14 N ion, static NMR spectra of 14 N (I = 1) in the laboratory frame were obtained. Temperature-dependent changes in the 14 N resonance frequency are attributable to alterations in the structural geometry, indicating a change in the quadrupole coupling constant of the 14 N nuclei. The spectra were obtained by the solid-state echo method using static NMR at a Larmor frequency of 28.90 MHz. Two resonance signals were expected from the quadrupole interactions of the 14 N nucleus with spin I = 1. The 14 N NMR spectra were shown at 225 and 270 K, and the resonance frequencies referenced  www.nature.com/scientificreports/ with respect to NH 4 NO 3 as a function of temperature are shown in Fig. 7. The line widths are very narrow at all temperatures. The two resonance signals for 14 N, which are attributable to NH 3 , superpose into one line at the transition point of 236 K. This single 14 N resonance line indicates that a phase transition takes place to a new phase with a higher symmetry than tetragonal 28 . In tetragonal phase below T C , the electric field gradient tensors at the N sites vary, reflecting changes in the atomic configuration around the nitrogen. But, there is no electric field gradient tensor at the 14 N site in the cubic structure because of the site symmetry of m3m.  13 C and 207 Pb nuclei varied with temperature. The temperature dependence of the chemical shifts was sensitive to the rotation of the PbBr 6 octahedra. From these results, it is evident that the structural environments of 13 C and 207 Pb change near T C . The change in 207 Pb chemical shift near T C can be explained by the rotation of PbBr 3 . This is consistent with the established nature of the phase transition. Additionally, the NMR line widths of 1 H, 13 C, and 207 Pb were 1.62, 2.77, and 206.24 ppm, respectively, and the relaxation time is proportional to the inverse of the line width. Although the chemical shifts of 13 C and 207 Pb abruptly varied near T C , the 1 H, 13 C, and 207 Pb T 1ρ values showed a similar trend with increasing temperature, and their T 1ρ values were continuous near T C . These  www.nature.com/scientificreports/ short relaxation times indicate ease of molecular motion. The TGA results also showed that CH 3 NH 3 PbBr 3 has a high thermal stability. In addition, the abrupt change occurring in the resonance frequency of the 14 N nuclei near T C is attributable to a structural phase transition. The NH 3 groups in the structure are coordinated by PbBr 6 , and thus atomic displacements in the environment of the 14 N nuclei with temperature are correlated with PbBr 6 . The electrostatic interactions governed by hydrogen-bonding interactions between the NH 3 + group in the CH 3 NH 3 cation and the PbBr 6 octahedra play an important role in the dynamics of the CH 3 NH 3 cations. Consequently, the main factor causing the phase transition from the tetragonal to cubic phase near T C is a change in the surroundings of the 207 Pb nuclei in the PbBr 6 octahedra and in the surroundings of C-N groups in the CH 3 NH 3 cations. Based on these results, the structural dynamics within the CH 3 NH 3 PbBr 3 perovskite structure are expected to have a significant effect on the operation mechanism of perovskite solar cells.