NMR and Raman Scattering Studies of Temperature- and Pressure-Driven Phase Transitions in CH3NH2NH2PbCl3 Perovskite

Three-dimensional methylhydrazinium (CH3NH2NH2+, MHy+) lead halides, related to the famous methylammonium (CH3NH3+, MA+) and formamidinium (CH(NH2)2+, FA) perovskites, are attractive optoelectronic materials crystallizing in polar structures. In this work, temperature-dependent 1H and 207Pb magic-angle spinning (MAS) NMR, Raman as well as high-pressure Raman studies of CH3NH2NH2PbCl3 (MHyPbCl3) are reported. Raman spectroscopy reveals many similarities between phonon properties of MHy lead halides and the MA and FA analogues. In particular, these families of compounds show an increase in wavenumber of cage modes when large I- ions are replaced by smaller Br- and then Cl- ones. They also show strong sensitivity of the CH3 torsional mode on size of the cavity occupied by MHy+ cation that decreases with decreasing size of the halide anion. The cage modes of MHyPbCl3 are, however, observed at significantly lower wavenumbers than similar modes of MAPbCl3 and FAPbCl3, indicating higher softness of MHyPbCl3. Temperature-dependent Raman and NMR studies demonstrate that the MHy+ cations in MHyPbCl3 are significantly less affected by the temperature-induced phase transition than the Pb-Cl framework. This suggests a displacive type of the phase transition dominated by tilting and deformation of the PbCl6 octahedra. Analysis of the 207Pb MAS NMR spectra reveals the presence of two differently distorted PbCl6 octahedra and diminishing (increasing) distortion of the less (more) distorted octahedra in the high-temperature phase. Pressure-dependent Raman studies reveal the presence of a single first-order pressure-induced phase transition between 0.72 and 1.27 GPa. Analysis of the spectra indicates that the driving forces for the pressure-induced phase transition in MHyPbCl3 are tilting and distortion of the PbCl6 octahedra accompanied by reorientation of MHy+ cations. Raman spectra do not show evidence of any additional phase transition or amorphization up to 6.95 GPa