Molecular Motions and Phase Transitions in Solid CH₃NH₃PbX₃ (X = C1, Br, I) as Studied by NMR and NQR

Abstract

The temperature dependence of

Universal Dynamics of Molecular Reorientation in Hybrid Lead Iodide Perovskites

“…The behavior for MAPbI 3 is non-monotonic due to the change in relaxation mechanisms from four-fold rotations (α-, β-phases) to three-fold rotations about C-N (γ-phase). 29 95 110 60…”

“…This hypothesis is corroborated by charge density analysis of X-ray diffraction experiments ( Figure S4). For each rotation mode about the principal axes of the molecule, the other two rotation angles are fixed (for φ 1 rotations, φ 2 = φ 3 = 0; for φ 2 rotations, φ 1 = (0, 90 29 Reported reorientation times from GHz spectroscopy, quasi-elastic neutron scattering (QENS), ab initio molecular dynamics (MD), and 14 N NMR are presented for comparison. 15,16,23,24,36,42,73 14 reorientation.…”

“…This represents a synthesis of the results of this work with the findings of complementary prior reports. 14,16,19,23,29,36,41 As FAPbI 3 is cooled, molecular motion slows and the inorganic octahedra tilt to improve coordination of the A-site cation and enhance electrostatic binding, but the molecular geometry appears to be incompatible with the preferred ground state octahedral tilting pattern. This leads to substantially more complex behavior in FAPbI 3 than in MAPbI 3 , with a disordered pseudo-cubic γ-phase with persistent dipole motion, 19 followed by a fragile glassy freezing into a disordered ground state.…”

“…Values for FAPbI 3 are from this work, while those for MAPbI 3 are drawn from the literature. 23,29 electric response and leads to a reduced activation barrier for rotation from 1 H NMR, since relatively facile three-fold rotations about the C-N axis become the dominant relaxation mechanism. While signatures of glassy freezing exist on cooling this compound below 100 K, they are significantly subtler than for FAPbI 3 , 14 which comports with the findings presented here.…”

“…Since the first preparation of the hybrid lead halide perovskites, 12 the behavior of the molecular cations has been studied by calorimetry, 13,14 infrared spectroscopy, 13,15,16 dielectric spectroscopy, 14,[17][18][19][20] Raman spectroscopy, 21 quasi-elastic and inelastic neutron scattering, [22][23][24][25][26] nuclear magnetic resonance (NMR) spectroscopy, [27][28][29][30][31][32] isotopic substitution, 33 and ab initio methods, [34][35][36][37][38][39] and these findings have been correlated to the structure evolution from crystallographic techniques. 19,[40][41][42][43][44][45][46] Most of these efforts have focused on MAPbX 3…”