Crystal structures and rotational dynamics of a two-dimensional metal halide perovskite (OA)₂PbI₄

The extended charge carrier lifetime in metal halide perovskites is responsible for their excellent optoelectronic properties. Recent studies indicate that the superb device performance in these materials is intimately related to the organic cation dynamics. Here, we focus on the investigation of the two-dimensional hybrid perovskite, (C₈H₁₇NH₃)₂PbI₄ (henceforth, OA⁺ = C₈H₁₇NH₃ ⁺). Using elastic and quasielastic neutron scattering techniques and group theoretical analysis, we studied the structural phase transitions and rotational modes of the C₈H₁₇NH₃ ⁺ cation in (OA)₂PbI₄. Our results show that, in the high-temperature orthorhombic (T > 310 K) phase, the OA⁺ cation exhibits a combination of a twofold rotation of the NH₃-CH₂ head group about the crystal c-axis with a characteristic relaxation time of ∼6.2 ps, threefold rotations (C₃) of NH₃ and CH₃ terminal groups, and slow librations of the other atoms. Contrastingly, only the C₃ rotation is present in the intermediate-temperature orthorhombic (238 K < T < 310 K) and low-temperature monoclinic (T < 238 K) phases.