Low-Temperature Persistent Disorder and Lattice Dynamics in a Luminescent 1D Hybrid Lead Halide: Implications and Insights

Hybrid organic–inorganic halides have traditionally been viewed as materials that adopt well-ordered structural phases at low temperatures. In this article, we report a one-dimensional perovskitoid aminoguanidinium lead iodide (AGAPbI₃) with a first-order phase transition at 400/369 K (during heating/cooling) that breaks away from this rule. Specifically, we demonstrate that the structural transformation to the low-temperature monoclinic C2/c phase does not entirely suppress the motions associated with the organic AGA⁺ cation, leading to a phenomenon which we call now a persistent disorder. Indeed, it is still possible to observe pronounced dynamics of its terminal NH₂ group at least, which gradually slows down upon cooling and impacts the PbI₆^4– octahedra. As a result, we observe an unusually high activation energy of 0.6 eV related to the low-temperature relaxation dynamics, which is approximately 1 order of magnitude higher than those observed in conventional hybrid halides. We illustrate that the ongoing dynamic processes profoundly influence the temperature-dependent third-harmonic generation response and photoluminescence, the latter of which is characterized by two broad emission peaks with large Stokes shifts. Lastly, we emphasize that AGA⁺ cations can adopt two symmetry-equivalent positions within the low-temperature phase of AGAPbI₃, revealing the ongoing transition between the low-temperature static and high-temperature dynamic disorder types in this hybrid compound.