Phase Behaviour and Dynamics of Organic Cations in Formamidinium Lead Iodide (FAPI) Using Machine-Learned Potentials

Hybrid halide perovskites have received tremendous attention due to their promising applications in photovoltaics and optoelectronics. Formamidinium lead iodide (FAPI) has emerged as one of the most interesting compounds, due to its tuneable stability. While it has already been extensively studied, there are remaining questions about its atomic structure, phase behaviour, and dynamics of FA cations, especially at low temperatures. In this context, molecular dynamics simulations can bring insights into the unanswered questions. However, due to the chemical complexity, it is not trivial to construct a model for atomistic simulations of perovskites. Recently, the promise of machine learning has been demonstrated in simulating finite-temperature dynamics of complex materials using completely atomistic approaches with accuracy comparable to first-principles simulations. <br/> <br/>In the present study, we generate a machine-learned potential for FAPI using the GPUMD package [1] to study the phase behaviour and dynamics of FA molecules. We carry out MD simulations to understand the structural phase space of the material. Our machine-learned potential successfully captures all the phase transitions reported in the literature: the first phase transition from a cubic (Pm-3m) to a tetragonal (P4/mbm) phase occurs at 310K and a second transition occurs at 100K to a disordered phase [2,3]. The low-temperature phase is not easily identified as it exhibits some disorder and local variation[4]. We analyse the octahedral tilting and preferred FA orientations, which further sheds light on understanding the nature of the low-temperature phase of the material. Further, to understand the local symmetry we study the rotational dynamics of the FA molecules. The FA molecules rotate freely at high temperatures in the cubic and tetragonal phases on the timescale of picoseconds. However, at lower temperatures (around lower phase transition) the rotational dynamics of FAs deviate from an Arrhenius behaviour, leading to the discovery of a disordered glassy state.<br/> <br/> <br/><b>References:</b><br/>[1] Z. Fan, Y. Wang, P. Ying, K. Song, J. Wang, Y. Wang, Z. Zeng, K. Xu, E. Lindgren, J.M. Rahm, A.J. Gabourie, J. Liu, H. Dong, J, Wu, Y. Chen, Z. Zhong, J. Sun, P. Erhart, Y. Su, and T. Ala-Nissila, J Chem Phys., <b>11,</b> 114801 (2022).<br/>[2] M.T. Weller, O. J. Weber, J.M. Frost, and A. Walsh, J. Phys. Chem. Lett., <b>16</b>, 3209-3212 (2015).<br/>[3] O. J. Weber, D. Ghosh, S. Gaines, P.F. Henry, A.B. Walker, M.S. Islam, and M.T.Weller, Chem. Mater., <b>30</b>, 3768-3778 (2018).<br/>[4] D. H. Fabini, T. A. Siaw, C. C. Stoumpos, G. Laurita, D. Olds, K. Page, J. H. Hu, M. G. Kanatzidis, S. Han, and R. Seshadri