Temperature-Induced Phase Transition in 2D Alkylammonium Lead Halide Perovskites: A Molecular Dynamics Study

Molecular dynamics simulations are utilized to unravel the temperature-driven phase transition in double-layered butylammonium (BA) methylammonium (MA) lead halide perovskite (BA)2(MA)Pb2I7, which holds great promise for a wide range of optoelectronics and sensor applications. The simulations successfully capture the structural transition from low to high symmetry phases with rising temperatures, consistent with experimental observations. The phase transition is initiated at two critical interfaces: the first is between the inorganic and organic layers, where the melting of N–H bonds in BA leads to a significant reduction in hydrogen bonding between BA and iodides, and the second is at the interface between the top and bottom organic layers, where the melting of the tail bonds in BA triggers the phase transition. Following this, BA cations exhibit a patterned and synchronized motion reminiscent of a conical pendulum, displaying a mix of ordered and disordered behaviors prior to evolving into a completely molten and disordered state. While the melting of BA cations is the primary driver of the phase transition, the rotational dynamics of MA cations also plays a critical role in determining the phase transition temperature, influenced by the BA–MA interaction. Such an interaction alters the polarization patterns of MA cations across the phase transition. In particular, an antiparallel polarization pattern is observed in the low-temperature phase. Additionally, displacive elements of the phase transition are identified in the simulations, characterized by the shear and distortion of the inorganic octahedra. Notably, at lower temperatures, the octahedral distortion follows a bimodal distribution, reflecting significant variations in distortion among octahedra. This variation is attributed to an anisotropic hydrogen bonding network between iodides and BA cations. Our study reveals the phenomena and mechanisms extending beyond the order–disorder transition mechanism, suggesting potential phase engineering through strategic tuning of organic and inorganic components.


Supplementary Movies
Moives are shared through figshare.comand can be accessed using the following link: https://doi.org/10.6084/m9.figshare.25457815 The captions of the movies are provided as follow: The hydrogen bonds in organic cations are not displayed in these snapshots.organic cations are not displayed in these snapshots.

Figure S2 :
Figure S2: Time-averaged atomic configurations of (BA) 2 (MA)Pb 2 I 7 at 1 K, obtained from a centered 200 ps interval around the respective snapshot.Distinct color schemes used for BA backbone bonds and Pb-I bonds in upper and lower inorganic layers to enhance visualization.The octahedral geometry is depicted in translucent white, and MA cations are omitted.

Figure S3 :
Figure S3: Time-averaged atomic configurations of (BA) 2 (MA)Pb 2 I 7 at 50 K, obtained from a centered 200 ps interval around the respective snapshot.Distinct color schemes used for BA backbone bonds and Pb-I bonds in upper and lower inorganic layers to enhance visualization.The octahedral geometry is depicted in translucent white, and MA cations are omitted.

Figure S4 :
Figure S4: Time-averaged atomic configurations of (BA) 2 (MA)Pb 2 I 7 at 100 K, obtained from a centered 200 ps interval around the respective snapshot.Distinct color schemes used for BA backbone bonds and Pb-I bonds in upper and lower inorganic layers to enhance visualization.The octahedral geometry is depicted in translucent white, and MA cations are omitted.

Figure S5 :
Figure S5: Time-averaged atomic configurations of (BA) 2 (MA)Pb 2 I 7 at 150 K, obtained from a centered 200 ps interval around the respective snapshot.Distinct color schemes used for BA backbone bonds and Pb-I bonds in upper and lower inorganic layers to enhance visualization.The octahedral geometry is depicted in translucent white, and MA cations are omitted.

Figure S6 :
Figure S6: Time-averaged atomic configurations of (BA) 2 (MA)Pb 2 I 7 at 200 K, obtained from a centered 200 ps interval around the respective snapshot.Distinct color schemes used for BA backbone bonds and Pb-I bonds in upper and lower inorganic layers to enhance visualization.The octahedral geometry is depicted in translucent white, and MA cations are omitted.

Figure S7 :
Figure S7: Time-averaged atomic configurations of (BA) 2 (MA)Pb 2 I 7 at 250 K, obtained from a centered 200 ps interval around the respective snapshot.Distinct color schemes used for BA backbone bonds and Pb-I bonds in upper and lower inorganic layers to enhance visualization.The octahedral geometry is depicted in translucent white, and MA cations are omitted.

Figure
Figure S8: Time-averaged atomic configurations of (BA) 2 (MA)Pb 2 I 7 at 300 K, obtained from a centered 200 ps interval around the respective snapshot.Distinct color schemes used for BA backbone bonds and Pb-I bonds in upper and lower inorganic layers to enhance visualization.The octahedral geometry is depicted in translucent white, and MA cations are omitted.
Time-averaged atomic positions during 20 ns simulated phase transition of (BA) 2 (MA)Pb 2 I 7 , wherein the temperature is increased from 125 K to 150 K. To enhance clarity, atomic oscillations were minimized by averaging 200 consecutive simulation trajectories, yielding a representation based on a 200 ps interval per snapshot.Pb-I bonds in the inorganic layers are distinguished by color: cyan for the upper layer and pink for the lower.The octahedral geometry, with centrally located Pb, is displayed in translucent white.Due to the accelerated rotation of MA cations at this temperature, their time-averaged conformations are indiscernible and thus omitted from the visualization.The hydrogen bonds in organic cations are not displayed in these snapshots.Supplementary Movie 2. Time-averaged atomic positions over a 20 ns simulation of (BA) 2 (MA)Pb 2 I 7 , wherein the temperature is increased from 125 K to 200 K, and the rotational degrees of freedom of C 3 -C 4 bond in BA cations remain constrained.To enhance clarity, atomic oscillations were minimized by averaging 200 consecutive simulation trajectories, yielding a representation based on a 200 ps interval per snapshot.Pb-I bonds in the inorganic layers are distinguished by color: cyan for the upper layer and pink for the lower.The octahedral geometry, with centrally located Pb, is displayed in translucent white.Due to the accelerated rotation of MA cations at this temperature, their time-averaged conformations are indiscernible and thus omitted from the visualization.The hydrogen bonds in organic cations are not displayed in these snapshots.snapshot.Pb-I bonds in the inorganic layers are distinguished by color: cyan for the upper layer and pink for the lower.The octahedral geometry, with centrally located Pb, is displayed in translucent white.Due to the accelerated rotation of MA cations at this temperature, their time-averaged conformations are indiscernible and thus omitted from the visualization.The hydrogen bonds in organic cations are not displayed in these snapshots.Supplementary Movie 6. Time-averaged atomic positions of (BA) 2 (MA)Pb 2 I 7 during simulated annealing.This visualization reveals the dynamics of (BA) 2 (MA)Pb 2 I 7 under simulated annealing from 80-100 ns at 175 K. To enhance clarity, atomic oscillations were minimized by averaging 200 consecutive simulation trajectories, yielding a representation based on a 200 ps interval per snapshot.Pb-I bonds in the inorganic layers are distinguished by color: cyan for the upper layer and pink for the lower.The octahedral geometry, with centrally located Pb, is displayed in translucent white.Due to the accelerated rotation of MA cations at this temperature, their time-averaged conformations are indiscernible and thus omitted from the visualization.The hydrogen bonds in organic cations are not displayed in these snapshots.Supplementary Movie 7. Time-averaged atomic positions of (BA) 2 (MA)Pb 2 I 7 during simulated annealing.This visualization reveals the dynamics of (BA) 2 (MA)Pb 2 I 7 under simulated annealing from 80-100 ns at 150 K. To enhance clarity, atomic oscillations were minimized by averaging 200 consecutive simulation trajectories, yielding a representation based on a 200 ps interval per snapshot.Pb-I bonds in the inorganic layers are distinguished by color: cyan for the upper layer and pink for the lower.The octahedral geometry, with centrally located Pb, is displayed in translucent white.Due to the accelerated rotation of MA cations at this temperature, their time-averaged conformations are indiscernible and thus omitted from the visualization.