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

Abstract

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.

Keywords: 2D perovskite; melting; molecular dynamics; order−disorder; phase transition.

Figure 1

Periodic supercell of (BA)₂(MA)Pb₂I₇, comprising 3264 particles. (a) Atomic structure with Pb (purple), I (dark gray), N (blue), C (light gray), and H (white) atoms. (b) Pb–I bonds in octahedra and the bonds in MA and BA, colored according to their respective atoms.

Figure 2

Temperature dependence of structural properties and vdW energy in (BA)₂(MA)Pb₂I₇. (a) Box lengths (X, Y, and Z) and volume. (b) Box angles (α, β, and γ). (c) Organic layer thickness. (d) vdW energy. Data time averaged over 20 ns (400–250 K), 50 ns (200–50 K), and 4 ns (1 K) for precision.

Figure 3

Atomic positions averaged over 200 ps within a triclinic structure at equilibrium at 125 K and atomic displacement vectors (arrows) from 150 to 125 K, scaled and color-coded based on displacement magnitude. Only BA cations’ backbone and the Pb–I skeleton of the lower inorganic layer are shown, omitting MA cations for clarity, with the Pb–I octahedra in translucent white. The left panels (a,c,e) show the Z, X, and Y displacement components of BA cations from different viewpoints. The right panels (b,d,f) display displacement components within the inorganic layer.

Figure 4

ACFs of several bonds in the BA cation’s backbone, N–C₁–C₂–C₃–C₄, and N–H bond over varying temperatures plotted against MD simulation runtime: (a) 125, (b) 150, (c) 175, (d) 200, (e) 250, and (f) 300 K.

Figure 5

Time-averaged structures of (BA)₂(MA)Pb₂I₇ at 125 and 175 K, derived from 200 ps intervals centered on selected snapshots. BA backbone and Pb–I bonds are color-coded in the respective layers, with Pb–I octahedra in semitransparent white and MA cations omitted for clarity. (a) Scatter plot of unaveraged trajectories at 125 K between 50 and 100 ns on the normalized XZ plane, excluding I atoms. In (c), a BA cation in trans–gauche conformation is highlighted with a dashed black oval. (f) XY projection of a snapshot at 175 K, illustrating instantaneous rotation directions of BA cations in the middle of the box with yellow and orange circular arrows for upper and lower inorganic layers, respectively, and a 180° phase delay indicated by dashed black outlines.

Figure 6

(a–c) N (blue) and C (red) in MA cations on the XY plane of the lower Pb–I framework over 50 ns at 50, 125, and 150 K, with yellow arrows marking the N–C bond average orientations. (d–f) N atom positions in BA cations (bottom in blue, top in cyan) and their averaged configurations, with green arrows indicating BA cation average head-to-tail orientations. Regions characterized by a higher persistence of H bonds are delineated by transparent rectangles with a red hue. (g) Trajectories of MA cations at 150 K. (h) H bonding average lifetimes between H atoms of in BA/MA cations and I atoms in octahedra against temperature, where the embedded figure is the illustration of possible H-bond between of BA/MA and Pb–I bonds.

Figure 7

Normalized distributions of (a,b) axial Pb–I–Pb bond angle deviation from 180°, (c,d) linking-equatorial I–Pb–I bond angle deviation from 180°, and (e,f) terminal-axial Pb–I bond angle deviation from 90°. The right panels correspond to the triclinic crystal system.