In the 2D perovskite structure of (BA) 2 (MA)Pb 2 I 7 , unlike its 3D counterpart, the rotational dynamics of MA cations are influenced not just by the Pb-I cages but also by the BA layers that encapsulate the MA cations from both the top and bottom. The interaction between BA and MA cations restricts the motion of MA cations, especially in the direction perpendicular to the inorganic layers, compared to the in-plane directions. This results in oval-shaped trajectories for the MA cations, as illustrated in Fig. . A representative relative orientation of MA and BA cations is shown in Fig. . This observation aligns with suggesting their quasi-liquid behavior at higher temperatures, similar to the behavior observed in (MA)PbI 3 at and above its phase transition temperature. To explore the impact of the rotational dynamics of MA cations on the phase transition, we conducted an MD simulation with the rotational freedoms of their N-C bonds deliberately constrained. It was found that under these restrictions, the phase transition shifted to a slightly higher temperature of 155 K. The process of such phase transition was captured in the Supplementary Movie 11. This indicates that while the melting of BA cations primarily drives the phase transition, the rotational dynamics of MA cations also contribute to the transition, likely through the interactions between BA and MA cations as discussed above.