Guest-induced phase transition leads to polarization enhancement in MHyPbCl₃

Hybrid perovskites offer more structural and chemical flexibility than traditional metal halide perovskites. In supertolerant 3D perovskites, a strong organic-inorganic sublattice coupling can influence several physical properties significantly. Here, we use first-principles calculations to explore the structural and polar properties of one such member, methylhydrazinium lead chloride, MHyPbCl₃, as it transitions from highly polar Phase-I to less polar Phase-II upon cooling. From a host/guest perspective, the two phases vary structurally in the guest (MHy) orientation and the two differently distorted host (lead halide) layers. Unlike most hybrid perovskites, this transition is primarily driven by the guest reorientation, further confirmed by a minimum energy path calculation. Maximally localized Wannier functions identify the atomistic origin of polarization enhancement as the host atoms, especially the more distorted octahedral layer. The guest, despite being the primary order parameter for transition, has a negligible contribution, in contrast to prior suggestions. The host distortion also induces significant feedback polarization on the guest molecule, which alters the density of states occupied by the guest at the band edge, leading to a nontrivial impact of the guest on optoelectronic properties.