  We present a detailed first-principles investigation of the structural and
polar properties of 3D hybrid perovskite, methylhydrazinium lead chloride,
MHyPbCl3, as it transitions from a highly polar Phase-I (high temperature, HT)
to less polar Phase-II (low temperature, LT) from the perspective of host/guest
interactions. Structurally, the two phases vary in the orientation of the guest
and the two differently distorted host layers. The transition character (TC)
across the path of phase transition and a relatively high guest-reorientation
barrier tells that the transition is primarily governed by guest-reorientation.
This overall guest/host transition is interesting because it leads to
enhancement in polarization with temperature which is quite unusual. Maximally
localized Wannier functions (MLWFs) have been used to probe into the atomistic
origin of this enhancement which establishes the leading role of the host
atoms, especially those lying in the more distorted octahedral layer, with only
a negligible contribution from the guest, despite the fact that it is the
primary effect leading to transition. Furthermore, we also find a significant
feedback polarization of ~9% that the host distortion induces on the guest.
This has a direct effect on the density of states occupied by the guest, which
shifts away from the band edge with the increase in the host distortion (for
Phase-II guest orientation). Thus the organic cations in 3D perovskites can
also have a non-trivial contribution to the optoelectronic properties and
exciton binding energies.
