Domain boundaries in the dipolar order in the perovskite material CH₃NH₃PbI₃

We investigate the ordering of the CH₃NH₃ dipoles in the material CH₃NH₃PbI₃. The dipoles are arranged in a simple cubic lattice. We perform numerical simulations in which we set the boundary conditions such that opposite sides of the simulated sample are ordered in different directions, hence simulating a domain boundary. We calculate the lowest energy state under this constraint. We find that at the level of dipole-dipole interactions, the dipole orientations tend to gradually transform between the two orientations at the two ends of the sample. When we take into consideration the finite spatial size of the CH₃NH₃ molecules and go beyond the point dipole approximation, we find that the domain boundary becomes sharper. For the parameters of CH₃NH₃PbI₃, our results indicate that the optimal energy structure has a boundary region of a width on the order of a single unit cell.