Entropy-Suppressed Ferroelectricity in Hybrid Lead-Iodide Perovskites

The actual nature of the electric polarization in hybrid lead-iodide perovskites is unveiled on the basis of ab initio and model results. A finite, albeit small electric polarization of few μC/cm2 is found to be pervasive in this system, due to the polar-uncompensated alignment of methylammonium dimers, at least for temperature lower than the activation energy of dimer rotations; however, the presence of a large number of structural local minima corresponding to differently oriented polarization directions counteracts the stabilization of an ordered ferroelectric phase at the macroscale. According to our estimate, only for temperatures lower than 40-50 K a clear ferroelectric behavior is displayed. At higher temperature the polarization is progressively suppressed and the ferroelectric ordering hindered by the large configurational entropy, giving rise to a superparaelectric-like behavior at the macroscale.