Title: Ferroelectric polarization and interplay with spin-orbit properties in perovskite halides.
Speaker: Dr. Alessandra Stroppa
Time: 2:00-4:00 p.m., 2016.6.27
Location: Room 216, Physical Building
Abstract: Ferroelectricity in halide perovskites currently represents a crucial issue, as it may have an important role for the enhancement of solar cells efficiency. Simulations of ferroelectric properties based on density functional theory are conceptually more demanding compared with “conventional" inorganic ferroelectrics due to the presence of both organic and inorganic components in the same compound. Here we present a detailed study focused on the prototypical CH3NH3PbI3 perovskite. By using density functional theory combined with symmetry mode analysis, we disentangle the contributions of the methylammonium cations and the role of the inorganic framework, therefore suggesting possible routes to enhance the polarization in this compound. Our estimate of the polarization for the tetragonal phase at low temperature is ∼4.42 μC/cm2, which is substantially lower than that of traditional perovskite oxides.
Furthermore, we show that the lead-free perovskite iodide (FA)SnI3, containing the planar formamidinium cation FA, (NH2CHNH2)+ ,is ferroelectric. In fact, the perpendicular arrangment of FA planes, leading to a 'weak' polarization, is energetically more stable than parallel arrangements of FA planes, being either antiferroelectric or 'strong' ferroelectric. Moreover, we show that the 'weak' and 'strong' ferroelectric states with the polar axis along different crystallographic directions are energetically competing. Therefore, at least at low temperature, an electric field could stabilize different polarized states, resulting in a highly tunable ferroelectricity. Intriguingly, the relatively strong spin-orbit couling in noncentrosymmetric (FA)SnI3 gives rise to a co-existence of Rashba and Dresselhaus effects and to a spin texture that can be induced, tuned and switched by an electric field controlling the ferroelectric state.
