Revealing hidden non-centrosymmetry in globally centrosymmetric 2D halide perovskites

Hybrid 2D halide perovskites are tunable semiconductors that exhibit diverse structural symmetry and spin-orbit coupling effects. We examine a series of globally centrosymmetric n = 2 Ruddlesden-Popper (RP) lead iodide perovskites containing six different A-site cations, (PA)2(A)Pb2I7 [PA = pentylammonium]. Despite local structural distortions caused by oversized A-cations, all six structures are globally centrosymmetric, confirmed through second harmonic generation (SHG) measurements. However, in three of these 2D perovskites, the hydrogen bonding interaction between the A-cation and perovskite cage results in individual monolayers that lack inversion symmetry. Band structure calculations reveal Rashba band splitting due to this monolayer polarization, which is strongest in (PA)2(EA)Pb2I7 (EA = ethylammonium). Applying high pressure unmasks the non-centrosymmetry in bulk crystals with hidden non-centrosymmetry, leading to substantial SHG signal. A survey of reported crystal structures reveals that such hidden non-centrosymmetry is prevalent among 2D perovskites. These unexplored centrosymmetric 2D perovskites could be utilized for spin-orbitronic applications.