Two-dimensional Ruddlesden–Popper (RP) halides are under intense investigation for photovoltaic and optoelectronic applications due to advantages in stability, diversity and tunability. Here we present results of a detailed investigation of the crystal structures of a prototypical groups of RP halides PEA2PbX4 (PEA= phenylethylammonium) using density functional theory calculations. We find PEA cations energetically prefer (by ~50 meV/f.u.) the same orientation within one PEA layer while the opposite in two adjacent PEA layers, which agrees with a most recent experiential study  but differs from other earlier reports, including one of our own . Orientations of PEA cations further mediate the distortion of PbI6 octahedral layer. Such trends are robust when using both Perdew–Burke–Ernzerhof (PBE) generalized gradient and strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximations with and without revised Vydrov–van Voorhis (rVV10) van der Waals functional. We investigate the origins of the preferred PEA cation orientation and PbI6 distortion direction based on analyses of the electronic structure. Our study provides insights into the nature of the chemical bonding determining the structure of layered hybrid Ruddlesden–Popper halides.
Acknowledgement: This work was supported by the Singapore Berkeley Research Initiative for Sustainable Energy(SinBeRISE) Program.
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