Hydrogen Bonding Analysis of Structural Transition-Induced Symmetry Breaking and Spin Splitting in a Hybrid Perovskite Employing a Synergistic Diffraction-DFT Approach

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) offer an outstanding opportunity for spin-related technologies owing in part to their tunable structural symmetry-breaking and distortions driven by organic-inorganic hydrogen (H) bonds. However, understanding how H-bonds tailor inorganic symmetry and distortions, and therefore enhance spin splitting for more effective spin manipulation, remains imprecise due to challenges in measuring H-atom positions using X-ray diffraction (XRD). The low scattering power and distorted aspherical electron density of H atoms cause XRD to inaccurately determine shorter N/C – H bond lengths (~0.8 – 0.9 Å) compared to expected values (~1.0 – 1.1 Å). In this work, we investigate a first-order structural transition at ~209 K upon cooling in (2-BrPEA)₂PbI₄ (2-BrPEA = 2-(2-bromophenyl)ethylammonium), from a partially disordered and globally centrosymmetric configuration for the organic cations to an ordered and non-centrosymmetric configuration.¹ While XRD generally establishes heavy atom coordinates, we utilize neutron diffraction for accurate H-atom position determination, demonstrating that the structural transition-induced asymmetric rearrangement of H-bonds removes inversion symmetry, but also amplifies the discrepancy between the adjacent inter-octahedral distortions through associated distinct degrees of H-bond strength to the equatorial I atoms. The asymmetric distortions lead to a substantial spin splitting (△E > 30 meV), representing one of the largest spin-splitting magnitudes in currently reported PbI₄^2--based 2D HOIPs. Compared with the neutron-based H-bonding analysis, while there is qualitative agreement, quantitatively inaccurate outcomes arise from the X-ray-based H-bonding analysis. We further show that H-atom-only density-functional theory (DFT) relaxation of the X-ray structure shifts H atoms to positions that are consistent with the neutron experimental data, already at the affordable level of a van der Waals corrected semilocal DFT approach. Additionally, this approach could be effectively applied to large databases of existing HOIP X-ray structures. This work establishes an important pathway to more generally improve H-atom and H-bonding analyses derived from quicker/less-expensive X-ray data, potentially enhancing knowledge of and control over asymmetric distortions and symmetry-breaking for advanced semiconductor material design.

[1] Xie, Y.; Koknat, G.; Weadock, N. J.; Wang, X.; Song, R.; Toney, M. F.; Blum, V.; Mitzi, D. B. Hydrogen Bonding Analysis of Structural Transition-Induced Symmetry Breaking and Spin Splitting in a Hybrid Perovskite Employing a Synergistic Diffraction-DFT Approach. J. Am. Chem. Soc. 2024, 146, 22509-22521.