Methylammonium Tetrel Halide Perovskite Ion Pairs and Their Dimers: The Interplay between the Hydrogen-, Pnictogen- and Tetrel-Bonding Interactions

The structural stability of the extensively studied organic-inorganic hybrid methylammonium tetrel halide perovskite semiconductors, MATtX(3) (MA = CH3NH3+; Tt = Ge, Sn, Pb; X = Cl, Br, I), arises as a result of non-covalent interactions between an organic cation (CH3NH3+) and an inorganic anion (TtX(3)(-)). However, the basic understanding of the underlying chemical bonding interactions in these systems that link the ionic moieties together in complex configurations is still limited. In this study, ion pair models constituting the organic and inorganic ions were regarded as the repeating units of periodic crystal systems and density functional theory simulations were performed to elucidate the nature of the non-covalent interactions between them. It is demonstrated that not only the charge-assisted N-H center dot center dot center dot X and C-H center dot center dot center dot X hydrogen bonds but also the C-N center dot center dot center dot X pnictogen bonds interact to stabilize the ion pairs and to define their geometries in the gas phase. Similar interactions are also responsible for the formation of crystalline MATtX(3) in the low-temperature phase, some of which have been delineated in previous studies. In contrast, the Tt center dot center dot center dot X tetrel bonding interactions, which are hidden as coordinate bonds in the crystals, play a vital role in holding the inorganic anionic moieties (TtX(3)(-)) together. We have demonstrated that each Tt in each [CH3NH3+center dot TtX(3)] ion pair has the capacity to donate three tetrel (sigma-hole) bonds to the halides of three nearest neighbor TtX(3)(-) units, thus causing the emergence of an infinite array of 3D TtX(6)(4-) octahedra in the crystalline phase. The TtX(4)(4-) octahedra are corner-shared to form cage-like inorganic frameworks that host the organic cation, leading to the formation of functional tetrel halide perovskite materials that have outstanding optoelectronic properties in the solid state. We harnessed the results using the quantum theory of atoms in molecules, natural bond orbital, molecular electrostatic surface potential and independent gradient models to validate these conclusions.

Automated summary

The structural stability of organic-inorganic hybrid perovskite semiconductors is determined by non-covalent interactions between the organic cations and inorganic anions. This study investigates the nature of these interactions using ion pair models and density functional theory simulations. The results reveal the importance of hydrogen and pnictogen bonds in stabilizing the ion pairs, as well as the role of tetrel bonding interactions in holding the inorganic anionic moieties together.