s–p Mixing in Stereochemically Active Lone Pairs Drives the Formation of 1D Chains of Lead Bromide Square Pyramids

Abstract

In lead­(II) halide compounds including virtually all lead halide perovskites, the Pb2+ 6s lone pair results in distorted octahedra, in accordance with the pseudo-Jahn–Teller effect, rather than generating hemihedral coordination polyhedra. Here, in contrast, we report the characterization of an organic–inorganic hybrid material consisting of one-dimensional edge-sharing chains of Pb–Br square pyramids, separated by [Mn­(DMF)6]2+ (DMF = dimethylformamide) octahedra. Molecular orbital analysis and density-functional theory calculations indicate that square pyramidal coordination about Pb2+ results from the occupancy of the empty ligand site by a Pb2+ lone pair that has both s and p orbital character rather than the exclusively 6s lone pair. These results demonstrate that a Pb2+ lone pair can be exploited to behave like a ligand in lead halide compounds, greatly expanding the realm of possible lead halide materials to include extended solids with nonoctahedral coordination environments.

The Tetrel Bond and Tetrel Halide Perovskite Semiconductors

“…Others have argued that PbO and PbS in both the rocksalt and litharge structures, which have distorted Pb 2+ octahedra, are not the result of chemically inert, stereochemically active lone pairs, but instead are the result of asymmetric electron densities that rely on direct electronic interaction with the coordinated anions [ 57 ]. Further discussion on the importance of stereochemically active lone pairs on Pb in Pb(II) halide compounds can be found elsewhere [ 58 ].…”

Broadband emission originating from the stereochemical expression of 6s2 lone pairs in two-dimensional lead bromide perovskites

“…The LPE is a second-order Jahn-Teller (SOJT) distortion, where structural distortions (i.e., deviating from the centrosymmetric coordination environment) lead to a stabilization of the 6s 2 electron pair by the sp orbital mixing of the Pb 2+ cation. 3,13 When the electronic energy gain is larger than the cost of breaking the inversion symmetry of the coordination environment, the structure becomes distorted. To confirm the presence of sp mixing on the Pb 2+ cation, we investigated the orbital projected density of states (DOS) and the valence electron density in the region of interest that is slightly below the Fermi energy (highlighted by the shaded areas in Fig.…”

“…This is the most common type of lonepair induced distortion observed in LHPs. 3 However, there is another distinct type of lone pair distortion observed in some other lead-containing solids including a few low-dimensional lead halide hybrids, 13,14 where the lone pair manifests itself by occupying a coordination site, resulting in an inherent, asymmetric coordination environment such as seven-coordinate lead halide polyhedra. [15][16][17] The low-dimensional lead halide hybrids generally exhibit a stoichiometry different from the typical LHPs and are formed by edge-and/or face-sharing polyhedral units.…”