Graphene-like conjugated pi-bond system in Pb1-xSnxSe

TL;DR

This study reveals a graphene-like conjugated pi-bond system in Pb1-xSnxSe, a topological crystalline insulator, through experimental and theoretical analysis, highlighting its unique electronic structure and bonding characteristics.

Contribution

It demonstrates the existence of a conjugated pi-bond system in a non-carbon crystal, expanding understanding of pi bonding beyond graphene.

Findings

Pi bonds observed in Pb1-xSnxSe via EELS and electron density mapping.

Pi bonds form a conjugated system among Pb atoms in the crystal.

The pi bond presence aligns with quantum chemistry calculations.

Abstract

Following the identification of the pi bond in graphene, in this work, a pi bond constructed through side-to-side overlap of half-filled 6pz orbitals was observed in a non-carbon crystal of Pb1-xSnxSe (x=0.34) (PSS), a prototype topological crystalline insulator (TCI) and thermoelectric material with a high figure-of-merit (ZT). PSS compounds with a rock-salt type cubic crystal structure was found to consist of sigma bond connected covalent chains of Pb(Sn)-Se with an additional pi bond that is shared as a conjugated system among the four nearest neighbor Pb pairs in square symmetry within all (001) monoatomic layers per cubic unit cell. The pi bond formed with half-filled 6pz orbitals between Pb atoms is consistent with the calculated results from quantum chemistry. The presence of pi bonds was identified and verified with electron energy-loss spectroscopy (EELS) through plasmonic excitations and electron density (ED) mapping via an inverse Fourier transform of X-ray diffraction.