Graphene-like conjugated π bond system in Pb_1-xSn_xSe

Following the identification of the π bond in graphene, in this work, a π bond constructed through side-to-side overlap of half-filled 6p_z orbitals was observed in a non-carbon crystal of Pb_1-xSn_xSe (x ∼ 0.34) (PSS), a prototype topological crystalline insulator and thermoelectric material with a high figure-of-merit. PSS compounds with a rock-salt type cubic crystal structure were found to consist of σ bond connected covalent chains of Pb(Sn)-Se with an additional π 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 π bond formed with half-filled 6p_z orbitals between Pb atoms is consistent with the calculated results from quantum chemistry. The presence of π bonds was identified and verified with electron energy-loss spectroscopy through plasmonic excitations and electron density mapping via an inverse Fourier transform of X-ray diffraction.