PbO and SnO have the same crystal structure and similar ionicity.
Can it be explained by orbital molecular theory?
I guess that different atomic radii of Pb and Sn result in different bonding strengths. Why not perform a DFT calculation? :-)
The bandgap depends on the interaction of potentials. For example, Graphene and-BN has the same structure, however, its bandgaps are totally different
In the periodic table, Pb is under Sn in the 4th group. Oxygen O is the 6th group.
SnO and PbO have different ionicity, with the ionicity of PbO approximately 33%.
The band gap depends on the match between the orbitals in the covalent bond.
Sn has electronic structure 5s2 5p2
Pb has electronic structure 6s2 6p2
O has electronic structure 2s2 2p4
Thus the pi bonding between 2p4 - 5p2 (SnO) is stronger than the pi bonding between 2p4 - 6p2 (PbO) and the band gap of SnO (0.7 eV) is lower than the band gap of PbO (1.9 eV)
Dear Cheng Chang in addition to the helpful expert answers which have already been provided, I just came across a very interesting article in which the issue has been discussed in detail. Moreover, the authors studied mixed-metal Sn1−xPbxO alloys which could have practiclal applications in energy conversion in solar cells, photocatalysis and hydrogen generation. For more information please have a look at the following relevant article:
Electronic properties of the Sn1−xPbxO alloy and band alignment of the SnO/PbO system: a DFT study
Article Electronic properties of the Sn(1−x)Pb(x)O alloy and band al...
This paper has been posted by the authors as public full text on RG so that you can freely access a pdf file of the article.
Good luck with your research and best wishes, Frank Edelmann
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