  Band gap anomaly is a well-known issue in lead chalcogenides PbX (X=S, Se,
Te, Po). Combining ab initio calculations and tight-binding (TB) method, we
have studied the band evolution in PbX, and found that the band gap anomaly in
PbTe is mainly related to the high onsite energy of Te 5s orbital and the large
s-p hopping originated from the irregular extended distribution of Te 5s
electrons. Furthermore, our calculations show that PbPo is an indirect band gap
(6.5 meV) semiconductor with band inversion at L point, which clearly indicates
that PbPo is a topological crystalline insulator (TCI). The calculated mirror
Chern number and surface states double confirm this conclusion.
