Ferroelectric instability and topological crystalline insulating nature in PbPo

We have investigated the lattice instability and the topological property of the electronic structure in PbPo in comparison with other IV-VI semiconductors, SnTe and PbTe. In the conventional exchange-correlation schemes of the density functional theory, the fcc structure of PbPo tends to be unstable in the presence of spin-orbit coupling (SOC) under [111] distortion so as to have a ferroelectric instability. This feature is revealed in the calculated phonon dispersion of PbPo by the phonon softening instability at $\mathbf{k}=\mathrm{\ensuremath{\Gamma}}$. But, in the modified Becke-Johnson (mBJ) potential scheme, we have shown that the tendency for SOC-driven ferroelectric instability is suppressed, and the fcc structure becomes stabilized. We have demonstrated that PbPo is a semiconductor having a band inversion at $\mathbf{k}=L$, which leads to a topological crystalline insulator (TCI) phase of PbPo, and the TCI and the ferroelectric states coexist for moderate [111] distortions.