Topological insulators constitute a new phase of matter protected by symmetries. Time-reversal symmetry protects strong topological insulators of the Z2 class, which possess an odd number of metallic surface states with dispersion of a Dirac cone. Topological crystalline insulators are less robust and merely protected by individual crystal symmetries. They show an even number of Dirac cones. In this work, it is demonstrated that Bi-doping of Pb1−xSnxSe (111) epilayers induces a quantum phase transition from a topological crystalline insulator to a Z2 topological insulator. This occurs because Bi-doping lifts the fourfold valley degeneracy and induces a gap at Γ, while the three Dirac cones at the M points of the surface Brillouin zone remain intact. We interpret this new phase transition as caused by a lattice distortion. It implies that the system becomes ferroelectric. Our results demonstrate that strong topological insulators can be switchable by distortions or electric fields.
 P. S. Mandal, G. Springholz, V. V. Volobuev, O. Caha, A. Varykhalov, E. Golias, G. Bauer, O. Rader, J. Sánchez-Barriga, Nat. Commun. 8, 968 (2017)