  We show that two different superconducting phases exist at high pressures in
the optimized tetradymite topological insulators Bi2Te2Se (BTS) and
Bi1.1Sb0.9Te2S (BSTS). The superconducting phases emerge at structural phase
transitions, the first at 8.4 GPa for BTS and 12.4 GPa for BSTS, and the second
at 13.6 GPa for BTS and 20.4 GPa, for BSTS. Electronic structure calculations
show that these phases do not have topological character. Comparison of our
results with prior work on Bi2Se3, Bi2Te3 and (Bi,Sb)2(Se,Te)3 allows us to
uncover a universal phase diagram for pressure-induced superconductivity in
tetradymites, providing a basis for understanding the relationships between
topological behavior, crystal structure, and superconductivity for these
materials.

Superconductivity