Cascade of pressure-driven phase transitions in the topological nodal-line superconductor PbTaSe2

We report a succession of pressure-tuned structural transitions in the topological nodal-line superconductor ${\mathrm{PbTaSe}}_{2}$, evidenced from synchrotron x-ray diffraction, elastic neutron scattering, Raman spectroscopy, and electrical transport measurements up to 56 GPa, accompanied by first-principles calculations to uncover the evolution of the underlying electronic structure. In contrast to the previously proposed small shift of the Pb Wyckoff coordinate in the sub-GPa regime, our study reveals that it is rather a transition from $P\overline{6}m2$ ($\ensuremath{\alpha}$ phase) to $P{6}_{3}mc$ ($\ensuremath{\beta}$ phase), subsequently followed by a transition to $P6/mmm$ ($\ensuremath{\gamma}$ phase) at $\ensuremath{\sim}7.5$ GPa and to $Pmmm$ ($\ensuremath{\delta}$ phase) at $\ensuremath{\sim}44$ GPa. In addition, the first-principles calculations unambiguously demonstrate the multiple types of topological fermions associated with these different pressure-driven structures. Collectively, our results not only present the intriguing structural transitions in this topological ${\mathrm{PbTaSe}}_{2}$ superconductor, they also provide the impetus to study topological phase transitions and their physical consequences in a broader class of topological materials.