Highly responsive ground state of PbTaSe2 : Structural phase transition and evolution of superconductivity under pressure

Transport and magnetic studies of ${\mathrm{PbTaSe}}₂$ under pressure suggest the existence of two superconducting phases with the low temperature phase boundary at $\ensuremath{\sim}0.25$ GPa that is defined by a very sharp, first order, phase transition. The first order phase transition line can be followed via pressure dependent resistivity measurements, and is found to be near 0.12 GPa near room temperature. Transmission electron microscopy and x-ray diffraction at elevated temperatures confirm that this first order phase transition is structural and occurs at ambient pressure near $\ensuremath{\sim}425$ K. The new, high temperature/high pressure phase has a similar crystal structure and slightly lower unit cell volume relative to the ambient pressure, room temperature structure. Based on first-principles calculations this structure is suggested to be obtained by shifting the Pb atoms from the $1a$ to $1e$ Wyckoff position without changing the positions of Ta and Se atoms. ${\mathrm{PbTaSe}}₂$ has an exceptionally pressure sensitive, structural phase transition with $\mathrm{\ensuremath{\Delta}}{T}_s/\mathrm{\ensuremath{\Delta}}P\ensuremath{\approx}\ensuremath{-}1400$ K/GPa near room temperature, and $\ensuremath{\approx}\ensuremath{-}1700$ K/GPa near 4 K. This first order transition causes a $\ensuremath{\sim}1$ K ($\ensuremath{\sim}25%$) steplike decrease in ${T}_c$ as pressure is increased through 0.25 GPa.