Superconducting properties of (NH3)yLixFeSe0.5Te0.5 under pressure.

We prepared two superconducting phases of (NH₃)_yLi_xFeSe_0.5Te_0.5, which show superconducting transition temperatures (T_c's) as high as 20.2 and 29.5 K at ambient pressure, here called the 'low-T_c phase' and 'high-T_c phase'. The temperature dependence of electrical resistance (R) was measured for the low-T_c phase of (NH₃)_yLi_xFeSe_0.5Te_0.5 over a pressure (p) range of 0–14 GPa, and for the high-T_c phase of (NH₃)_yLi_xFeSe_0.5Te_0.5 over 0–19 GPa, yielding double-dome superconducting T_c–p phase diagrams, i.e. two superconducting phases (SC-I and SC-II) were found for both the low-T_c and high-T_c phases under pressure. For the low-T_c phase, the maximum T_c was 20.2 K at 0 GPa for SC-I, and 19.9 K at 8.98 GPa for SC-II. For the high-T_c phase, the maximum T_c was 33.0 K at 1.00 GPa for SC-I, and 24.0 K at 11.5–13.2 GPa for SC-II. These results imply that the maximum T_c value of the high pressure phase (SC-II) does not exceed the maximum value of the SC-I, unlike what was shown in the T_c–p phase diagrams of (NH₃)_yLi_xFeSe and (NH₃)_yCs_xFeSe investigated previously. Nevertheless, the double-dome T_c–p phase diagram was found in metal-doped FeSe_0.5Te_0.5, indicating that this feature is universal in metal-doped FeSe_1−zTe_z. Moreover, no structural phase transitions were observed for either the low-T_c or high-T_c phases of (NH₃)_yLi_xFeSe_0.5Te_0.5 over the wide pressure range of 0–15.3 GPa, and the T_c-lattice constant (c) plots for both phases were recorded to determine the critical point separating SC-I and SC-II. [ABSTRACT FROM AUTHOR]