Superconducting properties of (NH₃) _y Li _x FeSe_0.5Te_0.5 under pressure

We prepared two superconducting phases of (NH₃) _y Li _x FeSe_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) _y Li _x FeSe_0.5Te_0.5 over a pressure (p) range of 0-14 GPa, and for the high-T _c phase of (NH₃) _y Li _x FeSe_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) _y Li _x FeSe and (NH₃) _y Cs _x FeSe 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) _y Li _x FeSe_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.