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

We prepared two superconducting phases of (NH 3 ) y Li x FeSe 0.5 Te 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 (NH 3 ) y Li x FeSe 0.5 Te 0.5 over a pressure ( p ) range of 0–14 GPa, and for the high- T c phase of (NH 3 ) y Li x FeSe 0.5 Te 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 (NH 3 ) y Li x FeSe and (NH 3 ) y Cs x FeSe investigated previously. Nevertheless, the double-dome T c – p phase diagram was found in metal-doped FeSe 0.5 Te 0.5 , indicating that this feature is universal in metal-doped FeSe 1− z Te z . Moreover, no structural phase transitions were observed for either the low- T c or high- T c phases of (NH 3 ) y Li x FeSe 0.5 Te 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.