Pressure dependence of superconductivity in low- and high-T-c phases of (NH3)(y)NaxFeSe

We prepared two superconducting phases, which are called low-T-c phase and high-T-c phase of (NH3)(y)NaxFeSe showing T-c's of 35 and 44 K, respectively, at ambient pressure, and studied the superconducting behavior and structure of each phase under pressure. The T-c of the 35 K at ambient pressure rapidly decreases with increasing pressure up to 10 GPa, and it remains unchanged up to 22 GPa. Finally, superconductivity was not observed down to 1.4 K at 29 GPa, i.e., T-c < 1.4K. The T-c of the 44 K phase also shows a monotonic decrease up to 15 GPa and it weakly decreases up to 25 GPa. These behaviors suggest no pressure-driven high-T-c phase (called SC-II) between 0 and 25 GPa for the low-T-c and high-T-c phases of (NH3)(y)NaxFeSe, differing from the behavior of (NH3)(y)CsxFeSe, which has a pressure-driven high-T-c phase (SC-II) in addition to the superconducting phase (SC-I) observed at ambient and low pressures. The T-c-c phase diagram for both low-T-c and high-T-c phases shows that the T-c can be linearly scaled with c (or FeSe plane spacing), where c is a lattice constant. The reason why a pressure-driven high-T-c phase (SC-II) was found for neither low-T-c nor high-T-c phases of (NH3)(y)NaxFeSe is fully discussed, suggesting a critical c value as the key to forming the pressure-driven high-T-c phase (SC-II). Finally, the precise T-c-c phase diagram is depicted using the data obtained thus far from FeSe codoped with a metal and NH3 or amine, indicating two distinct T-c-c lines below c = 17.5 angstrom.