Pressure dependence of superconductivity in low- and high- Tc phases of (NH3)yNaxFeSe

We prepared two superconducting phases, which are called ``low-${T}_{\mathrm{c}}$ phase'' and ``high-${T}_{\mathrm{c}}$ phase'' of ${(\mathrm{N}{\mathrm{H}}₃)}_y\mathrm{N}{\mathrm{a}}_x\mathrm{FeSe}$ showing ${T}_{\mathrm{c}}$'s of 35 and 44 K, respectively, at ambient pressure, and studied the superconducting behavior and structure of each phase under pressure. The ${T}_{\mathrm{c}}$ 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., ${T}_{\mathrm{c}}<1.4\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The ${T}_{\mathrm{c}}$ 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-${T}_{\mathrm{c}}$ phase (called ``SC-II'') between 0 and 25 GPa for the low-${T}_{\mathrm{c}}$ and high-${T}_{\mathrm{c}}$ phases of ${(\mathrm{N}{\mathrm{H}}₃)}_y\mathrm{N}{\mathrm{a}}_x\mathrm{FeSe}$, differing from the behavior of ${(\mathrm{N}{\mathrm{H}}₃)}_y\mathrm{C}{\mathrm{s}}_x\mathrm{FeSe}$, which has a pressure-driven high-${T}_{\mathrm{c}}$ phase (SC-II) in addition to the superconducting phase (SC-I) observed at ambient and low pressures. The ${T}_{\mathrm{c}}\ensuremath{-}c$ phase diagram for both low-${T}_{\mathrm{c}}$ and high-${T}_{\mathrm{c}}$ phases shows that the ${T}_{\mathrm{c}}$ can be linearly scaled with $c$ (or FeSe plane spacing), where $c$ is a lattice constant. The reason why a pressure-driven high-${T}_{\mathrm{c}}$ phase (SC-II) was found for neither low-${T}_{\mathrm{c}}$ nor high-${T}_{\mathrm{c}}$ phases of ${(\mathrm{N}{\mathrm{H}}₃)}_y\mathrm{N}{\mathrm{a}}_x\mathrm{FeSe}$ is fully discussed, suggesting a critical $c$ value as the key to forming the pressure-driven high-${T}_{\mathrm{c}}$ phase (SC-II). Finally, the precise ${T}_{\mathrm{c}}\ensuremath{-}c$ phase diagram is depicted using the data obtained thus far from FeSe codoped with a metal and $\mathrm{N}{\mathrm{H}}₃$ or amine, indicating two distinct ${T}_{\mathrm{c}}\ensuremath{-}c$ lines below $c=17.5\phantom{\rule{0.16em}{0ex}}\AA{}$.