Superconductivity in ( N H 3 ) y N a x FeS e 0.5 T e 0.5

Na-intercalated $\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ was prepared using the liquid $\mathrm{N}{\mathrm{H}}_{3}$ technique, and a superconducting phase exhibiting a superconducting transition temperature $({T}_{\mathrm{c}})$ as high as 27 K was discovered. This can be called the high-${T}_{\mathrm{c}}$ phase since a 21 K superconducting phase was previously obtained in ${(\mathrm{N}{\mathrm{H}}_{3})}_{y}\mathrm{N}{\mathrm{a}}_{x}\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$. The chemical composition of the high-${T}_{\mathrm{c}}$ phase was determined to be ${(\mathrm{N}{\mathrm{H}}_{3})}_{0.61(4)}\mathrm{N}{\mathrm{a}}_{0.63(5)}\mathrm{F}{\mathrm{e}}_{0.85}\mathrm{S}{\mathrm{e}}_{0.55(3)}\mathrm{T}{\mathrm{e}}_{0.44(2)}$. The x-ray diffraction patterns of both phases show that a larger lattice constant $c$ (i.e., $\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ plane spacing) produces a higher ${T}_{\mathrm{c}}$. This behavior is the same as that of metal-doped FeSe, suggesting that improved Fermi-surface nesting produces the higher ${T}_{\mathrm{c}}$. The high-${T}_{\mathrm{c}}$ phase converted to the low-${T}_{\mathrm{c}}$ phase within several days, indicating that it is a metastable phase. The temperature dependence of resistance for both phases was recorded at different magnetic fields, and the critical fields were determined for both phases. Finally, the ${T}_{\mathrm{c}}$ versus $c$ phase diagram was prepared for the metal-doped $\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$, which is similar to that of metal-doped FeSe, although the ${T}_{\mathrm{c}}$ is lower.