Superconductivity in (N H3)y N axFeS e0.5 T e0.5

Na-intercalated FeSe0.5Te0.5 was prepared using the liquid NH3 technique, and a superconducting phase exhibiting a superconducting transition temperature (Tc) as high as 27 K was discovered. This can be called the high-Tc phase since a 21 K superconducting phase was previously obtained in (NH3)yNaxFeSe0.5Te0.5. The chemical composition of the high-Tc phase was determined to be (NH3)0.61(4)Na0.63(5)Fe0.85Se0.55(3)Te0.44(2). The x-ray diffraction patterns of both phases show that a larger lattice constant c (i.e., FeSe0.5Te0.5 plane spacing) produces a higher Tc. This behavior is the same as that of metal-doped FeSe, suggesting that improved Fermi-surface nesting produces the higher Tc. The high-Tc phase converted to the low-Tc 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 Tc versus c phase diagram was prepared for the metal-doped FeSe0.5Te0.5, which is similar to that of metal-doped FeSe, although the Tc is lower.