摘要：We previously discovered multiple superconducting phases in the ammoniated Na-doped FeSe material, (NH3)yNaxFeSe. To clarify the origin of the multiple superconducting phases, the variation of T c was fully investigated as a function of x in (NH3)yNaxFeSe. The 32 K superconducting phase is mainly produced in the low-x region below 0.4, while only a single phase is observed at x = 1.1, with T c = 45 K, showing that the T c depends significantly on x, but it changes discontinuously with x. The crystal structure of (NH3)yNaxFeSe does not change as x increases up to 1.1, i.e. , the space group of I 4/mmm. The lattice constants, a and c , of the low- T c phase ( T c = 32.5 K) are 3.9120(9) and 14.145(8) Å, respectively, while a = 3.8266(7) Å and c = 17.565(9) Å for the high- T c phase (~46 K). The c increases in the high T c phase, implying that the T c is directly related to c . In (NH3)yLixFeSe material, the T c varies continuously within the range of 39 to 44 K with changing x. Thus, the behavior of T c is different from that of (NH3)yNaxFeSe. The difference may be due to the difference in the sites that the Na and Li occupy.
