We previously discovered multiple superconducting phases in the ammoniated Na-doped FeSe material, (NH 3)yNaxFeSe . To clarify the origin of the multiple superconducting phases, the variation of T c was fully investigated as a function of x in (NH 3)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 = 45K, showing that the T c depends significantly on x, but it changes discontinuously with x. The crystal structure of (NH 3)yNaxFeSe does not change as x increases up to 1.1, i.e., the space group of I4/mmm. The lattice constants, a and c, of the low-T c phase (T c =32.5K) 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 (∼46K). The c increases in the high T c phase, implying that the T c is directly related to c. In (NH 3) y Li x FeSe material, the T c varies continuously within the range of 39 to 44K with changing x. Thus, the behavior of T c is different from that of (NH 3)yNaxFeSe . The difference may be due to the difference in the sites that the Na and Li occupy.
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