The superconducting transition temperature T c of ammoniated metal-doped FeSe (NH 3 ) y M x FeSe (M: metal atom) has been scaled with the FeSe plane spacing, and it has been suggested that the FeSe plane spacing depends on the location of metal atoms in (NH 3 ) y M x FeSe crystals. Although the crystal structure of (NH 3 ) y Li x FeSe exhibiting a high T c (∼44 K) was determined from neutron diffraction, the structure of (NH 3 ) y M x FeSe exhibiting a low T c (∼32 K) has not been determined thus far. Here, we determined the crystal structure of (NH 3 ) y Cs 0.4 FeSe (T c = 33 K) through the Rietveld refinement of the x-ray diffraction (XRD) pattern measured with synchrotron radiation at 30 K. The XRD pattern was analyzed based on two different models, on-center and off-center, under a space group of I 4/mmm. In the on-center structure, the Cs occupies the 2a site and the N of NH 3 may occupy either the 4c or 2b site, or both. In the off-center structure, the Cs may occupy either the 4c or 2b site, or both, while the N occupies the 2a site. Only an on-center structure model in which the Cs occupies the 2a and the N of NH 3 occupies the 4c site provided reasonable results in the Rietveld analysis. Consequently, we concluded that (NH 3 ) y Cs 0.4 FeSe can be assigned to the on-center structure, which produces a smaller FeSe plane spacing leading to the lower T c .

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 Tc 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 Tc =  45 K, showing that the Tc 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 I4/mmm. The lattice constants, a and c, of the low-Tc phase (Tc = 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-Tc phase (~46 K). The c increases in the high Tc phase, implying that the Tc is directly related to c. In (NH3)yLixFeSe material, the Tc varies continuously within the range of 39 to 44 K with changing x. Thus, the behavior of Tc is different from that of (NH3)yNaxFeSe. The difference may be due to the difference in the sites that the Na and Li occupy.

This article reviews new superconducting phases of carbon-based materials. During the past decade, new carbon-based superconductors have been extensively developed through the use of intercalation chemistry, electrostatic carrier doping, and surface-proving techniques. The superconducting transition temperature T c of these materials has been rapidly elevated, and the variety of superconductors has been increased. This review fully introduces graphite, graphene, and hydrocarbon superconductors and future perspectives of high-T c superconductors based on these materials, including present problems. Carbon-based superconductors show various types of interesting behavior, such as a positive pressure dependence of T c. At present, experimental information on superconductors is still insufficient, and theoretical treatment is also incomplete. In particular, experimental results are still lacking for graphene and hydrocarbon superconductors. Therefore, it is very important to review experimental results in detail and introduce theoretical approaches, for the sake of advances in condensed matter physics. Furthermore, the recent experimental results on hydrocarbon superconductors obtained by our group are also included in this article. Consequently, this review article may provide a hint to designing new carbon-based superconductors exhibiting higher T c and interesting physical features.

Na-intercalated FeSe 0.5 Te 0.5 was prepared using the liquid NH 3 technique, and a superconducting phase exhibiting a superconducting transition temperature (T c ) as high as 27 K was discovered. This can be called the high-T c phase since a 21 K superconducting phase was previously obtained in (NH 3 ) y Na x FeSe 0.5 Te 0.5 . The chemical composition of the high-T c phase was determined to be (NH 3 ) 0.61(4) Na 0.63(5) Fe 0.85 Se 0.55(3) Te 0.44(2) . The x-ray diffraction patterns of both phases show that a larger lattice constant c (i.e., FeSe 0.5 Te 0.5 plane spacing) produces a higher T c . This behavior is the same as that of metal-doped FeSe, suggesting that improved Fermi-surface nesting produces the higher T c . The high-T c phase converted to the low-T 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 c versus c phase diagram was prepared for the metal-doped FeSe 0.5 Te 0.5 , which is similar to that of metal-doped FeSe, although the T c is lower.