As Jeffrey monologued that "The tetra-n-butyl and iso-amyl ammonium cations appear to provide the "best" and in fact the "only fit" consistent with formation" , no cation has yet appeared that gives a higher melting temperature at ambient pressure. However, the present 5 discovery provides a new aspect, that is, fitness of cation into the orderly hydrogen bond network does not appear to melting temperature of hydrates but gas capacity. What we have seen so far in semiclathrate hydrates may have been specialized for cation inclusion. The excessive fitting of the cation to the derived structures may, however, significantly reduce the gas capacity, as the framework no longer requires support by gas. Conversely, a melting point of 300 K had already 10 been successfully achieved in exchange for a half of the potential gas capacity. From a similar perspective, the HS-III structure can be regarded as a derived structure which sacrificed T cage and P cage to obtain gas capacity of small gases such as methane. These significant changes in properties are not limited to the frame of the FK phase as the present HS-I and the derived structures (Pmma and Imma) both belong to the HS-I in the FK theory. Another important finding is that this new structure achieves the first inversion of gas capture preference in the D cage for CO2 and CH4. This breakthrough challenges the previously accepted concept that clathrate hydrate selectivity cannot be drastically altered for similarly sized gas molecules, paving the way for innovative applications in gas separation technologies. In the future, revisiting this HS-I structure may enable attempts to customize it in these specific directions. As the final piece for the FK clathrate hydrate was obtained by the present study, by designing guest shape, it may be possible to form any mixture of primitive three FK phases to engineering the weak interactions and to reinforce the properties of clathrate hydrate for practical applications such as heat storage, CO2 capture and storage and gas transportation. Frank-Kasper phase in weakly-bound materials Over sixty years ago, Frank and Kasper indicated geometric constraints on the FK phases (1), and Matsumoto and Tanaka formed a hill of thermodynamic potential on the FK triangle map to find that the FK phases are strongly constrained by the host constituents and cannot be stabilized by simple geometric consistency alone. Probably due to this reason, Z phase in self-assembling soft matter had been missing, while it was found a few years ago by accurately adjusting length and interaction between components (43). This study showed that connecting some key blocks of potentially unstable phase by precursors, the T 2 P 2 cage by N4446Cl in this case, can realize it in a stable form. The present technique sheds light on how the clathrates in the broadest sense can be exploited to extend their unique properties. Fortunately, Group 14 elements have options for 20 the host elements and dopants that can cooperate to overcome geometric constraints, similar to how the host structure of the HS-I clathrate hydrates is achieved with the help of ion.