Stable Two-Dimensional Metallic State with Stacking Motif of "Spanning Overlap" in -[(CH3)2(C2H5)2N][Ni(dmit)22

The crystal structure of -[(CH3)2(C2H5)2N] [Ni(dmit)2]2, Ni2S20C18NH16 was determined by use of a four-circle diffractometer (at 294 K) and imaging plate (IP) system equipped with a He refrigerator (7 K). Crystal data: at 294 K, orthorhombic Pnma, a=14.014(4) ?, b=37.072(13) ?, c=6.519(2) ?, V= 3386.8(1.8) ?3, Z=4, R(Rw)=0.066(0.051) for 1284 reflections; at 7 K, orthorhombic P212121, a=13.760(45) ?, b=36.622(77) ?, c=6.500(2) ?, V=3275.5(1.4) ?3, Z=4, R(Rw)=0.062(0.077) for 3064 reflections. The magnetic susceptibility measurement of -[(CH3)2(C2H5)2N][Ni(dmit)2]2 showed that it was metallic down to 2 K with the Pauli-like magnetic susceptibility. This susceptibility behavior is quite different from that of the isomorphous -[(CH3)2(C2H5)2N][Ni(dmit)2]2. The tight-binding band structure calculation by extended Hückel methods indicated that -[(CH3)2(C2H5)2N][Ni(dmit)2]2 has the round two-dimensional Fermi surfaces at 7 K. The conductivity measurements showed no sign of the superconducting transition down to 0.1 K.