CRYSTAL AND ELECTRONIC-STRUCTURES OF THE 2-DIMENSIONAL TRANSITION-METAL-COMPLEX MOLECULE ALPHA-[(CH3)(2)(C2H5)(2)N][NI(DMIT)(2)](2) (DMIT=1,3-DITHIOL-2-THIONE-4,5-DITHIOLATE)

The crystal structure of -[(CH3)2(C2H5)2N][Ni(dmit)2]2 (dmit=1,3-dithiol-2- thione-4,5-dithiolate) was determined at 11 K by use of a newly developed x-ray system based on an imaging plate equipped with a closed-cycle He refrigerator. Despite the small splitting and the broadening of the x-ray-diffraction spots due to the drastic phase transition around 240 K, the crystal structure could be successfully determined. The space group of C2/c of the high-temperature phase changes to P21/c at low temperature. The arrangement of (CH3)2(C2H5)2N cations, which are disordered at room temperature, became ordered and there was a change of location of the Ni(dmit)2 anions. Monochromatic Laue photographs (Mo K) have revealed an additional subtle structural change. Except for the discontinuous change around 240 K, the resistivity exhibited a metallic behavior down to 2 K. An extended Huckel tight-binding band calculation, based on the 11-K structure and the x-ray evidence showing the appearance of the twofold superlattice, gave two-dimensional Fermi surfaces, which are consistent with those derived from experiments on magnetoresistance oscillation. © 1995 The American Physical Society.