Crystal and electronic structures of the two-dimensional transition-metal-complex molecule alpha -

Abstract

The crystal structure of \ensuremath{\alpha}-[(${\mathrm{CH}}_{3}$${)}_{2}$(${\mathrm{C}}_{2}$${\mathrm{H}}_{5}$${)}_{2}$N][Ni(dmit${)}_{2}$${]}_{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 P${2}_{1}$/c at low temperature. The arrangement of (${\mathrm{CH}}_{3}$${)}_{2}$(${\mathrm{C}}_{2}$${\mathrm{H}}_{5}$${)}_{2}$N cations, which are disordered at room temperature, became ordered and there was a change of location of the Ni(dmit${)}_{2}$ anions. Monochromatic Laue photographs (Mo K\ensuremath{\alpha}) 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.