A new semiconductor of organic crystal (BEDT-TTF)~2HgCl~3·TCE, [ (BEDT-TTF)= bis(ethylenedithio)tetrathiafulvalene, TCE=1,1,2- trichloroethane] has been synthesized by electro-crystallization method. Its structure was determined by four-circle X-ray diffraction method. ln(R/R~0) of its single crystal on bc-plane was measured to have a linear relationship to T^-^1. In (BEDT-TTF)~2HgCl~3·TCE crystal, the BEDT-TTF cationic radicals are arranged side-by-side equidistantly forming a kind of one-dimensional structure along the b axis. (HgCl~3)^- anions also form another kind of one-dimensional polymers (HgCl~3)~n^n^- along this direction. Therefore, (BEDT-TTF) ~2HgCl~3·TCE is an one-dimensional organic semiconductor. In order to reveal the conductivity-structure relationships, we have also synthesized the known organic superconductor β-(BEDT－TTF)~2I~3 and discussed why the conductivity of β-(BEDT－TTF)~2I~3 is superior to the of (BEDT-TTF)~2HgCl~3·TCE.

Key words: SUPERCONDUCTORS, CRYSTAL STRUCTURE, ELECTRICAL CONDUCTIVITY, FULVALENE (=BICYCLOPENTADIENYLI-DENE), TRICHLOROETHANE, MERCURY CHLORIDE