Two-dimensional radical-cationic Mott insulator based on an electron donor containing neither a tetrathiafulvalene nor tetrathiapentalene skeleton

We report the structure and physical properties of a new radical-cationic Mott insulator theta-(BEDT-BDT)PF6(BEDT-BDT: benzo(1,2-g:4,5-g']bis(thieno[2,3-b][1,4]dithiin)). The crystal structure belongs to a monoclinic system with a 1:1 donor/anion ratio. Based on this composition, the salt is considered a half-filled Mott insulator, in which the BEDT-BDT molecules form a theta mode arrangement. Owing to the strong electron correlation, the temperature dependence of the resistivity exhibits semiconducting behaviour. The static magnetic susceptibility first follows the Curie-Weiss law down to 15 K, but then follows the two-dimensional (2D) triangular Heisenberg model with J = 7.5 K below 15 K. Even at 2 K, this salt is paramagnetic and no magnetic order is formed. The tight-binding band calculation reveals a 2D Fermi surface. It is remarkable that theta-(BEDT-BDT)PF6 forms a 2D electronic structure without any 1,3-dithiol-2-ylidene units, unlike tetrathiafulvalene and tetrathiapentalene donors.