A New Genuine Mott Insulator: beta-(BEDT-TTF)TaF6

The structural, transport, and magnetic properties of the title compound have been investigated. This compound has a uniform donor stacking structure without dimerization, and the donor-to-anion ratio is 1 : 1. A large slip distance along the molecular long axis reduces the transfer integrals along the stacking and diagonal directions. The target compound is considered as a quasi-two-dimensional square lattice with an exactly half-filled band. The electrical resistivity is as large as 5.4 x 105 omega cm even at room temperature, and the charge gap is 0.55 eV. The electron spin resonance and magnetic torque reveal that the ground state is an antiferromagnetic ordered state below the Neel temperature of 10 K, and the spin-flop field is 0.82 T at 1.6 K. These results indicate that the target compound is a genuine Mott insulator.