Low temperature superstructure and transfer integrals in κ-(BEDT-TTF)2Cu[N(CN)2]X : X = Cl, Br

Low temperature crystal structures and band parameters were presented for the deuterized (d) and the hydrogenized (h) kappa - (BEDT-TTF)2Cu[N(CN)2]Cl and for deuterized kappa-(BEDT-TTF)2Cu[N(CN)2]Br. An enhanced molecular dimerization was observed for the Cl salts. Theoretical prediction by H. Kino and H. Fukuyama that the molecular dimerization stabilizes the Mott insulating state was confirmed. Below 200 K, X-ray diffuse scattering study revealed formation of two dimensional ax2c superstructure in the deuterized kappa -(BEDT-TTF)2CU[N(CN)2]Br. The structural disorder for the d-Br salt was observed in the superstructure and in the terminal ethylenes.