Anion size and isotope effects in (EDO-TTF)2XF6

(EDO-TTF)(2)PF6 is a quasi one-dimensional molecular conductor and undergoes a peculiar metal-insulator (MI) transition. The phase transition mechanism is cooperative among Peierls, charge ordering, and anion ordering transitions, which is accompanied with a distinct molecular deformation. (EDO-TTF)(2)XF6 (X = P (1h), As (2h), Sb (3h)) showed that the ransition temperature (T-MI) and the width of thermal hysteresis (Delta T-MI) were systematically changed depending on the anion size, i.e., the T-MI decreased in order of 279.0 K (1h), 270.8 K (2h), and 242 K (3h) and the Delta T-MI increased in order of 1.0 K (1h), 5.5 K (2h), and 14 K (3h). The deuterated salts, (EDO-TTF-d(2))(2)XF6 (X = P (1d) and As (2d)), showed higher T-MI of 282.0 (1d) and 274.0 K (2d) and narrower Delta T-MI of 0.0 (1d) and 5.0 K (2d) than the undeuterated 1h and 2h, respectively. X-ray analysis revealed the anisotropic thermal expansion of unit cell. The linear relationship between Delta T-MI and volume change at T-MI was found.