Discontinuous change of metal-insulator transition in the alloyed θ-system:: θ-(BEDT-TTF)2(Rb1-xCsx)Zn(SCN)4

The behaviors of the metal-insulator (MI) transitions for theta-(BEDT-TTF)(2) (Rb1-xCsx) Zn(SCN)(4) [x = 0.00, 0.07, 0.25, 0.34, 0.47, 0.61, 1.00] were investigated by transport and magnetic measurements. Although the unit cell volume (V) increases and the dihedral angle between the donor columns (theta) decreases in proportion to the Cs content (x), the MI transition temperature does not change continuously. For a I 0.07, the MI transitions occur around 200 K with a large hysteresis due to the structural modulation coupled with charge ordering. Magnetic behaviors in slow and rapid cooling runs are distinctly different. These transitions (x less than or equal to 0.07) are called the Rb-type. The MI transition temperature at x = 0.25 drops to 35 K abruptly and the transition temperature decreases slowly with increasing a: further. The transport and magnetic behaviors show a reduced hysteresis and no hysteresis is observed at x = 1. These features characterize the Cs-type transition (0.25 less than or equal to x). These results indicate that the mechanism of the MI transitions for the isostructural M = Rb and Cs of theta-(BEDT-TTF)(2)MZn(SCN)(4) is different from each other, influenced by the structural modulation feature.