Discontinuous Change of Metal-Insulator Transition in the Alloyed θ-system: θ-(BEDT-TTF)₂(Rb ₁-x Cs_x)Zn(SCN)₄

Abstract

The behaviors of the metal-insulator (MI) transitions for θ -(BEDT-TTF)₂(Rb_1-xCs_x)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 (θ ) decreases in proportion to the Cs content (x), the MI transition temperature does not change continuously. For x≤ 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≤ 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 x 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≤ x). These results indicate that the mechanism of the MI transitions for the isostructural M=Rb and Cs of θ -(BEDT-TTF)₂MZn(SCN)₄ is different from each other, influenced by the structural modulation feature.