Investigation of X-ray diffuse scattering in θ-(BEDT-TTF)2RbM′(SCN)4

The modulated structure of theta-(BEDT-TTF)(2)RbM'(SCN)(4); (M'=Zn, Co) were investigated by X-ray diffraction method. theta-(BEDTTTF)(2)RbM'(SCN)(4) are two-dimensional organic conductors which exhibit metal-insulator transition at 210 K. The insulating phase is known as a charge ordered state. Above T-MI = 210 K, diffuse rod of q=(1/3 k 1/4) was found. Below T-MI, there are some differences depending on the sample cooling speed. In the slow-cooled-state (0.2 K/min), the diffuse rod of q=(1/3 k 1/4) disappears and is replaced by superlattice of c*/2. In the rapid-cooled-state (5-6 K/min), the structural phase transition at T-MI becomes imperfect. The diffuse rod of q remains below T-MI and the lattice modulation of c*/2 is disordered in the direction of inter-conducting-layer.