Infrared and Raman studies of θ-(BEDT-TTF)2CsZn(SCN)4:: Comparison with the frozen state of θ-(BEDT-TTF)2RbZn(SCN)4

We present the optical conductivity and Raman spectra of theta-(BEDT-TTF)(2)CsZn(SCN)(4). The vibrational and vibronic bands in the 1200-1600cm(-1) region was assigned with the aid of the C-13- substituted compound. The nearly uniform charge distribution was found in a whole temperature range from 300 to 6 K. However, the space group symmetry I222 is locally broken already at room temperature. The broken symmetry suggests the resemblance to the high-temperature phase and frozen state of theta-(BEDT-TTF)(2)RbZn(SCN)(4), which consists of short-range ordered charge-ordering domains. The amplitude in the charge-ordering domain of theta-(BEDT-TTF)(2)CsZn(SCN)(4) is extremely small in a whole temperature range. At room temperature, hydrostatic pressure narrows the bandwidth of theta-(BEDT-TTF)(2)CsZn(SCN)(4) to enlarge the amplitude in short-ranged ordered domains. The long-range ordered charge ordering in theta-(BEDT-TTF)(2)CsZn(SCN)(4) Was found at 10 K under the hydrostatic pressure of 2.5 GPa.