Charge-ordering transition in orthorhombic and monoclinic single-crystals of θ-(BEDT-TTF)2TlZn(SCN)4 studied by vibrational spectroscopy -: art. no. 085114

We present a comparative study of the charge-ordered states in orthorhombic and monoclinic forms of theta-(BEDT-TTF)(2)TlZn(SCN)(4) [BEDT-TTF=bis(ethylenedithiolo) tetrathiafulvalene] by means of vibrational spectroscopy with the aid of electrical resistivity and x-ray diffraction experiments. Theoretical consideration is performed for the splitting of the vibronic modes of the Raman-active Cdouble bondC stretching of BEDT-TTF. Based on the assignment and factor group analysis of the vibronic modes, we examined the charge-ordering pattern. The orthorhombic form provides a horizontal stripe of the localized holes, which is the same result as in the isostructural theta-(BEDT-TTF)(2)RbZn(SCN)(4). The insulator-to-insulator phase transition of the monoclinic form has been shown to be accompanied by charge ordering. In contrast to the orthorhombic form, the charge-ordering pattern is a diagonal stripe. An unusual nonmetallic state in the high-temperature phase is discussed based on the line shape of the Raman spectrum.