Charge disproportionation in a semiconducting θ-type salt of BTM-TTP

An organic conductor θ-(BTM-TTP)2SbF6, where BTM-TTP denotes 2-[4,5-bis(methylthio)-1,3-dithiol-2-ylidene]-5-(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene, is semiconducting and shows Curie-Weiss-like magnetism, whereas the X-ray structure shows that all BTM-TTP molecules are crystallographically equivalent and the tight-binding band calculation affords the closed Fermi surface. We have studied the electrical properties and Raman spectra of θ-(BTM-TTP)2SbF6 in order to clarify the origin of the carrier localization. From the electrical resistivity measurement, the maximum of activation energy appeared at ca. 240 K. Raman spectroscopy reveals the charge disproportionation in θ-(BTM-TTP)2SbF6 from room temperature, and the degree of charge disproportionation is estimated to be +0.03∼0.13:+0.93 at 6 K.