We explore the nature of the metal-insulator transition in the two-dimensional organic compound -(BEDT-TTF)2Hg(SCN)2Cl by x-ray, electrical transport, ESR, Raman, and infrared investigations. Magnetic and vibrational spectroscopy concurrently reveal a gradual dimerization along the stacking direction , setting in already at the crossover temperature of 150 K from the metallic to the insulating state. A spin gap of meV is extracted. From the activated resistivity behavior below T = 55 K, a charge gap of meV is derived. At TCO = 72 K, the C=C vibrational modes reveal the development of a charge-ordered state with a charge disproportionation of . In addition to a slight structural dimerization, charge-order causes stripes most likely perpendicular to the stacks.
CCDC-1535580 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/.
The small interferences observed below 700 cm−1 are due to the multirefelction of the infrared window, which are temperature independent. Below T = 20 K, additional strong interferences appear in the very far-infrared region due to the transparency of the thin crystal.