The metal-insulator transition in the organic conductor ${\beta }^{\mathrm{\prime }\mathrm{\prime }}$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl

We explore the nature of the metal-insulator transition in ${\beta }^{\mathrm{\prime }\mathrm{\prime }}$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl by x-ray, electrical transport, ESR, Raman and infrared investigations. Magnetic and vibrational spectroscopy concurrently reveal a gradual dimerization along the stacking direction $a$, setting in already at the crossover temperature of 150K from the metallic to the insulating state. A spin gap ${\mathrm{\Delta }}_{\sigma }=47$meV is extracted. From the activated resistivity behavior below 55K a charge gap ${\mathrm{\Delta }}_{\rho }=60$meV is derived. At $T_{\mathrm{C}\mathrm{O}}=72$K the C=C vibrational modes reveal the development of a charge-ordered state with a charge disproportionation of $2{\delta }_{\rho }=0.34e$