Charge-sensitive vibrational modes in BEDT-TTF salts: Signatures of charge ordering and site charge

BEDT-TTF-based organic conductors host a number of ground states, tuned by electron repulsion. Knowledge of the charge distribution on the molecular sites in these materials is key to understanding the origin of different ground states. We survey and discuss the C=C stretching modes in BEDT-TTF based molecular conductors, which are extremely crucial in the characterization of charge-ordered insulators and are recently linked to superconductivity in some compounds. Focusing on the known examples of BEDT-TTF+0.5 salts, we analyze the reliability of the C=C stretching modes for the determination of charge ordering and absolute site charge. Considering the charge-ordered states, a prominent shift in frequency of 141 cm-1 per elementary charge e for ν27(b1u) and 98 cm-1/e for ν2(ag) can be clearly observed; however, the distribution resulting from different compounds spans over 20 cm-1. For nominal BEDT-TTF+0.5 compounds, the distribution of the resonance also extends around 20 cm-1, yielding an unexpectedly large uncertainty of Δρ ≈ (±0.04)e, which is presumably due to the influence of small differences in the structure. This highlights the limitations of charge-frequency relations to detect small deviations in absolute charge values on molecular lattice sites and emphasizes the use of the relations to estimate charge-ordering, rather than absolute site charge.