A New Charge-Ordered Molecular Conductor: <i>κ</i>-(BEDT-TTF)₂K⁺(18-crown-6)[Co^II(NCS)₄]∙(H₂O)

Abstract

A new molecular conductor, i.e., κ-(BEDT-TTF)2K+(18-crown-6)[CoII(NCS)4]∙(H2O), is semiconductive with substantial charge gap values (ΔE) of 0.57 eV (measured) and 0.37 eV (calculated). There is a full band separation despite formal average charge on BEDT-TTF of +0.5 and κ(kappa)-type packing of BEDT-TTF dimers that favors high conductivity. X-ray crystal structure analysis reveals complete charge ordering with full Coulomb charge on unique BEDT-TTF radical cations A (QA = +1), while unique molecules B are uncharged (QB = 0). Geometries of A (flat) and B (bent) differ considerably and are in accordance with the ascribing charges. Charge segregation is enhanced by forming tight face-to-face BEDT-TTF dimers AA (QAA = +2) and BB (QBB = 0). Strongly interacting double-charged dimers AA form “superstripes” running along a that are interleaved along b with chains of neutral dimers BB. Peculiar extremely thick (13.7 Å) four-decker insulating anion layers cast strong Coulomb potential onto the conductive layers predetermining charge localization in the latter.

Keywords

• single-molecule magnet (SMM)
• charge ordering (CO)
• stripe order
• metal-insulator (MI) transition
• molecular conductors
• electrocrystallization