Phase Transition from Mott Insulating Phase into the Charge Ordering Phase with Molecular Deformation in Charge-Transfer Salts κ-(ET)₄[M(CN)₆][N(C₂H₅)₄]·2H₂O (M = Co^III and Fe^III)

Two isostructural charge-transfer salts κ-(ET)₄[M(CN)₆][N(C₂H₅)₄]·2H₂O (ET = bis(ethylenedithio)tetrathiafulvalene and M = Co^III, Fe^III) were synthesized, and their crystal structures, Raman spectra, electrical conductivities, electron spin resonance (ESR), and static magnetic susceptibilities as a function of temperature were studied. At room temperature the salts contain two crystallographically independent ET molecules with an average charge of +0.5. Temperature dependence of the Raman spectra revealed that a charge disproportionation gradually occurs upon cooling, because new peaks originated from the ET⁺ and ET⁰ grow gradually. ESR, static magnetic susceptibility, and conductivity data, supported by band structure calculation, give us firm evidence of the phase transition from the Mott insulating phase with the fluctuation of charge order into the long-range charge ordered phase at around 150 K. The charge ordered ground state consists of (ET⁺)₂ dimers and (ET⁰)₂ dimers, where the neutral molecules are found to be bent. The mechanism of this phase transition is discussed on the basis of crystallographic, spectroscopic, and magnetic features at various temperatures.