Electronic structure of an organic charge-transfer complex, M 2P-TCNQF4 (M2P: 5,10-dihydro-5,10- dimethylphenazine; TCNQF4: 2,3,5,6-tetrafluoro-7,7,8,8- tetracyanoquinodimethane), was investigated by means of optical reflection spectroscopy and x-ray structural analyses. This is an ionic compound and has a unique quasi-two-dimensional (2D) crystal structure in which donor (D) molecules of M2P and acceptor (A) molecules of TCNQF4 stack respectively along the  direction, and D and A molecules also stack alternately along the  direction. We evaluated the transfer energy t(AA) between the neighboring A molecules, t(DD) between the neighboring D molecules along the  direction, and t(DA) between the neighboring D and A molecules along the  direction to be 43 meV, 29 meV, and 67 meV at room temperature, respectively. This demonstrates that an anisotropic 2D electronic structure is formed. By comparing the spectra of the imaginary part of the dielectric constant ε2 obtained from the polarized reflectivity spectra with the absorption spectrum of K-TCNQF4 and the ε2 spectra of M2P-PF6, which are composed of 1D A stacks and D stacks, respectively, it was revealed that in M2P-TCNQF4 the optical gap corresponds to the Mott gap transition from A- to A-, and the D+ to D+ transition and the A - to D+ transition or equivalently the charge-transfer (CT) transition located at the higher energies. Below the structural and magnetic phase transition temperature Tc ∼ 122 K, spin-singlet states are formed via displacements of both D and A molecules. The pattern of molecular displacements was found to be very unique, indicating that the phase transition cannot be attributed to a spin-Peierls-like mechanism. The nature of the phase transition is discussed from the temperature dependences of molecular displacements as well as of the Mott-gap transition and the CT transition.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - Aug 29 2011|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics