Optical and structural studies of a two-dimensional organic Mott insulator dimethylphenazine-tetrafluorotetracyanoquinodimethane

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 [100] direction, and D and A molecules also stack alternately along the [111] direction. We evaluated the transfer energy t(AA) between the neighboring A molecules, t(DD) between the neighboring D molecules along the [100] direction, and t(DA) between the neighboring D and A molecules along the [111] 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 T_c ∼ 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.