On the basis of the extended Hückel molecular orbital calculation, the intermolecular overlaps of the highest occupied molecular orbitals are calculated for α-(BEDT-TTF)2I3 reported by Bender et al., and for superconducting β-(BEDT-TTF)2I3 reported by Yagubskii et al. (BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene). α-(BEDT-TTF)2I3 is a two-dimensional semimetal or a narrow gap semiconductor. β-(BEDT-TTF)2I3 is a two-dimensional metal which has an almost isotropic closed Fermi surface.
Based on the extended Hückel molecular orbital calculations, the relation between the anisotropy of the band structure and the arrangement of the organic molecules is investigated for two organic donors, tetrathiafulvalene (TTF) and bis(ethylenedithio)tetrathiafulvalene (BEDT–TTF). The intermolecular overlaps of their HOMO are calculated while the intermolecular arrangements are varied. The maps of the overlaps thus obtained are then used to estimate the band-structure parameters of (TMTTF)2X and (BEDT–TTF)2ClO4(C2H3Cl3)0.5. The Fermi surface of (TMTTF)2X is quasi-one-dimensional and not closed. On the contrary, (BEDT–TTF)2ClO4(C2H3Cl3)0.5 is regarded as a two-dimensional semimetal.
The polarization energies of 44 organic solids were determined by ultraviolet photoelectron spectroscopy in the gaseous and solid states. Condensed polycyclic aromatic hydrocarbons with planar molecular structures were found to have a common value, 1.7 eV, independent of their molecular sizes and also their crystal structures. The common value is approximately interpreted by the first-order expression for the polarization energy. A large variation of values in the range 0.9-3.0 eV was obtained for several compounds. Among them, molecules with intricate structures have smaller values and those with large molecular polarizabilities have larger values than the common value. These results indicate that the polarization energy of an organic solid is mainly determined by two factors: the molecular polarizability and the molecular packing in the solid. Intermolecular interactions in the solid, other than the van der Waals force, also contribute to the value.
The crystal structures of (BEDT–TTF)2[MHg(SCN)4] (M=K at 298 and 104 K and M=NH4 at 298 K) were determined by X-ray analyses. A donor layer and an anion sheet stack alternately along the b-axis. The packing pattern of the donor is close to α-phase, while an anion sheet constructs a thick two-dimensional polymer plane parallel to (010). The linkage of (Remark: Graphics omitted.) (or NH4) (Remark: Graphics omitted.) in an anion sheet is very unique. The band structure calculation indicates both a closed and an open Fermi surfaces, resembling that of κ-(BEDT–TTF)2[Cu(NCS)2].
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