An introduction to molecular organic materials - Molecular Organic Materials - page 71

Figure 1.30. Theoretical band structure of (TMTTF) 2 ReO 4 along the − X and

− Y directions. Courtesy of Dr E. Canadell.

by the separation between two adjacent molecules, as represented in Fig. 1.28(c).
In this case the materials should behave as metals as previously discussed. The
presence of anions (which ultimately induce the circulation of charge) doubles the
lattice parameter, so that one would expect two bands. Figure 1.30 shows the band
structure of (TMTTF) 2 ReO 4 as calculated with the extended Huckel method.
We clearly identify the two bands expected due to the doubling of the lattice pa-
rameter aswell as the dimerization gap along the Xdirection, which corresponds

to the real-space a -direction, the stacking direction. Because of the orbital overlap
along a , the dominant band dispersion is found along the

−

− X direction. However,

we also observe some dispersion along the Y direction, which means that the

system is not purely 1D but slightly 2D. This small degree of two-dimensionality
prevents the Peierls transition and permits the BFS to become superconductors.
Since the BFS are mixed valence materials (formally one charge to be distributed
for two organic molecules) we have in fact 3 N ion electrons to be accommodated in
two bands, with 4 N ion electrons allowed (hence the term three-quarter filled bands),
which predicts metallic behaviour. The real structure is 3D because of lateral in-
teractions and slight displacements of molecules with regard to each other, which
means that not all BFS are metals, but the important point is that we summarize most
of the dominating physics with an extremely simple model. As will be discussed

−