Structural genealogy of BEDT-TTF-based organic conductors I.: Parallel molecules:: β and β" phases

A method is proposed to systematize a number of structural modifications of BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene)-based organic conductors and related materials. Analysis of actual crystal structures indicates that most crystal structures are constructed of two essential building blocks: "ring-over-bond" (RB) and "ring-over-atom" (RA) overlap modes. Several different ways to pile up these elements lead to various structures which are conventionally designated as beta, beta', beta ", theta, alpha, and alpha "-phases. In the beta- and beta "-phases, introduction of "dislocations" along the stacking axis generates a number of modifications. Here dislocations mean interactions of two donor molecules which have larger displacements along the molecular long axis than the standard RE and RA modes. Systematic nomenclature to distinguish these modifications is proposed. Transfer integrals are, however, not very sensitive to the existence of dislocations, so that the Fermi surfaces of these multiple phases are derived from the fundamental structure by folding the first Brillouin zone.