Anion Effects in (BEDT-TTF)2X Salts: Symmetry Induced Superstructures

The structural and electrical properties of a series of (BEDT-TTF)2X salts are systematized in terms of a substructure-superstructure relationship which is induced by variation in anion symmetry and pictorially represented as a crystallographic family tree. Importantly, it is recognized that the ambient-pressure superconductors α-(BEDT-TTF)2I3, α-(BEDT-TTF)2IBr2, and α-(BEDT-TTF)2AuI2 have crystal structures coincident with the parent subcell of this crystallographic family tree. Moreover, all first-generation supercells (doubling of any one subcell vector) are exclusively populated by salts with non-centrosymmetric anions of tetrahedral symmetry, and, apparently, second-generation supercells (doubling of any two subcell vectors) are populated by salts with centrosymmetric anions of octahedral symmetry. Finally, trends in structural properties with anion size are briefly considered.

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