Relative stability of donor and acceptor stacks against Peierls distortion in the tetrathia- and tetraselenafulvalene-tetracyanoquinodimethane family of organic metals

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Abstract
While most of the known organic conductors of tetrathiafulvalene-tetracyanoquinodimethane type are characterized by having the Peierls instability driven by acceptor stacks there exist few compounds in which the Peierls instability is driven by donor stacks. We present tetramethyltetraselenafulvalene-2,5-dimethyltetracyanoquinodimethane as an example of a compound belonging to the second class. Magnetic data are presented to establish that indeed the transition in this compound is donor driven. We analyze the effects of the magnitude of the unit cell on the relative tendency of the donor and acceptor stacks to undergo a Peierls distortion. We compare the effect of the unit-cell expansion on the magnitude of the electronphonon coupling and the metal-insulator transition temperature, when this expansion is produced by alkyl groups and by selenium atoms.