Electronic and structural instabilities of mixed-stack organic charge-transfer salts

Electronic (valence) and structural (Peierls) instabilities occur either separately or together in organic charge-transfer (CT) crystals with mixed stacks of donors (D) and acceptors (A). A Peierls-Hubbard model, HCT, with site D, A energies and linear electron-phonon coupling in a harmonic lattice is shown to provide a unified microscopic description and to distinguish between thermal and quantum Peierls transitions. Vibrational, magnetic and structural data are interpreted as thermal Peierls transitions in two ionic CT salts and as dimerized ground states with spin solitons in other salts. Quantum Peierls transitions are identified in largely neutral salts, and a neutral-ionic crossover is discussed in a salt with dipolar disorder that suppresses the Peierls instability. The coincident Peierls and neutral-ionic transition of tetrathiafulvalene-chloranil (TTF-CA) is another special case of HCT.