Neutral-Ionic Transition in Organic Charge-transfer Salts

7 Neutral - ionic transition in organic charge -transfer salts 

The generalization of the Hiickel model (1) to a donor-acceptor stack is through a site energy, ep = (—1)PA, that lowers the energy of electrons on odd-numbered D sites for A 0. In principle we have different on-site U in (3) for donors and acceptors, but a single U is usually assumed. The Coulomb terms in (4) now have zp = 2 and 0 at odd and even sites, respectively, because neutral D and A require two and no electrons. The complete basis for a stack is identical to the PPP basis 

The neutral-ionic transition (NIT) at t = 0 occurs abruptly[94] when the Madelung energy M of the ionic lattice exceeds the energy I — A to transfer an electron form D to A. Long-range Coulomb interactions are treated self-consistently as part of A in the modified Hubbard model[95], 

As seen in Fig. 7, crossovers[93] for periodic and antiperiodic boundary conditions separate the neutral and ionic regimes. The dashed line is the extrapolated behavior of the infinite chain. The narrow ionic region at small U becomes a point at U = 0 for the noninteracting system that can readily be solved exactly. Finite 

Experimental and theoretical studies of valence transitions are current topics with many open points. Different roles are expected for Coulomb or interchain interactions and coupling to molecular or lattice phonon in various systems. Direct solutions of models are one of several methods for studying the NIT. Magnetic, dielectric and thermal properties can be obtained exactly, albeit for finite systems of increasing size. Extrapolations to infinite chains require care and guidance from related analytical results. 

---