GROUND-STATE OPTICAL-PROPERTIES OF CHARGE-TRANSFER CRYSTALS CLOSE TO THE NEUTRAL-IONIC INTERFACE - TETRATHIAFULVALENE-2,5-DICHLORO-P-BENZOQUINONE

The crystal structure and the optical data of the (1:1) charge-transfe r (CT) complex of tetrathiafulvalene (TTF) with 2,5-dichloro-p-benzoqu inone (2,5Cl2BQ) are reported, yielding a full characterization of the ground state properties. The complex crystallizes in the triclinic sp ace group P1BAR, Z=1. TTF and 2,5Cl2BQ form mixed regular stacks along the b axis. The degree of ionicity rho is approximately 0.18, and the CT integral approximately 0.22 eV. A detailed analysis of the optical data yields estimates of the strength of electron-molecular vibration coupling and of intersite electron-electron interactions. The latter has also been computed in the atom-in-molecule fractional charge appro ximation. The TTF-2,5Cl2BQ structure and microscopic parameters at amb ient conditions are found to be rather similar to those of the extensi vely studied complex of TTF with tetrachloro-p-benzoquinone (CA). The different behavior of the two complexes under pressure (both exhibit a neutral-ionic phase transition, but rho changes discontinuously for T TF-CA and continuously for TTF-2,5Cl2BQ) is ascribed to the competitiv e role of the Madelung energy and of the CT integral.