TUNABLE PHASE-TRANSITIONS IN CONDUCTIVE CU(2,5-DIMETHYL-DICYANOQUINONEDIIMINE)(2) RADICAL-ION SALTS

The conduction process of the copper salts of DCNQI and the origin of the phase transition inducable in different ways, is explained in this review by an admixture of the copper d states to the DCNQI p pi band. This admixture depends on structural processes which shift the charge transfer delta from the copper moiety to the DCNQI stack to larger va lues with reduced lattice dimensions. Reaching exactly delta = 4/3 (ba nd filling degree rho 2/3), the trimerization in the copper stack lead s to the disproportion of the mixed valence state of Cu by the localiz ation of the d states as Cu2+. A charge density wave is formed, openin g a gap at the Fermi level E(F). These statements are acquired by sele ctive deuteration, selective alloying, applying pressure and by simult aneous electron spin resonance (ESR) and conductivity experiments in t he ESR equipment under pressure as a function of temperature. For all systems, when reaching critical values of the unit cell volume, phase transitions take place. We could establish a general phase diagram whi ch is based on an effective pressure scale including internal (chemica l) and external applied pressure.