Tunable Phase Transitions in Conductive Cu(2,5-Dimethyl-Dicyanoquinonediimine)$\mathsf{_2}$ 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 values with reduced lattice dimensions. Reaching exactly $\delta = 4/3$ (band filling degree $\rho = 2/3$), the trimerization in the copper stack leads to the disproportion of the mixed valence state of Cu by the localization of the d states as Cu$^{2+}$. A charge density wave is formed, opening a gap at the Fermi level $E_{\rm F}$. These statements are acquired by selective deuteration, selective alloying, applying pressure and by simultaneous electron spin resonance (ESR) and conductivity experiments in the 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 which is based on an effective pressure scale including internal (chemical) and external applied pressure