Conductive Cu(2,5-dimethyl-dicyanoquinonediimine)2 radical ion salts: Systems with none, one, or two phase transitions.

Phase transitions in the radical ion salts of Cu(2,5-dimethyl-dicyanoquinonediimine)2 (Cu(DCNQI)2) can be achieved either by ‘‘external’’ pressure or by ‘‘internal’’ pressure. The latter is introduced by chemical modifications at the DCNQI-molecules (e.g., deuteration) or by alloying deuterated Cu(2,5-(CD3)2-DCNQI)2 (d6) with undeuterated Cu(2,5-(CH3)2-DCNQI)2 (h8), giving the mixture (h8/d6) in different ratios. In this work we present simultaneous conductivity (σ) and electron spin resonance (ESR) experiments on differently deuterated Cu(DCNQI)2-systems and on alloys (h8/d6) under external pressure. The anticoincidence of σ and ESR allows the determination of phase transition temperatures even in the absence of electrical contacts. For each system an individual phase diagram is established. Introducing an effective pressure peff=p0+p with p0 being the ‘‘internal’’ (chemical) pressure, a general phase diagram could be constructed by determining the individual p0 value for all systems. For the alloys (h8/d6) the relation ∂p0/∂q=5.0 bar/% (q is the percentage of d6) could be evaluated. The occurrence of a phase transition is associated with a change in the lattice parameters. For that, a structural model of a v-shaped temperature dependence of the unit cell volume V is suggested. For thermodynamic considerations, V is used as order parameter. If this order parameter V crosses critical values Vcbig and Vcsmall from above (cooling) or below (heating), phase transitions from conducting to insulating phases or vice versa are induced. This model explains the phase transition temperatures, the re-entry and the hysteresis effects of all systems qualitatively. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]