CONDUCTIVE CU(2,5-DIMETHYL-DICYANOQUINONEDIIMINE)(2) RADICAL-ION SALTS - SYSTEMS WITH NONE, ONE, OR 2 PHASE-TRANSITIONS

Phase transitions in the radical ion salts of Cu(2,5-dimethyl-dicyanoq uinonediimine)(2) (Cu(DCNQI)(2)) can be achieved either by ''external' ' pressure or by ''internal'' pressure. The latter is introduced by ch emical modifications at the DCNQI-molecules (e.g., deuteration) or by alloying deuterated Cu(2,5-(CD3)(2)-DCNQI)(2) (d(6)) with undeuterated Cu(2,5-(CH3)(2)-DCNQI)(2) (h(8)), giving the mixture (h(8)/d(6)) in d ifferent ratios. In this work we present simultaneous conductivity (si gma) and electron spin resonance (ESR) experiments on differently deut erated Cu(DCNQI)(2)-systems and on alloys (h(8)/d(6)) under external p ressure. The anticoincidence of sigma and ESR allows the determination of phase transition temperatures even in the absence of electrical co ntacts. For each system an individual phase diagram is established. In troducing an effective pressure p(eff)=p(0)+p with p(0) being the ''in ternal'' (chemical) pressure, a general phase diagram could be constru cted by determining the individual p(0) value for all systems. For the alloys (h(8)/d(6)) the relation partial derivative p(0)/partial deriv ative(q)=5.0 bar/% (q is the percentage of d(6)) could be evaluated. T he occurrence of a phase transition is associated with a change in the lattice parameters. For that. a structural model of a v-shaped temper ature dependence of the unit cell volume V is suggested. For thermodyn amic considerations, V is used as order parameter. If this order param eter V crosses critical values V-cbig and V-csmall from above (cooling ) or below (heating), phase transitions from conducting to insulating phases or vice versa are induced. This model explains the phase transi tion temperatures, the re-entry and the hysteresis effects of all syst ems qualitatively. (C) 1996 American Institute of Physics.