J. Phys. Chem. 1993,97, 12030-12033
12030
Copper(2+) States as Mediators for Metallic Conductivity in DCNQI Radical Anion Salts J. U. von Schiitz,'*t Dagmar Bauer,? S. Hiinig,s K. Sinzger,t and H. C. Wolft Physikalisches Institut der Universitat Stuttgart, Stuttgart, Germany, and Institut f i r Organische Chemie der Universitat Wurzburg, Wurzburg, Germany Received: July 7, 1993"
By conductivity, ESR, and susceptibility measurements on fully and partially deuterated dimethyl-DCNQI (2,5-Me2DCNQI, DCNQI = N,N'-dicyanoquinonediimine) radical ion salts with copper as counterions, we have found giant phase transitions from a metallic to a semiconducting state a t Tc= 70 K which are accompanied by a steplike increase in the susceptibility and a change from a Pauli- to a CurieWeiss behavior. Concomitantly, below Tc a narrow ESR signal of Cu2+ spins occurs which is absent in the metallic range. Approaching a 3d-antiferromagnetic transition at 7 K, the ESR line broadens and is lost.We explain the peculiarities of these copper salts via the coexistence of conducting and relaxed areas. In the relaxed areas the previously fluctuating Cu2+states are localized and ordered, suppressing the conductivity. The coincidence (in temperature) of charge ordering and the developing of a charge density wave are responsible for the giant changes in u and x in the deuterated salts.
Introduction
Cu ion states enter in a dominant but rather unknown manner into the propertiesof some new conductors/superconductors.They play an essential role in the pair-forming process in the HTCsuperconductors, and there are Cu ions incorporated in those superconducting organic metals which have the highest Tcof this class so far.' The systems we are dealing with, the DCNQI radical ion salts with metallic counterions, are not superconducting (yet) but are generally highly conductive (up to lo6 S cm-l). The radical ion salts with copper as counterions are outstanding in comparison to all salts with other metallic counterions which all are quasi-one-dimensional systems? (i) The Cu salts are quasithree-dimensional metals with anisotropies q/c1 < 50 as determined by NMR,3 or even
