UNEXPECTED ISOTOPE EFFECT IN C-13-SUBSTITUTED (C-EQUIVALENT-TO-N) MOLECULAR CONDUCTOR (DME-DCNQI)2CU DME-DCNQI=2,5-DIMETHYL-N,N'-DICYANOQUINONEDIIMINE)

The introduction of C-13 atoms into two cyano groups in the mixed-vale nce molecular conductor (DMe-DCNQI)2Cu, where DMe-DCNQI is 2,5-dimethy l-N,N'-dicyanoquinonediimine, non-deuterated and selectively deuterate d at the methyl groups, was carried out. Isotope purity for C-13 was h igher than 98 atom%. The impurity content (Li, Na, K, Mg, Ti, Mn, Fe, Co, Ni, Zn, Si) was less than 0.002 wt.% (inductively coupled plasma m ass spectrometry method). Electrical resistivity measurements have sho wn apparent and reproducible shifts of the temperatures for phase tran sitions in each C-13-containing salt. This unexpected isotope effect i ndicates that the electronic state of (DMe-DCNQI)2Cu is quite sensitiv e to perturbations in the coordination around Cu.