Electron Localization Induced by Disordered Anions in an Organic Conductor.

We report on a new organic conductor κ″-(ET) 2 Cu[N(CN) 2 ]Br ( κ″-Br ), which is the first polymorph of an organic superconductor κ-(ET) 2 Cu[N(CN) 2 ]Br ( κ-Br ), where ET denotes bis (ethylenedithio)tetrathiafulvalene. κ″-Br has a similar κ-type arrangement of ET molecules to κ -Br , but, in contrast to the orthorhombic κ -Br , which has ordered polyanion chains, presents a monoclinic crystal structure with disordered polymeric anion chains. To elucidate the electronic state of κ″-Br , we performed band calculations as well as transport, magnetic, and optical measurements. The calculated band dispersion, magnitude of electron correlation, and room-temperature optical conductivity spectra of κ″-Br were comparable to those of κ -Br . Despite these similarities, the κ″-Br salt exhibited a semiconducting behavior. The electron spin resonance and Raman spectroscopies indicated that there is neither magnetic nor charge order in κ″-Br , suggesting the occurrence of Anderson localization due to disordered anion layers.