Département de chimie moléculaire

Charge transport and dielec. measurements were carried out on compacted powder and single-crystal samples of bistable RbxMn[Fe(CN)6]y·zH2O in the two valence-tautomeric forms (MnIIFeIII and MnIIIFeII) as a function of temperature (120-350 K) and frequency (10-2-106 Hz). The complex conductivity data reveal universal conductivity behavior and obey the Barton-Nakajima-Namikawa relation. The charge transport is accompanied by dielec. relaxation that displays the same thermal activation energy as the conductivity Surprisingly, the activation energy of the conductivity was found very similar in the two valence-tautomeric forms (∼0.55 eV), and the conductivity change between the two phases is governed mainly by the variation of the preexponential factor in each sample. The phase transition is accompanied by a large thermal hysteresis of the conductivity and the dielec. constant In the hysteresis region, however, a crossover occurs in the charge transport mechanism at T {\textless} ∼220 K from an Arrhenius-type to a varying activation energy behavior, conferring an unusual double-loop shape to the hysteresis.