Observation of a metallic resistivity behavior below 60 K in annealed molybdenum blue bronzes

When annealing molybdenum blue bronzes in the presence of a metal (Ag or Cu) a metallic resistivity behavior is observed below T ≈ 60 K in the three crystal directions. Nevertheless, DC conductivity and dielectric constant measurements still clearly exhibit the presence of a charge density wave (CDW). Electronic Paramagnetic Resonance (EPR) studies of annealed samples revealed a high concentration of Mo5+ sites with respect to untreated samples suggesting that due to some chemical reactions between the metal and the blue bronze some Mo6+ sites have been change into Mo5+ sites. We have discussed possible interpretations of such anomaly