Metallicity in crystals of the quasi-one-dimensional rhodate Ba1.2 Rh8 O16

Single crystals of the quasi-one-dimensional oxide Ba1.2 Rh8 O16 have been grown, crystallizing in the hollandite-type structure. These crystals show a metalliclike resistivity, with ρ∼0.56 mΩcm at 300 K. The thermopower is positive, with S∼20 μV/K at 300 K. A peak in the thermopower is observed at T∼100 K, leading to a maximum power factor equal to 30 μW/cm K2 at 75 K, which is comparable to the one of the Nax CoO2 single crystal at 300 K. The metalliclike character of this compound is interpreted by considering the large rhodium oxidation state (∼3.7) responsible for the large charge-carrier concentration (∼ 1022 cm-3) estimated by Hall measurements. The origin of the peak in the S (T) at low T is also discussed.