Metallicity in crystals of the quasi-one-dimensional rhodateBa1.2Rh8O16

Abstract

Single crystals of the quasi-one-dimensional oxide ${\text{Ba}}_{1.2}{\text{Rh}}_{8}{\text{O}}_{16}$ have been grown, crystallizing in the hollandite-type structure. These crystals show a metalliclike resistivity, with $\ensuremath{\rho}\ensuremath{\sim}0.56\text{ }\text{m}\ensuremath{\Omega}\text{ }\text{cm}$ at 300 K. The thermopower is positive, with $\text{S}\ensuremath{\sim}20\text{ }\ensuremath{\mu}\text{V}/\text{K}$ at 300 K. A peak in the thermopower is observed at $T\ensuremath{\sim}100\text{ }\text{K}$, leading to a maximum power factor equal to $30\text{ }\ensuremath{\mu}\text{W}/\text{cm}\text{ }{\text{K}}^{2}$ at 75 K, which is comparable to the one of the ${\text{Na}}_{x}{\text{CoO}}_{2}$ single crystal at 300 K. The metalliclike character of this compound is interpreted by considering the large rhodium oxidation state $(\ensuremath{\sim}3.7)$ responsible for the large charge-carrier concentration $(\ensuremath{\sim}{10}^{22}\text{ }{\text{cm}}^{\ensuremath{-}3})$ estimated by Hall measurements. The origin of the peak in the $S(T)$ at low $T$ is also discussed.