Thermopower in the quadruple perovskite ruthenates

S. Hébert, R. Daou, and A. Maignan

Phys. Rev. B 91, 045106 – Published 6 January 2015

Abstract

The electrical resistivity and thermoelectric power of the paramagnetic $A C u_{3} R u_{4} O_{12}$ quadruple perovskite ruthenates with $A = Na, Ca, C a_{0.5} L a_{0.5}$ , and $La$ have been measured from $T = 5 K$ up to 900 K, with the formal Ru oxidation state changing from $3.75 (A = L a^{3 +})$ to $4.25 (A = N a^{+})$ . In this series of distorted metallic perovskites, resistivity curves present a metallic behavior, with no sign of saturation up to 900 K, typical of a bad metal. Two regimes are evidenced for the Seebeck coefficient $S$ : at high $T$ , a quasilinear increase of $S$ is observed up to 900 K, to a common value of $\sim 29 – 32 μ V K^{- 1}$ at 900 K, while for $T < 200 K$ , strong differences of $S$ are observed which can be directly correlated with the evolution of the paramagnetic Pauli susceptibility χ. The comparison of these quadruple perovskites with $SrRu O_{3}$ and $CaRu O_{3}$ perovskites show that at high $T$ , similar $S$ values are obtained, but the $S (T)$ dependences are completely different in the whole $T$ range. In perovskites, the constant $S$ of $\sim 25 – 35 μ V / K$ was explained by the spin degeneracy term calculated from the generalized Heikes formula, while in the quadruple perovskites the high $T$ linear $S (T)$ is typical of Mott's formula, used for metals. The origin of the similar $S$ values measured at 900 K could thus be coincidental but remains to be understood. From the comparison of $S (T)$ at low $T$ among all these ruthenates, a clear connection between the low $T$ slope of $S$ and the magnetic behavior is evidenced, with a large slope in the case of ferromagnetic $SrRu O_{3}$ , in contrast to a very small $d S / d T$ in the case of the Pauli paramagnetic $NaC u_{3} R u_{4} O_{12}$ , which possesses the smallest susceptibility.