Thermopower in the quadruple perovskite ruthenates

The electrical resistivity and thermoelectric power of the paramagnetic ACu(3)Ru(4)O(12) quadruple perovskite ruthenates with A = Na, Ca, Ca0.5La0.5, and La have been measured from T = 5 K up to 900 K, with the formal Ru oxidation state changing from 3.75 (A = La3+) to 4.25 (A = Na+). 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 similar to 29-32 mu 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 chi. The comparison of these quadruple perovskites with SrRuO3 and CaRuO3 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 similar to 25-35 mu 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 SrRuO3, in contrast to a very small dS/dT in the case of the Pauli paramagnetic NaCu3Ru4O12, which possesses the smallest susceptibility.