Two new magnetic hollandites A1.5Ru6.1Cr1.9O16 (A = Sr, Ba): magnetoresistance and thermopower
The magnetic and transport properties of two hollandites, Sr1.5Ru6.1Cr1.9O16 and Ba1.5Ru6.1Cr1.9O16, crystallizing in the I4/m space group, have been investigated. The structural study shows a lack of cation ordering, and the mixed Cr and Ru occupation in the edge shared MO6 octahedra is responsible for magnetic and electrical transport properties dominated by the localized nature of the carriers, in clear contrast with the metallic and Pauli magnetism observed in ruthenium hollandites. In particular, a complex magnetic behavior together with a cluster-glass behavior and semi-conducting resistivity are observed. Interestingly, both compounds exhibit negative magneto-resistance, reaching −18.6% at 5 K and 9 T in Ba1.5Ru6.1Cr1.9O16, and −4% in Sr1.5Ru6.1Cr1.9O16. This magneto-resistance is directly associated with the variable-range hopping nature of transport and hence with the existence of localized carriers induced by the Cr for Ru substitution. In contrast, the thermopower at high temperature shows a behavior dominated by the Ru cation network, with S ∼ 30 μV K−1, consistent with those measured for other ruthenates such as SrRuO3 and CaCu3Ru4O12 quadruple perovskites, or in the KRu4O8 hollandite. The comparison of these two hollandites with the KRu4O8 hollandite enables a direct investigation of the impact of localization on the thermopower in these materials.