Effect of ruthenium substitution in layered sodium cobaltate NaxCoO2: Synthesis, structural and physical properties

Solid-state synthesis of Na0.71Co1−xRuxO2 compositions shows that ruthenium can be substituted for cobalt in the hexagonal Na0.71CoO2 phase up to x=0.5. The cell expands continuously with increasing ruthenium content. All mixed Co–Ru phases show a Curie–Weiss behaviour with no evidence of magnetic ordering down to 2 K. Unlike the parent phase Na0.71CoO2, ruthenium-substituted phases are all semiconducting. They exhibit high thermoelectric power, with a maximum of 165 μV/K at 300 K for x=0.3. The Curie constant C and Seebeck coefficient S show a non-monotonic evolution as a function of ruthenium content, demonstrating a remarkable interplay between magnetic properties and thermoelectricity. The presence of ruthenium has a detrimental effect on water intercalation and superconductivity in this system. Applying to Ru-substituted phases the oxidative intercalation of water known to lead to superconductivity in the NaxCoO2 system yields a 2-water layer hydrate only for x=0.1, and this phase is not superconducting down to 2 K.