Magnetism and spin entropy in Ru doped Na0.5CoO2.

The effect of Ru doping on the magnetic coupling among Co ions and on the Seebeck effect in Na0.5CoO2 was systematically studied using density functional theory. It was found that the Ru dopant takes the 4+ oxidation state and replaces a Co4+ ion. In addition, the remaining Co4+ ions in Na0.5CoO2:Ru were stabilized in a low spin state. Magnetically, the Ru dopants couple in a ferrimagnetic manner with Co ions in the host lattice. Due to the higher electronic degeneracy of Ru4+ dopants, the Seebeck coefficient in Na0.5CoO2 is predicted to be higher than that of the pristine compound.