Charge and spin ordering in insulating Na0.5CoO2: effects of correlation and symmetry

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 Na_0.5CoO₂ 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 Na_0.5CoO₂: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 Na_0.5CoO₂ is predicted to be higher than that of the pristine compound.

Dopant incorporation site in sodium cobaltate's host lattice: a critical factor for thermoelectric performance

NaxCoO2 that comprises alternating Na and CoO layers has exotic magnetic and thermoelectric properties that could favorably be manipulated by adding dopants or varying Na concentration. In this work, we investigated the structural and electronic properties of Sr and Sb doped NaxCoO2 (x = 0.50, 0.625, 0.75 and 0.875) through comprehensive density functional calculations. We found that Sr dopants always occupy a site in the Na layer while Sb dopants always substitute a Co ion in the host lattice regardless of Na concentration. This conclusion withstood when either generalized gradient approximation (GGA) or GGA + U method was used. By residing on the Na layer, Sr dopants create charge and mass inertia against the liquid-like Na layer and, therefore, improve the crystallinity and decrease the electrical resistivity through better carrier mobility. On the other hand, by substituting Co ions, Sb dopants reduce the electrical conductivity and therefore decrease the Seebeck coefficient.

Phase diagram of NaxCoO2 studied by Gutzwiller density-functional theory

The ground state of NaxCoO2 (0.0<x<1.0) is studied by the local density approximation plus the Gutzwiller approach, where charge transfer and orbital fluctuations are all self-consistently treated ab initio. In contrast to previous studies, which are parameter-dependent, we characterized the phase diagram as (1) Stoner magnetic metal for x>0.6 due to a_{1g} van Hove singularity near the band top, (2) correlated nonmagnetic metal without e_{g};{'} pockets for 0.3<x<0.6, and (3) e_{g};{'} pockets appear for x<0.3, and additional magnetic instability is revealed. Experimental quasiparticle properties are well explained, and the a_{1g}-e_{g};{'} anticrossing is attributed to spin-orbital coupling.