Interplay between magnetism and Na concentration in NaxCoO2

Through comprehensive density functional calculations, the crystallographic, magnetic and electronic properties of NaxCoO2 (x = 1, 0.875, 0.75, 0.625 and 0.50) were investigated. We found that all Na ions in NaCoO2 and Na0.875CoO2 share the basal coordinates with O ions. However, as x decreases, some of Na ions move within the basal plane in order to reduce the in-plane Na-Na electrostatic repulsion. Magnetically, there was strong tendency for type A antiferromagnetism in the Na0.75CoO2 system, while all other Na deficient systems had a weaker ferromagnetic (FM) tendency. The results on magnetism were in excellent agreement with the experiments.