Ground state structure of sodium ions in NaxCoO2:: A combined Monte Carlo and first-principles approach

We have determined the ground state structures of sodium ions in NaxCoO2 by a combination of the Monte Carlo method and the first-principles method. We predict the ground state structures at x=1/4, 1/3, 1/2, 3/5, 5/7, 3/4, and 13/16. Our results on the stable phases explain well the experimental structures. At high concentration, the phase at x=13/16 is found to be one state belonging to an infinite series of ground states of two interpenetrating triangular sublattices in the interaction parameter space.