Structure prediction and phase diagram calculation of NaxCoO2 by Ab Initio methods

The unusual electronic properties of NaxCoO2 make it a material of considerable interest. At high sodium concentration it displays a remarkable combination of high electronic conductivity and high Seebeck coefficient. Understanding these phenomena requires a detailed understanding of the local structure, since different Na-vacancy orderings are strongly coupled to the electronic structure and Co3+/Co4+ arrangement. In this study, we investigate the charge ordering and Na-vacancy ordering using first principles electronic structure methods within the GGA and GGA+U approximations. Na ordering is determined not only by a competition between Na site energies difference and Na-Na repulsion, but also by the Co-Na interlayer interaction induced by charge localization. We believe that in particular at high Na content a quantitative understanding of the coupling is essential in understanding the remarkable electronic properties. Phase diagram calculations of NaxCoO2 to understand the phase stability in the system will be presented.