An investigation of the sodium patterning in NaxCoO2 „0.5ÏxÏ1... by density functional theory methods

Extensive experimental work has been carried out to characterize the stable Na-vacancy ordering patterns at various compositions of layered NaxCoO2. However, contradictions and debates prevail in the literature, particularly at high Na concentrations x 0.5. Understanding of the exotic electronic properties in this system requires a thorough understanding of the Na-vacancy structural orderings. Using density functional theory in the generalized gradient approximation GGA , combined with a cluster expansion structure prediction algorithm we have found an intricate set of Na-vacancy ordered ground states in NaxCoO2 0.5 x 1 . We demonstrate a newly predicted ordering pattern between 0.67 x 0.71. By comparing the first principles electronic structure methods within the GGA and GGA+U Hubbard U correction approximations, we demonstrate that at certain Na concentration the stable ordering is affected by charge localization on the Co layer through coupling between the Na and Co lattices. © 2008 American Institute of Physics. DOI: 10.1063/1.2839292