Ground-state phase diagram of NaxCoO2: correlation of Na ordering with CoO2 stacking sequences

We have proposed a Hamiltonian that takes into account both Na-Na interactions and coupling between Na ions and CoO2 layers. By a combination of the Monte Carlo and first-principles approaches, all the possible stacking sequences of CoO2 layers together with the Na ordering have been obtained. In particular, an infinite series of ground states of Na ordering has been predicted in P3-NaxCoO2. We have obtained the ground-state phase diagram with the variation of Na concentration, which explains well the complex variation of the CoO2 stacking sequences. Our calculations show that the ordering of Na ions to minimize the Coulomb interaction is the main cause of the variation of the CoO2 stacking sequences.