Charge and orbital ordering in Na0.5CoO2

The ground state of Na0.5CoO2 has been calculated using the full potential local orbital method and local density approximation plus Hubbard UU (LDA+U); the results demonstrate that charge and orbital ordering evidently exist in the present system in association with the antiferromagnetic state. Notable structural features observed between 300 and 30 K have been carefully examined using in situ TEM investigations, a superstructure with a wave vector of Q1=a∗/2Q1=a∗/2, becoming commonly visible below Tc1=88K, can be interpreted as the charge/orbital ordering on the Co1 and Co2 sites. Moreover, we have also observed another notable superstructure with Q2=a∗/4Q2=a∗/4 below the phase transition of Tc2=53K, which suggests a more complicated orbital ordered state existing at lower temperatures.