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 U ( 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 Q 1 = a ∗ / 2 , becoming commonly visible below T c 1 = 88 K , can be interpreted as the charge/orbital ordering on the Co1 and Co2 sites. Moreover, we have also observed another notable superstructure with Q 2 = a ∗ / 4 below the phase transition of T c 2 = 53 K , which suggests a more complicated orbital ordered state existing at lower temperatures.