Superspace group approach to the crystal structure of Na _0.5CoO₂

The crystal structure of layered cobalt oxide Na_0.5CoO ₂ has been determined by means of the (3 + 1)-dimensional superspace group approach. Structural parameters have been refined with a superspace group of Ccmm(1p0)000 using powder neutron diffraction data collected below room temperature. The compound can be regarded as a misfit-layered cobalt oxide which consists of [Na] and [CoO₂] subsystems, with a refined misfit parameter p = bcoO₂/b_Na = 0.5011(7) at 296 K. A parallel set of zigzag arrangements, i.e., the positional modulation, of Na ions has been revealed along the b-axis. The temperature-dependent alteration of Co-O distances shows discontinuous characteristics below the phase transition temperature around 80 K, which indicates the charge-separation of Co ions. On the basis of the bond valence sum calculation, a one-dimensional charge-separation pattern of Co ions with charges of +3.5 + δ and +3.5 - δ, perpendicular to the Na zigzag chains, is established.