Figure 2

X-ray diffraction measurements of sodium ordering in Na_0.8CoO₂. (a) Long-range order in the fully ordered tri-vacancy stripe phase gives sharp superlattice reflections. A tri-vacancy cluster comprises three Na-ion vacancies and three Na ions promoted from 2d (blue) to 2b (red) sites. Hexagonal unit cell and supercell vectors are shown (black). Green vectors link equivalent sites in successive stripes. (b) In the partially disordered tri-vacancy stripe phase, the stripes are ordered long range, but the locations of the tri-vacancy clusters are not correlated between stripes, and there is a reduction in intensity of some of the higher order superlattice peaks. (c) In the disordered phase there are no sharp superlattice reflections, and the characteristic diffuse scattering pattern arises from short-range ordering of multi-vacancy clusters. The calculated intensities are from the supercell of the ordered tri-vacancy stripe phase²⁵ (a), a section of crystal with long-range ordering of stripes, but with random translations of the tri-vacancy clusters within stripes (b), and from a Monte Carlo simulation of a disordered mixture of di-, tri- and quadri-vacancy clusters (c).