Short-range cation ordering in ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{Ni}}_{2\mathrm{-}\mathit{x}}$${\mathrm{O}}_2$

The series ${\mathrm{Li}}_{\mathit{x}}$ ${\mathrm{Ni}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{O}}_2$ undergoes a structural phase transition near ${\mathit{x}}_{\mathit{c}}$=0.62, from cubic (x0.62). The transition is driven by ordering of the cations into layers of Li and Ni, which are perpendicular to one of the cubic 〈111〉 directions. Here we report synchrotron x-ray data for 18 samples with compositions in the range 0.2≤x≤1.0. The degree of cation ordering for samples near the critical composition was found to be sensitive to the annealing rate used during sample preparation. Quantitative estimates for the coherence lengths of short- and long-range-ordered domains are extracted from the data. The order parameter and cell constants are also determined as a function of composition x. Some implications of the domain structure on interpretation of electronic properties and magnetic ordering are briefly discussed.