The composition and synthesis conditions of the (Li/Na)CoO₂ phase with an ordered 1:1 Li/Na stacking alternating with CoO₂ slabs were determined from a careful study of the P2-Na_0.7CoO₂−O3-LiCoO₂ system. An in situ X-ray diffraction (XRD) thermal study emphasizes the metastable character of this phase that can be stabilized only by very fast quenching. Its composition, (Li_0.42Na_0.37)CoO₂, is significantly different from the ideally expected one, (Li_0.50Na_0.35)CoO₂, and its structure, confirmed by Rietveld refinement of the XRD pattern, presents an ideal alternate ordering of lithium, cobalt, and sodium layers within OP4-type oxygen packing. The presence of vacancies in both alkali-ion layers was confirmed by electrochemical intercalation of lithium and sodium. For the first time, a new type of layered oxide exhibiting OPP9-type oxygen packing was evidenced. Between the CoO₂ slabs, alkali ions are intercalated in the following order: Li_octa−Na_prism−Na_prism. This material crystallizes in the <i>R</i><i>m</i> space group with <i>a</i>_hex = 2.828 Å and <i>c</i>_hex = 46.85 Å cell parameters.< Leer menos