Résumé
Alkali transition-metal layered compounds usually contain only one type of alkali cation between the edge-shared octahedra layers. Herein, the ternary phase diagram A2Ni2TeO6 (A = Li, Na, K) was explored through solid-state synthesis and new alkali-mixed compositions showing alternation of distinct alkali layers are obtained. Such intergrowth structures are synthesized either by a single high-temperature treatment from raw chemicals or through reaction between layered precursors, the latter involving a solid-state process triggered at moderate temperatures. The in-depth characterization of the multiple cationic orderings is performed by combining powder diffraction techniques (X-rays and neutrons), high-resolution transmission electron microscopy, and solid-state NMR spectroscopy. In addition to the Ni/Te honeycomb ordering, alternation of lithium layers with sodium or potassium layers is observed for compositions (Li/Na)2Ni2TeO6 or (Li/K)2Ni2TeO6, respectively. Crystal structure solving was achieved by stacking building blocks of the respective single alkali layered oxides and unveiled a complex out-of-plane ordering of honeycomb layers. Moreover, a solid-state reaction between Li2Ni2TeO6 and NaKNi2TeO6 enables preparation of the new phase Li∼1Na∼0.5K∼0.5Ni2TeO6, a unique example containing up to three alkali cations and exhibiting a more complex stacking with sodium and potassium cations occupying the same layer. This investigation confirms that the chemical versatility of layered alkali transition-metal compounds could also occur on the alkali layer. Following the research methodology described here, we revisit the crystal chemistry of alkali transition-metal layered materials by exploring alkali ion substitutions previously thought infeasible, in order to find new alkali-mixed compositions.