Solid-state design of crystalline alkali-mixed layered compounds
Romain BERTHELOT
Charles Gerhardt Institute of Montpellier, CNRS, UM, ENSCM, France
Solid-state chemists have long explored alkali transition metal layered compounds as a playground for discovering new compositions and tuning crystal structures and physical or electrochemical properties. In these materials, cation substitution within the transition metal layers is generally facilitated by the similar ionic radii of many elements in octahedral coordination. By contrast, modifying the interlayer space is far more constrained, as differences in size and site symmetry hinder the incorporation of multiple alkali elements.
Here, we show that these limitations can be overcome through carefully designed synthesis strategies. A range of layered oxides and sulfides is explored, yielding new mixed-alkali compositions with two distinct cation distributions: either a disordered arrangement within a single interlayer or an ordered stacking of chemically distinct alkali layers.
Resolving these complex structures remains a central challenge, particularly with regard to locating alkali ions and accounting for stacking defects. To address this, we combine complementary techniques: X-ray and neutron powder diffraction to probe long-range order, alongside high-resolution transmission electron microscopy and solid-state NMR to access local structural environments. Density functional theory calculations further rationalize the stability of the observed arrangements.
Together, these results uncover new structural motifs in layered materials and provide guidelines for the targeted design of mixed-alkali transition metal layered compounds.
Contact: Romain Wernert/Thierry Brousse (ST2E)
