Bonding, Ion Mobility, and Rate-Limiting Steps in Deintercalation Reactions with ThCr2Si2-type KNi2Se2

Here, we study the nature of metal–metal bonding in the ThCr2Si2 structure type by probing the rate-limiting steps in the oxidative deintercalation of KNi2Se2. For low extents of oxidation, alkali ions are removed exclusively to form K1–xNi2Se2. For greater extents of oxidation, the rate of the reaction decreases dramatically, concomitant with the extraction of both potassium and nickel to form K1–xNi2–ySe2. The appreciable mobility of transition metal ions is unexpected, but illustrates the relative energy scales of different defects in the ThCr2Si2 structure type. Furthermore, the fully oxidized compounds, K0.25Ni1.5Se2, spontaneously convert from the tetrahedral [NiSe4]-containing ThCr2Si2 structure to a vacancy-ordered NiAs structure with [NiSe6] octahedra. From analysis of the atom positions and kinetic data, we have determined that this transformation occurs by a continuous, low-energy pathway via subtle displacements of Ni atoms and buckling of the Se sublattice. These results have profound implications for our understanding of the stability, mobility, and reactivity of ions in materials.