Effect of composition on charge exchange, lattice expansion, and staging in potassium-ammonia graphite intercalation compounds

We have studied the charge exchange and compositional dependence of the sandwich thickness of stage-1 alkali-ammonia ternary graphite intercalation compounds K(${\mathrm{NH}}_3$ ${)}_{\mathrm{x}}$ ${\mathrm{C}}_{\mathrm{y}}$, 0≤x≤4.33, 12≤y≤24. A model of the sandwich energy is presented which explicitly accounts for x-dependent charge exchange and size or stiffness effects and is in excellent agreement with experimental measurements of the dependence of the (00l) x-ray diffraction patterns on ammonia vapor pressure. From this model we find that for the stage-1 compound K(${\mathrm{NH}}_3$ ${)}_{4.33}$ ${\mathrm{C}}_{24}$, f=0.95 and that the ${\mathrm{NH}}_3$ molecules solvate some of the electron charge which was originally donated to the carbon layers in the ${\mathrm{KC}}_{24}$ starting material. In addition, the ${\mathrm{NH}}_3$ molecules form planar fourfold-coordinated K(${\mathrm{NH}}_3$ ${)}_4$ clusters and hence also solvate the ${\mathrm{K}}^{+}$ ions in graphite galleries. We suggest that the K(${\mathrm{NH}}_3$ ${)}_4$ clusters together with ‘‘spacer’’ ${\mathrm{NH}}_3$ molecules constitute the two-dimensional structural analog of the well-studied bulk, three-dimensional metal-ammonia solutions.