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(NH₃{)}ₓCy, 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(NH₃{)}4.33C₂₄, f=0.95 and that the NH₃ molecules solvate some of the electron charge which was originally donated to the carbon layers in the KC₂₄ starting material. In addition, the NH₃ molecules form planar fourfold-coordinated K(NH₃{)}₄$ clusters and hence also solvate the ${K}⁺$ ions in graphite galleries. We suggest that the K(${NH}₃)₄ clusters together with ``spacer'' NH₃ molecules constitute the two-dimensional structural analog of the well-studied bulk, three-dimensional metal-ammonia solutions.