We report the results of optical reflectivity studies of the stage-1 and -2 graphite-${\mathrm{KH}}_{\mathrm{x}}$ intercalation compounds prepared by direct reaction of highly ordered pyrolytic graphite and KH powder. The stage-1 and -2 binary graphite-K compounds are studied for comparison. The optical data are analyzed in terms of a model involving two-dimensional (2D) graphitic π electrons and three-dimensional (3D) nearly free K(4s) electrons. The model is used to interpret the observed values of the free-carrier unscreened plasma frequencies and the position of the interband absorption threshold to determine experimental values for the Fermi level ($E_F$) in the carbon π band(s) and the fractional occupation of the K(4s) band. For the hydrides, we find quantitative evidence that the hydrogen states lie below $E_F$. Thus, hydrogen is present as ${\mathrm{H}}^{\mathrm{-}}$, acting as an acceptor, thereby compensating the electron donation to the π bands from the K(4s) states. This assumption and the optical data for the stage-1 and -2 hydrides results in a [H]/[K] ratio of 0.8, in excellent agreement with chemical analyses reported by Guérard and co-workers, and leads to very small values for the fractional K(4s) band occupation $f_{\mathrm{K}}$<0.03 electrons per K atom. Within the framework of a superimposed 2D (π) and 3D [K(4s)] rigid-band model, our experimental results support an empty K(4s) band (i.e., $f_{\mathrm{K}}$=0) in stage-2 ${\mathrm{C}}_{24}$K. In stage-1 ${\mathrm{C}}_8$K, the rigid-band model yields large values for $f_{\mathrm{K}}$ ($f_{\mathrm{K}}$>0.5 electrons per K atom), unless the value of the optical mass of the electrons in the K(4s) states is larger than ∼2. The ${\mathrm{C}}_8$K results are also discussed in terms of more sophisticated energy-band calculations.

• Received 17 February 1987

DOI:https://doi.org/10.1103/PhysRevB.35.9790