X-ray study of the structure of K1-xRbxC8.

Abstract

The mixed layered compounds ${\mathrm{K}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Rb}}_{\mathrm{x}}$${\mathrm{C}}_{8}$ represent quasi-two-dimensional alloys interleaved by graphite hexagonal planes. We report extensive x-ray scattering investigations of the various structural aspects of these compounds. These include the variation of the layer separation with the alloy composition, the in-plane order and disorder of the alkali-metal ions in the graphite host, the stacking sequence of the alkali-metal layers, and the change of the carbon-carbon bond length as a function of the composition. We find that the interlayer separation exhibits a slight deviation from linearity independent of the graphite material used (pyrolytic graphite or single crystals), with a maximum deviation centered at x=(2/3. In the plane, the mixed alkali-metal layers form a commensurate (2\ifmmode\times\else\texttimes\fi{}2)R0\ifmmode^\circ\else\textdegree\fi{} superstructure with complete site disorder among the K and Rb ions. Extensive low-temperature scans have not revealed any tendency of the mixed layer to either super order or phase separate. The stacking sequence is independent of the alloy composition and is identical to the stacking sequence in ${\mathrm{KC}}_{8}$ as well as in ${\mathrm{RbC}}_{8}$. The carbon-carbon bond length appears not to change with the alloy composition, indicating that the charge transfer between the alkali-metal s band and the graphite \ensuremath{\pi} band remains constant with composition and is independent of the layer separation.