A X-Ray Study of the Structure of POTASSIUM(1-X)-RUBIDIUM(X)-CARBON(Y)

The mixed layered compounds K_ {1-x}Rb_{x}C_8 and K _{1-x}Rb _{x}C_ {24} represent quasi two-dimensional alloys interleaved by graphite hexagonal planes. We report extensive x-ray scattering investigations of the structural aspects of these compounds. In stage 1 K_{1-x }Rb_{x}C _8, 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 layers form a (2 x 2)R0 ^{o} superstructure with complete site disorder among the K and Rb metal atoms. Extensive low temperature investigations have not revealed any tendency of the mixed layer to either super-order to phase separate. The stacking sequence is independent of alloy composition and is identical to the stacking sequence in KC_8 as well as in RbC_8. The carbon-carbon bond length appears not to change with alloy composition. By comparing the diffusion constants from a macroscopic diffusion experiment with self diffusion constants obtained from quasi-elastic neutron scattering data, we found that the substitutional intercalation process must be drastically hindered by a barrier at the perimeter of the sample. The stage 2 K_{1-x}Rb_{x} C_{24} (x = 0.52, 0.80) compounds were synthesized for the first time. Below the ordering temperature (T_{u} = 149 and 159K, respectively) peaks corresponding to a discommensuration domain structure are observed, which correspond to the compositional average of the unmixed compounds. By taking single crystal Laue photographs, we have determined the position and orientation of the peaks near the origin. We propose a simple physical model to interpret the x-ray data. Below a second transition temperature T_{L}cong 100K, the domain structure becomes unstable against a composition modulation.