Electron diffraction from superlattices in graphite-rubidium intercalation compounds

Electron-diffraction results are reported in the temperature range $110<T<900$ K for graphite-Rb intercalation compounds based on highly oriented pyrolytic graphite host material. Annealed stage $n=1$ samples show the simple graphite pattern below {~} 300 K, corresponding to a p(2×2)R 0^∘ in-plane intercalate superlattice ordering with α,β,γ,δ interlayer intercalate stacking order. A reversible transition is made at {~} 300 K to a p(2×2)R 0^∘ superlattice, but lacking α,β,γ,δ stacking. The higher-stage compounds ($n=2,3,4,7$) exhibit commensurate p(√7×√7)R±19.1^∘ superlattices below a temperature TU which decreases with increasing stage index from 170 K for $n=2$. As T is increased above TU, the higher-stage compounds exhibit a reversible transition to another ordered structure which is present until the transition to a high-temperature phase, observed at 620 K for $n=2$. The structural transition at TU between the two ordered phases is accompanied by a dramatic change in the bright-field real-image micrograph, taken on the same portion of the sample as the electron-diffraction patterns. The observation of a second ordered phase above TU in the Rb compounds is in contrast with the disordered phase reported previously for second-stage K compounds.