Electron diffraction from superlattices in graphite-rubidium intercalation compounds

Abstract

Electron-diffraction results are reported in the temperature range $110<T<\mathrm{}900\mathrm{}$ K for graphite-Rb intercalation compounds based on highly oriented pyrolytic graphite host material. Annealed stage $n=1\mathrm{}$ samples show the simple graphite pattern below ∼ 300 K, corresponding to a $p(2\mathrm{}\times 2)R 0°$ in-plane intercalate superlattice ordering with $\alpha$,$\beta$,$\gamma$,$\delta$ interlayer intercalate stacking order. A reversible transition is made at ∼ 300 K to a $p(2\mathrm{}\times 2)R 0°$ superlattice, but lacking $\alpha$,$\beta$,$\gamma$,$\delta$ stacking. The higher-stage compounds ($n=2,3,4,7\mathrm{}$) exhibit commensurate $p(\sqrt{7}\times \sqrt{7})R\pm 19.1°$ superlattices below a temperature $T_U$ which decreases with increasing stage index from 170 K for $n=2\mathrm{}$. As $T$ is increased above $T_U$, 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 $n=2\mathrm{}$. The structural transition at $T_U$ 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 $T_U$ in the Rb compounds is in contrast with the disordered phase reported previously for second-stage K compounds.