Electron diffraction from superlattices in graphite-rubidium intercalation compounds

Abstract

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 \ensuremath{\sim} 300 K, corresponding to a $p(2\ifmmode\times\else\texttimes\fi{}2)R 0\ifmmode^\circ\else\textdegree\fi{}$ in-plane intercalate superlattice ordering with $\ensuremath{\alpha}$,$\ensuremath{\beta}$,$\ensuremath{\gamma}$,$\ensuremath{\delta}$ interlayer intercalate stacking order. A reversible transition is made at \ensuremath{\sim} 300 K to a $p(2\ifmmode\times\else\texttimes\fi{}2)R 0\ifmmode^\circ\else\textdegree\fi{}$ superlattice, but lacking $\ensuremath{\alpha}$,$\ensuremath{\beta}$,$\ensuremath{\gamma}$,$\ensuremath{\delta}$ stacking. The higher-stage compounds ($n=2,3,4,7$) exhibit commensurate $p(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R\ifmmode\pm\else\textpm\fi{}19.1\ifmmode^\circ\else\textdegree\fi{}$ superlattices below a temperature ${T}_{U}$ which decreases with increasing stage index from 170 K for $n=2$. 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 $n=2$. 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.