Structural phase transition of high-stage MoCl5 graphite intercalation compounds.

Abstract

Structural phase transitions of high-stage ${\mathrm{MoCl}}_{5}$ graphite intercalation compounds (GIC's) have been investigated by using specific heat, x-ray (00L) and (HK0) diffraction, electron diffraction, and neutron (HKL) diffraction techniques. Each ${\mathrm{MoCl}}_{5}$ intercalate layer is uncorrelated with respect to the other ${\mathrm{MoCl}}_{5}$ intercalate layers but is modulated by the graphite layers. The stage-3 ${\mathrm{MoCl}}_{5\mathrm{\ensuremath{-}}}$GIC undergoes a structural phase transition at ${\mathit{T}}_{\mathit{c}}$=480 K, where the specific heat shows a \ensuremath{\lambda}-type anomaly and the c-axis repeat distance exhibits an abrupt increase as temperature is increased. The ${\mathrm{MoCl}}_{5}$ intercalate layer consists of the orthorhombic phase and the hexagonal phase. The orthorhombic phase forms a plane body-centered rectangular lattice with a=6.21\ifmmode\pm\else\textpm\fi{}0.02 \AA{} and b=17.93\ifmmode\pm\else\textpm\fi{}0.02 \AA{} at room temperature. The rotation angle between the a axis of the rectangular lattice and the graphite 〈100〉 direction of real space is \ensuremath{\theta}=30\ifmmode^\circ\else\textdegree\fi{}+\ensuremath{\Omega}, with \ensuremath{\Omega}=\ifmmode\pm\else\textpm\fi{}8.1\ifmmode^\circ\else\textdegree\fi{} below ${\mathit{T}}_{\mathit{c}}$, and gradually changes to \ensuremath{\theta}=30\ifmmode^\circ\else\textdegree\fi{} well above ${\mathit{T}}_{\mathit{c}}$. An explanation of the phase transition at ${\mathit{T}}_{\mathit{c}}$ is presented in terms of a rectangular superlattice that is commensurate with the graphite lattice.