Structural phase transition of high-stage ${\mathrm{MoCl}}_5$ graphite intercalation compounds

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 ${\mathrm{MoCl}}_{5\mathrm{-}}$GIC undergoes a structural phase transition at ${\mathit{T}}_{\mathit{c}}$=480 K, where the specific heat shows a λ-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±0.02 Å and b=17.93±0.02 Å at room temperature. The rotation angle between the a axis of the rectangular lattice and the graphite 〈100〉 direction of real space is θ=30°+Ω, with Ω=±8.1° below ${\mathit{T}}_{\mathit{c}}$, and gradually changes to θ=30° 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.